28: Invertebrates Flashcards
Phylum Porifera, Phylum Cnidaria, Superphylum Lophotrochozoa, Superphylum Ecdysozoa, Superphylum Deuterostomia (326 cards)
1
Q
What percentage of animal species are invertebrates?
A
Nearly 97%.
2
Q
What is an amoebocyte?
A
A sponge cell with multiple functions, including nutrient delivery, egg formation, sperm delivery, and cell differentiation. AKA archaeocyte.
3
Q
What is a choanocyte?
A
A sponge cell that functions to generate a water current and to trap and ingest food particles via phagocytosis. AKA collar cell.
4
Q
What is a gemmule?
A
An environmentally-resistant structure produced by asexual reproduction in adult freshwater sponges wherein the typical sponge morphology is inverted.
5
Q
What are invertebrata?
A
A category of animals that do not possess a cranium or vertebral column. AKA invertebrates.
6
Q
What is mesohyl?
A
A collagen-like gel containing suspended cells that perform various functions in the sponge.
7
Q
What is an osculum?
A
A large opening in the sponge’s body through which water leaves.
8
Q
What is an ostium?
A
A pore present on the sponge’s body through which water enters.
9
Q
What is a pinacocyte?
A
An epithelial-like cell that forms the outermost layer of sponges and encloses the mesohyl.
10
Q
What is Porifera?
A
A phylum of animals with no true tissues, but a porous body with rudimentary endoskeleton.
11
Q
What is a sclerocyte?
A
A cell that secretes silica spicules into the mesohyl.
12
Q
What is a spicule?
A
A structure made of silica or calcium carbonate that provides structural support for sponges.
13
Q
What is a spongocoel?
A
A central cavity within the body of some sponges.
14
Q
How does sponge motility change depending on life cycle stage?
A
Sponge larvae are able to swim; however, adults are non-motile and spend their life attached to a substratum.
15
Q
Which structures in sponge bodies enable water movement throughout the sponge?
A
Structures such as canals, chambers, and cavities enable water to move through the sponge to nearly all body cells.
16
Q
Why is water vital to sponges?
A
Water is vital to sponges for excretion, feeding, and gas exchange.
17
Q
How can the morphology of sponges be described?
A
The morphology of the simplest sponges takes the shape of a cylinder with a large central cavity, the spongocoel, occupying the inside of the cylinder. Water can enter into the spongocoel from numerous pores in the body wall. Water entering the spongocoel is extruded via a large common opening called the osculum. However, sponges exhibit a range of diversity in body forms, including variations in the size of the spongocoel, the number of osculi, and where the cells that filter food from the water are located.
18
Q
Which class is the exception to tissue specialization in sponges?
A
Hexactinellids have specialized tissues.
19
Q
What are the general components found in sponges?
A
Pinacocytes form the outermost layer of sponges and enclose the mesohyl. The gel-like consistency of mesohyl acts like an endoskeleton and maintains the tubular morphology of sponges. In addition to the osculum, sponges have multiple pores called ostia on their bodies that allow water to enter the sponge.
20
Q
How are ostia formed?
A
In some sponges, ostia are formed by porocytes, single tube-shaped cells that act as valves to regulate the flow of water into the spongocoel. In other sponges, ostia are formed by folds in the body wall of the sponge.
21
Q
Where are choanocytes located in sponges?
A
Choanocytes are present at various locations, depending on the type of sponge, but they always line the inner portions of some space through which water flows (the spongocoel in simple sponges, canals within the body wall in more complex sponges, and chambers scattered throughout the body in the most complex sponges). Whereas pinacocytes line the outside of the sponge, choanocytes tend to line certain inner portions of the sponge body that surrounds the mesohyl.
22
Q
Which group of protists are choanocytes similar to and what is the significance?
A
Choanocytes and choanoflagellates are similar in appearance, which suggests that sponges and choanoflagellates are closely related and likely share a recent common ancestry.
23
Q
Where are choanocytes embedded within sponges?
A
The cell body is embedded in mesohyl and contains all organelles required for normal cell function, but protruding into the “open space” inside of the sponge is a mesh-like collar composed of microvilli with a single flagellum in the center of the column.
24
Q
How do choanocytes facilitate water movement throughout a sponge?
A
The cumulative effect of the flagella from all choanocytes aids the movement of water through the sponge: drawing water into the sponge through the numerous ostia, into the spaces lined by choanocytes, and eventually out through the osculum (or osculi).
25
How do choanocytes facilitate feeding in sponges?
Food particles, including waterborne bacteria and algae, are trapped by the sieve-like collar of the choanocytes, slide down into the body of the cell, are ingested by phagocytosis, and become encased in a food vacuole.
26
How do choanocytes facilitate sexual reproduction in sponges?
Choanocytes will differentiate into sperm for sexual reproduction, where they will become dislodged from the mesohyl and leave the sponge with expelled water through the osculum.
27
What are the two crucial cells in sponges?
Choanocytes and amoebocytes.
28
How did amoebocytes get their name?
They are named for the fact that they move throughout the mesohyl in an amoeba-like fashion.
29
What are some of the functions of amoebocytes?
Delivering nutrients from choanocytes to other cells within the sponge, giving rise to eggs for sexual reproduction (which remain in the mesohyl), delivering phagocytized sperm from choanocytes to eggs, and differentiating into more-specific cell types.
30
What are some of the specific cell types that amoebocytes differentiate into?
Collencytes and lophocytes, which produce the collagen-like protein to maintain the mesohyl, sclerocytes, which produce spicules in some sponges, and spongocytes, which produce the protein spongin in the majority of sponges. These cells produce collagen to maintain the consistency of the mesohyl.
31
Why are spicules found in the mesohyl of sponges?
In some sponges, sclerocytes secrete small spicules into the mesohyl, which are composed of either calcium carbonate or silica, depending on the type of sponge. These spicules serve to provide additional stiffness to the body of the sponge. Additionally, spicules, when present externally, may ward off predators.
32
How is the presence and composition of spicules and spongin used to classify sponges?
Class Calcarea contains calcium carbonate spicules and no spongin, class Hexactinellida contains six-rayed siliceous spicules and no spongin, and class Demospongia contains spongin and may or may not have spicules; if present, those spicules are siliceous. Spicules are most conspicuously present in class Hexactinellida, the order consisting of glass sponges.
33
What is an example of large spicules?
Some of the spicules may attain giant proportions (in relation to the typical size range of glass sponges of 3 to 10 mm) as seen in *Monorhaphis chuni*, which grows up to 3 m long.
34
What are some example species of the three classes of sponges?
*Clathrina clathrus* belongs to class Calcarea, *Staurocalyptus* spp. belongs to class Hexactinellida, and *Acarnus erithacus* belongs to class Demospongia.
35
How do sponges perform digestion?
Their food is trapped when water passes through the ostia and out through the osculum. Bacteria smaller than 0.5 microns in size are trapped by choanocytes, which are the principal cells engaged in nutrition, and are ingested by phagocytosis. Particles that are larger than the ostia may be phagocytized by pinacocytes. In some sponges, amoebocytes transport food from cells that have ingested food particles to those that do not. For this type of digestion, in which food particles are digested within individual cells, the sponge draws water through diffusion. The limit of this type of digestion is that food particles must be smaller than individual cells.
36
How is diffusion used in sponges?
Several major body functions in sponges (gas exchange, circulation, excretion) are performed by diffusion between the cells that line the openings within the sponge and the water that is passing through those openings. All cell types within the sponge obtain oxygen from water through diffusion. Likewise, carbon dioxide is released into seawater by diffusion. In addition, nitrogenous waste produced as a byproduct of protein metabolism is excreted via diffusion by individual cells into the water as it passes through the sponge.
37
What is sponge fragmentation?
Where a piece of sponge breaks off, settles on a new substrate, and develops into a new individual.
38
What is sponge budding?
Where a genetically identical outgrowth grows from the parent and eventually detaches or remains attached to form a colony.
39
What are the modes of asexual reproduction in sponges?
Fragmentation, budding, and the formation of gemmules.
40
What are gemmules made of?
In gemmules, an inner layer of amoebocytes is surrounded by a layer of collagen (spongin) that may be reinforced by spicules. The collagen that is normally found in the mesohyl becomes the outer protective layer.
41
Why are gemmules useful?
In freshwater sponges, gemmules may survive hostile environmental conditions like changes in temperature and serve to recolonize the habitat once environmental conditions stabilize. Gemmules are capable of attaching to a substratum and generating a new sponge. Since gemmules can withstand harsh environments, are resistant to desiccation and remain dormant for long periods, they are an excellent means of colonization for a sessile organism.
42
Are sponges monoecious or dioecious?
Sponges are monoecious (hermaphroditic), which means that one individual can produce both gametes (eggs and sperm) simultaneously.
43
When does gamete production occur in sponges?
In some sponges, production of gametes may occur throughout the year, whereas other sponges may show sexual cycles depending on water temperature. Sponges may also become sequentially hermaphroditic, producing oocytes first and spermatozoa later.
44
How do oocytes and spermatozoa arise in sponges?
Oocytes arise by the differentiation of amoebocytes and are retained within the spongocoel, whereas spermatozoa result from the differentiation of choanocytes and are ejected via the osculum.
45
How do spermatozoa reach the oocytes of other sponges?
Ejection of spermatozoa may be a timed and coordinated event, as seen in certain species. Spermatozoa carried along by water currents can fertilize the oocytes borne in the mesohyl of other sponges.
46
Where are sponge larvae found?
Early larval development occurs within the sponge, and free swimming larvae are then released via the osculum.
47
Are adult sponges motile?
Sponges are generally sessile as adults and spend their lives attached to a fixed substratum. They do not show movement over large distances like other free-swimming marine invertebrates. However, sponge cells are capable of creeping along substrata via organizational plasticity. Under experimental conditions, researchers have shown that sponge cells spread on a physical support demonstrate a leading edge for directed movement. It has been speculated that this localized creeping movement may help sponges adjust to microenvironments near the point of attachment. It must be noted, however, that this pattern of movement has been documented in laboratories, but it remains to be observed in natural sponge habitats.
48
What is Cnidaria?
A phylum of animals that are diploblastic and have radial symmetry.
49
What is a cnidocyte?
A specialized stinging cell found in cnidarians.
50
What is the epidermis?
The outer layer (from ectoderm) that lines the outside of the animal.
51
What is extracellular digestion?
When food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb nutrients.
52
What is the gastrodermis?
The inner layer (from endoderm) that lines the digestive cavity.
53
What is a gastrovascular cavity?
An opening that serves as both a mouth and an anus, which is termed an incomplete digestive system.
54
What is a medusa?
A free-floating cnidarian body plan with mouth on underside and tentacles hanging down from a bell.
55
What is the mesoglea?
The non-living, gel-like matrix present between ectoderm and endoderm in cnidarians.
56
What is a nematocyst?
A harpoon-like organelle within a cnidocyte with pointed projectile and poison to stun and entangle prey.
57
What is a polyp?
A stalk-like sessile life form of cnidarians with mouth and tentacles facing upward, usually sessile but may be able to glide along surfaces.
58
What does it mean to be polymorphic?
Possessing multiple body plans within the lifecycle of a group of organisms.
59
What percentage of cnidarians are marine species?
Nearly all, about 99%.
60
How do cnidarians use toxins?
Cnidarians contain specialized cells known as cnidocytes containing organelles called nematocysts. These cells are present around the mouth and tentacles, and serve to immobilize prey with toxins contained within the cells. Nematocysts contain coiled threads that may bear barbs. The outer wall of the cell has hairlike projections called cnidocils, which are sensitive to touch. When touched, the cells are known to fire coiled threads that can either penetrate the flesh of the prey or predators of cnidarians or ensnare it. These coiled threads release toxins into the target and can often immobilize prey or scare away predators.
61
What are the two body plans of Cnidaria?
Polyp (or “stalk”) and medusa (or “bell”).
62
What is an example of each cnidarian body plan type?
An example of the polyp form is *Hydra* spp.; perhaps the most well-known medusoid animals are the jellies (jellyfish).
63
How motile are the cnidarian body plans and where are their oral sides oriented?
Polyp forms are sessile as adults, with a single opening to the digestive system (the mouth) facing up with tentacles surrounding it. Medusa forms are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell.
64
What is an example of a polymorphic cnidarian?
The colonial hybrid called an *Obelia*.
65
What are the two types of sessile polyps in *Obelia*?
The first is the gastrozooid, which is adapted for capturing prey and feeding; the other type of polyp is the gonozooid, adapted for the asexual budding of medusa.
66
How do gonozooids produce new polyps?
When the reproductive buds of gonozooids mature, they break off and become free-swimming medusae, which are dioecious (either male or female). The male medusa makes sperm, and the female medusa makes eggs. After fertilization, the zygote develops into a blastula, which develops into a planula larva. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed.
67
What are the membrane layers of cnidarians?
All cnidarians show the presence of two membrane layers in the body that are derived from the endoderm and ectoderm of the embryo. The outer layer (from ectoderm) is called the epidermis and lines the outside of the animal, whereas the inner layer (from endoderm) is called the gastrodermis and lines the digestive cavity. Between the two membrane layers is a non-living, jelly-like mesoglea connective layer.
68
How complex is the anatomy of cnidarians, in terms of tissues and organs?
In terms of cellular complexity, cnidarians show the presence of differentiated cell types in each tissue layer, such as nerve cells, contractile epithelial cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as the presence of intercellular connections. However, the development of organs or organ systems is not advanced in this phylum.
69
How is the cnidarian nervous system organized?
The nervous system is primitive, with nerve cells scattered across the body. This nerve net may show the presence of groups of cells in the form of nerve plexuses or nerve cords. The nerve cells show mixed characteristics of motor as well as sensory neurons. The predominant signaling molecules in these primitive nervous systems are chemical peptides, which perform both excitatory and inhibitory functions. Despite the simplicity of the nervous system, it coordinates the movement of tentacles, the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.
70
How do cnidarians perform digestion?
The cnidarians perform extracellular digestion in a gastrovascular cavity.
71
How is gas exchange and circulation in cnidarians performed?
Cnidarian cells exchange oxygen and carbon dioxide by diffusion between cells in the epidermis with water in the environment, and between cells in the gastrodermis with water in the gastrovascular cavity. The lack of a circulatory system to move dissolved gases limits the thickness of the body wall and necessitates a non-living mesoglea between the layers. There is no excretory system or organs, and nitrogenous wastes simply diffuse from the cells into the water outside the animal or in the gastrovascular cavity. Because there is no circulatory system, nutrients must move from the cells that absorb them in the lining of the gastrovascular cavity through the mesoglea to other cells.
72
How many species of Cnidaria are there?
The phylum Cnidaria contains about 10,000 described species.
73
What are the classes of Cnidaria?
Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa.
74
Which cnidarians are included in the class Anthozoa?
The class Anthozoa includes all cnidarians that exhibit a polyp body plan only; in other words, there is no medusa stage within their life cycle.
75
What are some motile and non-motile examples of the cnidarian classes?
The anthozoans (sea anemones and corals) are all sessile species, whereas the scyphozoans (jellyfish) and cubozoans (box jellies) are swimming forms. The hydrozoans contain sessile forms and swimming colonial forms like the Portuguese man o' war.
76
What are some examples of anthozoans?
Sea anemones, sea pens, and corals.
77
How many species of anthozoans are there?
There is an estimated number of 6100 described species.
78
What are some morphological characteristics of sea anemones?
Sea anemones are usually brightly colored and can attain a size of 1.8 to 10 cm in diameter. These animals are usually cylindrical in shape and are attached to a substrate. A mouth opening is surrounded by tentacles bearing cnidocytes.
79
How is the mouth of a sea anemone linked to the gastrovascular cavity?
The mouth of a sea anemone is surrounded by tentacles that bear cnidocytes. The slit-like mouth opening and pharynx are lined by a groove called a siphonophore. The pharynx is the muscular part of the digestive system that serves to ingest as well as egest food, and may extend for up to ⅔ the length of the body before opening into the gastrovascular cavity.
80
How is the gastrovascular cavity of sea anemones divided?
The gastrovascular cavity of sea anemones is divided into several chambers by longitudinal septa called mesenteries. Each mesentery consists of one ectodermal and one endodermal cell layer with the mesoglea sandwiched in between. Mesenteries do not divide the gastrovascular cavity completely, and the smaller cavities coalesce at the pharyngeal opening. The adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas exchange.
81
What do anemones feed on?
Sea anemones feed on small fish and shrimp, usually by immobilizing their prey using the cnidocytes.
82
How do sea anemones establish mutualistic relationships with hermit crabs?
Some sea anemones establish a mutualistic relationship with hermit crabs by attaching to the crab's shell. In this relationship, the anemone gets food particles from prey caught by the crab, and the crab is protected from the predators by the stinging cells of the anemone.
83
How are clownfish able to establish mutualistic relationships with sea anemones?
Anemone fish, or clownfish, are able to live in the anemone since they are immune to the toxins contained within the nematocysts.
84
How do anthozoans reproduce?
Anthozoans remain polypoid throughout their lives and can reproduce asexually by budding or fragmentation, or sexually by producing gametes. Both gametes are produced by the polyp, which can fuse to give rise to a free-swimming planula larva. The larva settles on a suitable substratum and develops into a sessile polyp.
85
Which cnidarians are in the class Scyphozoa?
Class Scyphozoa includes all the jellies and is exclusively a marine class of animals.
86
How many species of Scyphozoa are there?
Class Scyphozoa includes about 200 known species
87
Which body plan type is found in scyphozoans?
The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present. Scyphozoans display a characteristic bell-like morphology.
88
How large are scyphozoans?
Members can range from 2 to 40 cm in length but the largest scyphozoan species, *Cyanea capillata*, can reach a size of 2 m across.
89
How are the body plans of jellies organized?
In the jellyfish, a mouth opening is present on the underside of the animal, surrounded by tentacles bearing nematocysts. Scyphozoans live most of their life cycle as free-swimming, solitary carnivores. The mouth leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli. In some species, the digestive system may be further branched into radial canals. Like the septa in anthozoans, the branched gastrovascular cells serve two functions: to increase the surface area for nutrient absorption and diffusion; thus, more cells are in direct contact with the nutrients in the gastrovascular cavity.
90
Where is the nervous system found in scyphozoans?
Nerve cells are scattered all over the body. Neurons may even be present in clusters called rhopalia.
91
Which muscles do scyphozoans use to swim?
They possess a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water.
92
How do scyphozoans reproduce?
Scyphozoans are dioecious animals with separate sexes. The gonads are formed from the gastrodermis and gametes are expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form known as scyphistoma. These forms may produce additional polyps by budding or may transform into the medusoid form. The life cycle of these animals can be described as polymorphic, because they exhibit both a medusal and polypoid body plan at some point in their life cycle.
93
Which cnidarians are in the class Cubozoa?
This class includes jellies that have a box-shaped medusa, or a bell that is square in cross-section; hence, are colloquially known as “box jellyfish”.
94
How large are cubozoans?
They may achieve sizes of 15 to 25 cm.
95
Besides their bell, how do cubozoans differ from scyphozoans?
Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles. This is the most venomous group of all the cnidarians.
96
What are some characteristics of cubozoan body plans?
The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. These animals are further classified into orders based on the presence of single or multiple tentacles per pedalium. In some cases, the digestive system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey.
97
What forms do cubozoans take?
Cubozoans exist in a polypoid form that develops from a planula larva. These polyps show limited mobility along the substratum and, like scyphozoans, may bud to form more polyps to colonize a habitat. Polyp forms then transform into the medusoid forms.
98
What is *Malo kingi*?
A tiny cubozoan jelly which is thimble-shaped and, like all cubozoan jellies, has four muscular pedalia to which the tentacles attach. *M. kingi* is one of two species of jellies known to cause Irukandji syndrome, a condition characterized by excruciating muscle pain, vomiting, increased heart rate, and psychological symptoms.
99
How many species of hydrozoans are there?
Hydrozoa includes nearly 3200 species; most are marine, although some freshwater species are known.
100
Which body plan types are found in hydrozoans?
Animals in this class are polymorphs, and most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable.
101
How is the body plan of the hydrozoan polyp form organized?
The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal.
102
What are some examples of colonial hydrozoans?
Many hydrozoans form colonies that are composed of a branched colony of specialized polyps that share a gastrovascular cavity, such as in the colonial hydroid *Obelia*. Colonies may also be free-floating and contain medusoid and polypoid individuals in the colony as in *Physalia* (the Portuguese man o' war) or *Velella* (by-the-wind sailor).
103
What are some examples of solitary hydrozoans?
Members of *Hydra* are solitary polyps and members of *Gonionemus* are solitary medusae.
104
What is the distinguishing characteristic of hydrozoans?
The true characteristic shared by all the diverse species of hydrozoans is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue.
105
What is Annelida?
A phylum of vermiform animals with metamerism.
106
What is a captacula?
A tentacle-like projection that is present in tusk shells to catch prey.
107
What is a clitellum?
A specialized band of fused segments, which aids in reproduction.
108
What is a conispiral?
Shell shape coiled around a horizontal axis.
109
What is a corona?
A wheel-like structure on the anterior portion of the rotifer that contains cilia and moves food and water toward the mouth.
110
What is a ctenidium?
A specialized gill structure in mollusks.
111
What is a mantle?
A specialized epidermis that encloses all visceral organs and secretes shells. AKA pallium.
112
What is a mastax?
A jawed pharynx unique to the rotifers.
113
What is metamerism?
A series of body structures that are similar internally and externally, such as segments.
114
What is Mollusca?
A phylum of protostomes with soft bodies and no segmentation.
115
What is nacre?
A calcareous secretion produced by bivalves to line the inner side of shells as well as to coat intruding particulate matter.
116
What is Nemertea?
A phylum of dorsoventrally flattened protostomes known as ribbon worms.
117
What is a parapodium?
A fleshy, flat appendage that protrudes in pairs from each segment of polychaetes.
118
What is a pilidium?
A larval form found in some nemertine species.
119
What does it mean to be planospiral?
A shell shape coiled around a vertical axis.
120
What is a planuliform?
A larval form found in phylum Nemertea.
121
What is a radula?
A tongue-like organ with chitinous ornamentation.
122
What is a rhynchocoel?
A cavity present above the mouth that houses the proboscis.
123
What is a schizocoelom?
A coelom formed by groups of cells that split from the endodermal layer.
124
What is a seta/chaeta?
A chitinous projection from the cuticle.
125
What is a trochophore?
The first of the two larval stages in mollusks.
126
What is a veliger?
The second of the two larval stages in mollusks.
127
What are some characteristics of embryonic development in lophotrochozoans?
Animals belonging to the superphylum Lophotrochozoa are protostomes and have schizocoeloms.
128
How do schizocoeloms develop?
In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band and then splits internally to form the coelom; this protostomic coelom is hence termed schizocoelom.
129
What characteristics distinguish lophotrochozoans?
As lophotrochozoans, the organisms in this superphylum possess either a lophophore or trochophore larvae. The lophophores include groups that are united by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth. Trochophore larvae are characterized by two bands of cilia around the body.
130
What are some examples of lophophorates?
Lophophorata include the flatworms and several other phyla.
131
Why are lophophorates and trochozoans grouped into the lophotrochozoan clade?
These clades are upheld when RNA sequences are compared.
132
How many germ layers do lophotrochozoans have?
The lophotrochozoans are triploblastic and possess an embryonic mesoderm sandwiched between the ectoderm and endoderm.
133
What kind of symmetry do lophotrochozoans exhibit?
Lophotrochozoans are bilaterally symmetrical.
134
What is cephalization?
The concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.
135
How did cephalization evolve?
The development of bilateral symmetry precipitated the evolution of cephalization.
136
Are flatworms free-living or parasitic?
The flatworms include many free-living and parasitic forms, including important parasites of humans.
137
What are the two lineages of Platyhelminthes?
Catenulida and Rhabditophora.
138
What are the Catenulida?
The Catenulida, or “chain worms”, is a small clade of just over 100 species. These worms typically reproduce asexually by budding, however, the offspring do not fully detach from the parents, and resemble a chain in appearance.
139
How can the tissues of flatworms be described?
Flatworms have three embryonic tissue layers that give rise to surfaces that cover tissues (from ectoderm), internal tissues (from mesoderm), and line the digestive system (from endoderm). The epidermal tissue is a single layer of cells or a layer of fused cells (syncytium) that covers a layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include mesenchymal cells that contain collagen and support secretory cells that secrete mucus and other materials at the surface.
140
Do flatworms have coeloms?
The flatworms are acoelomates, so their bodies are solid between the outer surface and the cavity of the digestive system.
141
How can flatworms be described according to their food sources?
The free-living species of flatworms are predators or scavengers. Parasitic forms feed on the tissues of their hosts.
142
How can the digestive system of flatworms be described?
Most flatworms have a gastrovascular cavity rather than a complete digestive system. In such animals, the “mouth” is also used to expel waste materials from the digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system.
143
How can the excretory system of flatworms be described?
Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes.
144
How can the nervous system of flatworms be described?
The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells.
145
Why are flatworms flat?
There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms.
146
How can the body plan of a planarian be described?
The planarian is a flatworm that has a gastrovascular cavity with one opening that serves as both mouth and anus. The excretory system is made up of tubules connected to excretory pores on both sides of the body. The nervous system is composed of two interconnected nerve cords running the length of the body, with cerebral ganglia and eyespots at the anterior end.
147
What are the classes of flatworms?
Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda. The relationships among members of these classes is being reassessed, with the turbellarians in particular now viewed as a paraphyletic group.
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What is an example of a turbellarian?
Class Turbellaria includes the Bedford's flatworm (*Pseudobiceros bedfordi*), which is about 8 to 10 cm in length.
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What is an example of a monogenean?
The parasitic class Monogenea includes *Dactylogyrus* spp. *Dactylogyrus*, commonly called a gill fluke, is about 0.2 mm in length and has two anchors that it uses to latch onto the gills of host fish.
150
What are some examples of trematodes?
The Trematoda class includes *Fascioloides magna* and *Fasciola hepatica*, also known as the common liver fluke.
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What is an example of a cestode?
Class Cestoda includes tapeworms such as *Taenia saginata*, which infects both cattle and humans and can reach 4 to 10 meters in length.
152
How can the turbellarians be described?
The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments. The ventral epidermis of turbellarians is ciliated and facilitates their locomotion. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment.
153
How can the monogeneans be described?
The monogeneans are ectoparasites, mostly of fish, with simple lifecycles that consist of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. The parasite has only one host and that host is usually only one species. The worms may produce enzymes that digest the host tissues or simply graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the male gametes develop first and so cross-fertilization is quite common.
154
How can trematodes be described?
The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex lifecycles that involve a primary host in which sexual reproduction occurs, and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk.
155
What is an example of a human disease caused by trematodes?
Trematodes are responsible for serious human diseases including schistosomiasis, a blood fluke. The disease infects an estimated 200 million people in the tropics, leading to organ damage and chronic symptoms like fatigue. Infection occurs when the human enters the water and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the primary snail host.
156
How can cestodes be described?
The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but contain reproductive structures, both male and female. Tapeworms do not possess a digestive system; instead, they absorb nutrients from the food matter passing them in the host's intestine.
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How do tapeworms reproduce?
Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point, they are “mature” and all structures except fertilized eggs have degenerated. Most reproduction occurs by cross-fertilization. The proglottid detaches from the body of the worm and is released into the feces of the organism. The eggs are eaten by an intermediate host. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are transmitted by eating uncooked or poorly cooked pork, beef, and fish.
158
What are rotifers?
The rotifers are microscopic (about 100 µm to 30 mm) group of mostly aquatic organisms that get their name from the corona, a rotating, wheel-like structure that is covered with cilia at their anterior end.
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How are rotifers classified?
Although their taxonomy is currently in flux, one treatment places the rotifers in three classes: Bdelloidea, Monogononta, and Seisonidea. The classification of the group is currently under revision, however, as more phylogenetic evidence becomes available. It is possible that the “spiny headed worms” currently in phylum Acanthocephala will be incorporated into this group in the future.
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How are the bodies of rotifers organized?
The body form of rotifers consists of a head (which contains the corona), a trunk (which contains the organs), and the foot. Rotifers are typically free-swimming and truly planktonic organisms, but the toes or extensions of the foot can secrete a sticky material forming a holdfast to help them adhere to surfaces. The head contains sensory organs in the form of a bi-lobed brain and small eyespots near the corona.
161
How do rotifers feed?
The rotifers are filter feeders that will eat dead material, algae, and other microscopic living organisms, and are therefore very important components of aquatic food webs. Rotifers obtain food that is directed toward the mouth by the current created from the movement of the corona. The food particles enter the mouth and travel to the mastax (pharynx with jaw-like structures). Food then passes by digestive and salivary glands, and into the stomach, then into the intestines. Digestive and excretory wastes are collected in a cloacal bladder before being released out the anus.
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Do rotifers have coeloms?
Rotifers are pseudocoelomates.
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Where are rotifers found?
Rotifers are commonly found in freshwater and some saltwater environments throughout the world.
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How many species of rotifer are there?
About 2200 species of rotifers have been identified.
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What are some sexual characteristics of rotifers?
Rotifers are dioecious organisms (having either male or female genitalia) and exhibit sexual dimorphism (males and females have different forms). Many species are parthenogenic and exhibit haplodiploidy, a method of sex determination in which a fertilized egg develops into a female and an unfertilized egg develops into a male. In many dioecious species, males are short-lived and smaller with no digestive system and a single testis. Females can produce eggs that are capable of dormancy for protection during harsh environmental conditions.
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What is the colloquial name for nemertini?
Ribbon worms.
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Where are nemertini found?
Most species of phylum Nemertea are marine, predominantly benthic or bottom dwellers, however, nemertini have been recorded in freshwater and terrestrial habitats as well.
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How many species of nemertini are there?
There are an estimated 900 species known.
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How do nemertini feed?
Most nemerteans are carnivores, feeding on worms, clams, and crustaceans. Some species are scavengers, and some nemertini species, like *Malacobdella grossa*, have also evolved commensal relationships with some mollusks. Some species have devastated commercial fishing of clams and crabs. Nemerteans have almost no predators and two species are sold as fish bait.
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How can the morphology of nemertini be described?
Ribbon worms vary in size from 1 cm to several meters. They show bilateral symmetry and remarkable contractile properties. Because of their contractability, they can change their morphological presentation in response to environmental cues. Animals in phylum Nemertea show a flattened morphology, that is, they are flat from front to back, like a flattened tube. Nemertea are soft and unsegmented animals.
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What is an example of a nemertean?
The proboscis worm (*Parborlasia corrugatus*) is a scavenger that combs the sea floor for food.
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What is a unique characteristic of nemertini?
A unique characteristic of Nemertea is the presence of a proboscis enclosed in a rhynchocoel. The proboscis serves to capture food and may be ornamented with barbs in some species. The rhynchocoel is a fluid-filled cavity that extends from the head to nearly two-thirds of the length of the gut in these animals. The proboscis may be extended or retracted by the retractor muscle attached to the wall of the rhynchocoel.
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How can the digestive system of nemertini be described?
The nemertini show a very well-developed digestive system. A mouth opening that is ventral to the rhynchocoel leads into the foregut, followed by the intestine. The intestine is present in the form of diverticular pouches and ends in a rectum that opens via an anus. Gonads are interspersed with the intestinal diverticular pouches and open outwards via genital pores.
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How can the circulatory system of nemertini be described?
A circulatory system consists of a closed loop of a pair of lateral blood vessels. The circulatory system is derived from the coelomic cavity of the embryo. Some animals may also have cross-connecting vessels in addition to lateral ones. Although these are called blood vessels, since they are of coelomic origin, the circulatory fluid is colorless. Some species bear hemoglobin as well as other yellow or green pigments. The blood vessels are connected to the rhynchocoel. The flow of fluid in these vessels is facilitated by the contraction of muscles in the body wall. A pair of protonephridia, or primitive kidneys, is present in these animals to facilitate osmoregulation. Gaseous exchange occurs through the skin in the nemertini.
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How can the nervous system of nemertini be described?
Nemertini have a ganglion or “brain” situated at the anterior end between the mouth and the foregut, surrounding the digestive system as well as the rhynchocoel. A ring of four nerve masses called “ganglia” composes the brain in these animals. Paired longitudinal nerve chords emerge from the brain ganglia and extend to the posterior end. Ocelli or eyespots are present in pairs, in multiples of two in the anterior portion of the body. It is speculated that the eyespots originate from neural tissue and not from the epidermis.
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What is the life cycle of nemertini?
Animals in phylum Nemertea show sexual dimorphism, although freshwater species may be hermaphroditic. Eggs and sperm are released into the water, and fertilization occurs externally. The zygote then develops into a planuliform larva. In some nemertine species, a pilidium larva may develop inside the young worm, from a series of imaginal discs. This larval form, characteristically shaped like a deerstalker cap, devours tissues from the young worm for survival before metamorphosing into the adult-like morphology.
177
How predominant are mollusks?
The phylum Mollusca is the predominant phylum in marine environments. It is estimated that 23% of all known marine species are mollusks; there are over 85,000 described species, making them the second most diverse phylum of animals.
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What does the name “mollusca” connote?
The name “mollusca” signifies a soft body, since the earliest descriptions of mollusks came from observations of unshelled cuttlefish.
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Where are mollusks found?
Mollusks are predominantly a marine group of animals; however, they are known to inhabit freshwater as well as terrestrial habitats.
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What are some common morphological characteristics of mollusks?
Mollusks display a wide range of morphologies in each class and subclass, but share a few key characteristics, including a muscular foot, a visceral mass containing internal organs, and a mantle that may or may not secrete a shell of calcium carbonate.
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What are some characteristics of the muscular foot of mollusks?
Mollusks have a muscular foot, which is used for locomotion and anchorage, and varies in shape and function, depending on the type of mollusk under study. In shelled mollusks, this foot is usually the same size as the opening of the shell. The foot is a retractable as well as an extendable organ. The foot is the ventral-most organ, whereas the mantle is the limiting dorsal organ.
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Do mollusks have coeloms?
Mollusks are eucoelomates, but the coelomic cavity is restricted to a cavity around the heart in adult animals. The mantle cavity develops independently of the coelomic cavity.
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What is the visceral mass in mollusks?
The visceral mass is present above the foot, in the visceral hump. This includes digestive, nervous, excretory, reproductive, and respiratory systems.
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What respiratory organs do mollusks have?
Mollusk species that are exclusively aquatic have gills for respiration, whereas some terrestrial species have lungs for respiration.
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What is the purpose of a radula?
A tongue-like organ called a radula, which bears chitinous tooth-like ornamentation, is present in many species, and serves to shred or scrape food.
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What is the purpose of the mantle?
The mantle (AKA the pallium) is the dorsal epidermis in mollusks; shelled mollusks are specialized to secrete a chitinous and hard calcareous shell.
187
What is the life cycle of a mollusk?
Most mollusks are dioecious animals and fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails and slugs, or in cephalopods. In some mollusks, the zygote hatches and undergoes two larval stages - trochophore and veliger - before becoming a young adult; bivalves may exhibit a third larval stage, glochidia.
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What are some general forms of mollusks?
Mollusks have a dramatic variety of form, ranging from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately sculpted and colored shells.
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What are the classes in the phylum Mollusca?
Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.
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What are some characteristics of the class Aplacophora?
Class Aplacophora ("bearing no plates") includes worm-like animals primarily found in benthic marine habitats. These animals lack a calcareous shell but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs).
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What are some characteristics of the class Monoplacophora?
Members of the class Monoplacophora ("bearing one plate") possess a single, cap-like shell that encloses the body. The morphology of the shell and underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of *Neopilina galatheae* in 1952. Today, scientists have identified nearly two dozen extant species.
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What are some characteristics of the class Polyplacophora?
Animals in the class Polyplacophora ("bearing many plates") are commonly known as “chitons” and bear an armor-like eight-plated shell. These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Respiration is facilitated by ctenidia (gills) that are present ventrally. These animals possess a radula that is modified for scraping. The nervous system is rudimentary with only buccal or “cheek” ganglia present at the anterior end. Eyespots are absent in these animals. A single pair of nephridia for excretion is present.
193
What are some characteristics of the class Bivalvia?
Class Bivalvia ("two shells") includes clams, oysters, mussels, scallops, and geoducks. Members of this class are found in marine as well as freshwater habitats. As the name suggests, bivalves are enclosed in a pair of shells (valves are commonly called “shells”) that are hinged at the dorsal end by shell ligaments as well as shell teeth. The overall morphology is laterally flattened, and the head region is poorly developed. Eyespots and statocysts may be absent in some species. Since these animals are suspension feeders, a radula is absent in this class of mollusks. Respiration is facilitated by a pair of ctenidia, whereas excretion and osmoregulation are brought about by a pair of nephridia. Bivalves often possess a large mantle cavity. In some species, the posterior edges of the mantle may fuse to form two siphons that serve to take in and exude water.
194
How do pearls form?
One of the functions of the mantle is to secrete the shell. Some bivalves like oysters and mussels possess the unique ability to secrete and deposit a calcareous nacre or “mother of pearl” around foreign particles that may enter the mantle cavity. This property has been commercially exploited to produce pearls.
195
What are some characteristics of the class Gastropoda?
Animals in the class Gastropoda ("stomach foot") include well-known mollusks like snails, slugs, conches, sea hares, and sea butterflies. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell. Shells may be planospiral (like a garden hose wound up), commonly seen in garden snails, or conispiral (like a spiral staircase), commonly seen in marine conches.
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What are some morphological characteristics of gastropods?
The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling. Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropod species. The mantle cavity encloses the ctenidia as well as a pair of nephridia.
197
What are conotoxins?
Marine snails of the genus *Conus* attack prey with a venomous sting. The toxin released, known as conotoxin, is a peptide with internal disulfide linkages. Conotoxins can bring about paralysis in humans, indicating that this toxin attacks neurological targets. Some conotoxins have been shown to block neuronal ion channels.
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What are some potential applications of conotoxins in pharmacology?
Conotoxins are an exciting area of potential pharmacological development, since these peptides may be possibly modified and used in specific medical conditions to inhibit the activity of specific neurons. For example, these toxins may be used to induce paralysis in muscles in specific health applications, similar to the use of botulinum toxin. Since the entire spectrum of conotoxins, as well as their mechanisms of action, are not completely known, the study of their potential applications is still in its infancy. Most research has focused on their use to treat neurological diseases. They have also shown some efficacy in relieving chronic pain, and the pain associated with conditions like sciatica and shingles.
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What are some characteristics of the class Cephalopoda?
Class Cephalopoda ("head foot" animals) include octopi, squids, cuttlefish, and nautilus. Cephalopods are a class of shell-bearing animals as well as mollusks with a reduced shell. They display vivid coloration, typically seen in squids and octopi, which is used for camouflage. All animals in this class are carnivorous predators and have beak-like jaws at the anterior end. All cephalopods show the presence of a very well-developed nervous system along with eyes, as well as a closed circulatory system. The foot is lobed and developed into tentacles, and a funnel, which is used as their mode of locomotion. Suckers are present on the tentacles in octopi and squid. Ctenidia are enclosed in a large mantle cavity and are serviced by large blood vessels, each with its own heart associated with it; the mantle has siphonophores that facilitate exchange of water.
200
How is locomotion performed in cephalopods?
Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion. This is called “jet” propulsion.
201
How is the excretory system of cephalopods organized?
A pair of nephridia is present within the mantle cavity.
202
What is the life cycle of cephalopods?
Sexual dimorphism is seen in this class of animals. Members of a species mate, and the female then lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period. Reproduction in cephalopods is different from other mollusks in that the egg hatches to produce a juvenile adult without undergoing the trochophore and veliger larval stages.
203
What is a defense mechanism of some cephalopods?
Cephalopods such as squids and octopuses produce sepia or a dark ink, which is squirted upon a predator to assist in a quick getaway.
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What are some characteristics of shells in cephalopods?
In the shell-bearing *Nautilus* spp., the spiral shell is multi-chambered. These chambers are filled with gas or water to regulate buoyancy. The shell structure in squids and cuttlefish is reduced and is present internally in the form of a squid pen and cuttlefish bone, respectively.
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What are some characteristics of the class Scaphopoda?
Members of the class Scaphopoda ("boat feet") are known colloquially as “tusk shells” or “tooth shells”, as evident when examining *Dentalium*, one of the few remaining scaphopod genera. Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals bear a single conical shell, which has both ends open. The head is rudimentary and protrudes out of the posterior end of the shell. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Captaculae serve to catch and manipulate prey. Ctenidia are absent in these animals.
206
Which animals does the phylum Annelida include?
The phylum Annelida includes segmented worms, which includes earthworms, polychaete worms, and leeches.
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Where are annelids found?
They are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats.
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What is the origin of the name “Annelida”?
The name of the phylum is derived from the Latin word *annellus*, which means a small ring.
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How do annelids interact with other species in their environments?
Annelids show parasitic and commensal symbioses with other species in their habitat.
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How many species of annelids are there?
Approximately 16,500 species have been described in phylum Annelida.
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How do annelid blastopores develop?
Annelids show protostomic development in embryonic stages.
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Why are annelids called “segmented worms”?
Because of their key characteristic of metamerism, or true segmentation.
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What are some characteristics of annelid morphology?
Annelids display bilateral symmetry and are worm-like in overall morphology. Annelids have a segmented body plan wherein the internal and external morphological features are repeated in each body segment. Metamerism allows animals to become bigger by adding “compartments” while making their movement more efficient. This metamerism is thought to arise from identical teloblast cells in the embryonic stage, which give rise to identical mesodermal structures. The overall body can be divided into head, body, and pygidium (or tail). The clitellum is a reproductive structure that generates mucus that aids in sperm transfer and gives rise to a cocoon within which fertilization occurs; it appears as a fused band in the anterior third of the animal.
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What are some characteristics of annelid anatomy?
The epidermis is protected by an acellular, external cuticle, but this is much thinner than the cuticle found in the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are located interior to the epidermis. Chitinous hairlike extensions, anchored in the epidermis and projecting from the cuticle, called setae/chaetae are present in every segment. Annelids show the presence of a true coelom, derived from embryonic mesoderm and protostomy. Hence, they are the most advanced worms. A well-developed and complete digestive system is present in earthworms (oligochaetes) with a mouth, muscular pharynx, esophagus, crop, and gizzard being present. The gizzard leads to the intestine and ends in an anal opening. Each segment is limited by a membranous septum that divides the coelomic activity into a series of compartments.
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What are some characteristics of the circulatory system of annelids?
Annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the alimentary canal as well as capillaries that service individual tissues. In addition, these vessels are connected by transverse loops in every segment.
216
How does gas exchange occur in annelids?
Annelids lack a well-developed respiratory system, and gas exchange occurs across the moist surface.
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How does excretion occur in annelids?
Excretion is facilitated by a pair of metanephridia (a type of primitive “kidney” that consists of a convoluted tubule and an open, ciliated funnel) that is present in every segment towards the ventral side.
218
What are some characteristics of the nervous system of annelids?
Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx. The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment.
219
What are some reproductive characteristics of annelids?
Annelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious with temporary or seasonal gonads that develop (as in polychaetes). However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation.
220
What are the classes of the phylum Annelida?
Phylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earth worms, leeches, and their relatives).
221
How do earthworms differ from other annelids?
Earthworms are the most abundant members of the class Oligochaeta, distinguished by the presence of the clitellum as well as few, reduced chaetae ("oligo" = “few”; “chaetae” = “hairs”).
222
How do the chaetae differ between polychaetes and oligochaetes?
The number and size of chaetae are greatly diminished in Oligochaeta compared to the polychaetes. The many chaetae of polychaetes are also arranged within fleshy, flat, paired appendages that protrude from each segment called parapodia, which may be specialized for different functions in the polychaetes.
223
What are two subclasses of Oligochaeta?
The class Oligochaeta includes the subclass Hirudinia and the subclass Branchiobdella.
224
What are some examples of Hirudinea?
The subclass Hirudinea includes leeches such as *Hirudo medicinalis* and *Hemiclepsis marginata*.
225
How do leeches differ from other annelids?
A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and a lack of chaetae. Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates.
226
What are some examples of Branchiobdella?
The subclass Branchiobdella includes species like *Branchiobdella balcanica sketi* and *Branchiobdella astaci*, worms that show similarity with leeches as well as oligochaetes.
227
What is Arthropoda?
A phylum of animals with jointed appendages.
228
What does it mean to be biramous?
It refers to two branches per appendage.
229
What is a cephalothorax?
A fused head and thorax in some species.
230
What is a chelicera?
A modified first pair of appendages in subphylum Chelicerata.
231
What is a cuticle in animals?
The tough, external layer possessed by members of the invertebrate class Ecdysozoa that is periodically molted and replaced.
232
What is a cypris?
A larval stage in the early development of crustaceans.
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What is a hemocoel?
The internal body cavity seen in arthropods.
234
What does it mean to be hermaphrodite?
It refers to an animal where both male and female gonads are present in the same individual.
235
What is a nauplius?
A larval stage in the early development of crustaceans.
236
What is Nematoda?
A phylum of worm-like animals that are triploblastic, pseudocoelomates that can be free-living or parasitic.
237
What is an oviger?
An additional pair of appendages present on some arthropods between the chelicerae and pedipalps.
238
What is a pedipalp?
The second pair of appendages in Chelicerata.
239
What does it mean to be uniramous?
It refers to one branch per appendage.
240
What is a zoea?
A larval stage in the early development of crustaceans.
241
How diverse are the ecdysozoans?
The superphylum Ecdysozoa contains an incredibly large number of species. This is because it contains two of the most diverse animal groups: phylum Nematoda (the roundworms) and phylum Arthropoda (the arthropods).
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What is the distinguishing characteristic of ecdysozoans?
The most prominent distinguishing feature of ecdysozoans is their tough, external covering called the cuticle. The cuticle provides a tough, but flexible exoskeleton that protects these animals from water loss, predators, and other aspects of the external environment. All members of this superphylum periodically molt, or shed their cuticle as they grow. After molting, they secrete a new cuticle that will last until their next growth phase. The process of molting and replacing the cuticle is called ecdysis, which is how the superphylum derived its name.
243
What are some basic characteristics of nematodes?
Nematodes are triploblastic, bilaterally symmetrical, and pseudocoelomates, and consist of both free-living and parasitic forms.
244
How many species of nematodes are there?
Phylum Nematoda includes more than 28,000 species with an estimated 16,000 being parasitic in nature.
245
What is the origin of the name Nematoda?
It is derived from the Greek word “Nemos”, which means “thread”.
246
Where are nematodes found?
Nematodes are present in all habitats with a large number of individuals of each species present in each.
247
Which nematode species is used as a model organism?
The free-living nematode, *Caenorhabditis elegans* has been extensively used as a model system in laboratories all over the world.
248
How does the morphology of nematodes differ from that of cnidarians?
In contrast with cnidarians, nematodes show a tubular morphology and circular cross-section. They are pseudocoelomates and show the presence of a complete digestive system with a distinct mouth and anus. This is in contrast with the cnidarians, where only one opening is present (an incomplete digestive system).
249
What are some characteristics of the cuticle of nematodes?
The cuticle of nematodes is rich in collagen and chitin, and forms an external “skeleton” outside the epidermis. The cuticle also lines many of the organs internally, including the pharynx and rectum.
250
How is the epidermis of nematodes organized?
The epidermis can be either a single layer of cells or a syncytium.
251
What are some characteristics of the morphology of nematodes?
The overall morphology of nematodes is cylindrical. The head is radially symmetrical. A mouth opening is present at the anterior end with three or six lips as well as teeth in some species in the form of cuticle extensions. Some nematodes may present other external modifications like rings, head shields, or warts. Rings, however, do not reflect true internal body segmentation. The mouth leads to a muscular pharynx and intestine, which leads to a rectum and anal opening at the posterior end. The muscles of nematodes differ from those of most animals: they have a longitudinal layer only, which accounts for the whip-like motion of their movement.
252
How does excretion occur in nematodes?
In nematodes, specialized excretory systems are not well-developed. Nitrogenous wastes may be lost by diffusion through the entire body or into the pseudocoelom, where they are removed by specialized cells. Regulation of water and salt content of the body is achieved by renette glands, present under the pharynx in marine nematodes.
253
What are some characteristics of the nervous system of nematodes?
Most nematodes possess four longitudinal nerve cords that run along the length of the body in dorsal, ventral, and lateral positions. The ventral nerve cord is better developed than the dorsal or lateral cords. All nerve cords fuse at the anterior end, around the pharynx, to form head ganglia or the “brain” of the worm (which take the form of a ring around the pharynx) as well as at the posterior end to form the tail ganglia. In *C. elegans*, the nervous system accounts for nearly one-third of the total number of cells in the animal.
254
How does reproduction occur in nematodes?
Nematodes employ a variety of reproductive strategies that range from monoecious to dioecious to parthenogenic, depending upon the species under consideration. *C. elegans* is a monoecious species and shows development of ova contained in a uterus as well as sperm contained in the spermatheca. The uterus has an external opening known as the vulva. The female genital pore is near the middle of the body, whereas the male's is at the tip. Specialized structures at the tail of the male keep him in place while he deposits sperm with copulatory spicules. Fertilization is internal, and embryonic development starts very soon after fertilization. The embryo is released from the vulva during the gastrulation stage. The embryonic development stage lasts for 14 hours; development then continues through four successive larval stages with ecdysis between each stage - L1, L2, L3, and L4 - ultimately leading to the development of a young male or female adult worm. Adverse environmental conditions like overcrowding and lack of food can result in the formation of an intermediate larval stage known as the dauer larva.
255
What is the origin of the use of *Caenorhabditis elegans* as a model system?
*C. elegans* was brought into the focus of mainstream biological research by Dr. Sydney Brenner. Since 1963, Dr. Brenner and scientists worldwide have used this animal as a model system to study various physiological and developmental mechanisms.
256
Why is *C. elegans* attractive as a model system?
*C. elegans* is a free-living organism found in soil. It is easy to culture this organism on agar plates (10,000 worms/plate), it feeds on *Escherichia coli* (another long-term resident of biological laboratories worldwide), and therefore, it can be readily grown and maintained in a laboratory. The biggest asset of this nematode is its transparency, which helps researchers to observe and monitor changes within the animal with ease. It is also a simple organism with fewer than 1000 cells and a genome of 20,000 genes. It shows chromosomal organization of DNA into five pairs of autosomes plus a pair of sex chromosomes, making it an ideal candidate to study genetics. Since every cell can be visualized and identified, this organism is useful for studying cellular phenomena like cell-cell interactions, cell-fate determinations, cell division, apoptosis, and intracellular transport.
257
How does the life cycle of *C. elegans* contribute to its usefulness in laboratories?
One tremendous asset is the short life cycle of the worm. It takes only 3 days to achieve the “egg to adult to daughter egg”, so tracking genetic changes is easier in this animal. The total life span of *C. elegans* is 2 to 3 weeks; hence, age-related phenomena are easy to observe.
258
How does the cellular makeup of *C. elegans* contribute to its usefulness in laboratories?
The position and number of the 959 cells present in adult hermaphrodites of the organism is constant. This feature is extremely significant when studying cell differentiation, cell-cell communication, and apoptosis.
259
How do the genetics of *C. elegans* contribute to its usefulness in laboratories?
*C. elegans* is amenable to genetic manipulations using molecular methods.
260
How has the use of *C. elegans* contributed to understanding of biological processes?
Biologists worldwide have created information banks and groups dedicated to research using *C. elegans*. Their findings have led, for example, to better understandings of cell communication during development, neuronal signaling and insight into lipid regulation (which is important in addressing health issues like the development of obesity and diabetes). In recent years, studies have enlightened the medical community with a better understanding of polycystic kidney disease. This simple organism has led biologists to complex and significant findings, growing the field of science in ways that touch the everyday world.
261
How do some nematodes act as parasites?
A number of common parasitic nematodes serve as prime examples of parasitism. These animals exhibit complex lifecycles that involve multiple hosts, and they can have significant medical and veterinary impacts. Humans may become infected by *Dracunculus medinensis*, known as guinea worms, when they drink unfiltered water containing copepods. Hookworms, such as *Ancyclostoma* and *Necator*, infest the intestines and feed on the blood of mammals, especially in dogs, cats, and humans. Trichina worms (*Trichinella*) are the causal organism of trichinosis in humans, often resulting from the consumption of undercooked pork; *Trichinella* can infect other mammalian hosts as well. *Ascaris*, a large intestinal roundworm, steals nutrition from its human host and may create physical blockage of the intestines. The filarial worms, such as *Dirofilaria* and *Wuchereria*, are commonly vectored by mosquitoes, which pass the infective agents among mammals through their blood-sucking activity. *Dirofilaria immitis*, a blood-infective parasite, is the notorious dog heartworm species. *Wuchereria bancrofti* infects the lymph nodes of humans, resulting in the non-lethal but deforming condition called elephantiasis, in which parts of the body become swelled to gigantic proportions due to obstruction of lymphatic drainage and inflammation of lymphatic tissues.
262
What is the origin of the name Arthropoda?
The name “arthropoda” means “jointed legs” (in Greek, “arthros” means “joint” and “podos” means “leg”).
263
How predominant are arthropods?
An enormous number of invertebrates are included in the phylum Arthropoda, which dominate the animal kingdom with an estimated 85% of known species included in this phylum and many arthropods yet undocumented. Phylum Arthropoda is the largest phylum in the animal world, and insects form the single largest class within this phylum.
264
What characteristics distinguish arthropods?
The principal characteristics of all the animals in Arthropoda are functional segmentation of the body and presence of jointed appendages. Arthropods also show the presence of an exoskeleton made principally of chitin, which is a waterproof, tough polysaccharide.
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What are the basic classifications of arthropods?
Arthropods are eucoelomate, protostomic organisms.
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What are the classes of the phylum Arthropoda?
Trilobitomorpha (trilobites, all extinct), Hexapoda (insects and relatives), Myriapoda (millipedes, centipedes, and relatives), Crustaceans (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Chelicerata (horseshoe crabs, arachnids, scorpions, and daddy longlegs).
267
What are trilobites?
Trilobites are an extinct group of arthropods found chiefly in the pre-Cambrian era that are probably most closely related to the Chelicerata.
268
What are some characteristics of the segments of arthropods?
A unique feature of animals in the arthropod phylum is the presence of a segmented body and fusion of sets of segments that give rise to functional body regions called tagma. Tagma may be in the form of a head, thorax, and abdomen, or a cephalothorax and abdomen, or a head and trunk.
269
What are some characteristics of the circulatory systems of arthropods?
A central cavity, called the hemocoel (or blood cavity) is present, and the open circulatory system is regulated by a tubular or single-chambered heart. Respiratory systems vary depending on the group of arthropod: insects and myriapods use a series of tubes (tracheae) that branch through the body, open to the outside through openings called spiracles, and perform gas exchange directly between the cells and air in the tracheae, whereas aquatic crustaceans utilize gills, terrestrial chelicerates employ book lungs, and aquatic chelicerates use book gills. The book lungs of arachnids (scorpions, spiders, ticks and mites) contain a vertical stack of hemocoel wall tissue that somewhat resembles the pages of a book. Between each of the “pages” of tissue is an air space. This allows both side of the tissue to be in contact with the air at all times, greatly increasing the efficiency of gas exchange. The gills of crustaceans are filamentous structures that exchange gases with the surrounding water.
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What are some characteristics of the excretory systems of arthropods?
Groups of arthropods differ in the organs used for excretion, with crustaceans possessing green glands and insects using Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the body of nitrogenous waste.
271
What are some characteristics of the cuticles of arthropods?
The cuticle is the covering of an arthropod. It is made up of two layers: the epicuticle, which is a thin, waxy water-resistant outer layer containing no chitin, and the layer beneath it, the chitinous procuticle. Chitin is a tough, flexible polysaccharide. In order to grow, the arthropod must shed the exoskeleton during a process called ecdysis ("to strip off"); this is a cumbersome method of growth, and during this time, the animal is vulnerable to predation.
272
What are some characteristics of the anatomy of hexapods?
The name Hexapoda denotes the presence of six legs (three pairs) in these animals as differentiated from the number of pairs present in other arthropods. Hexapods are characterized by the presence of a head, thorax, and abdomen, constituting three tagmata. The thorax bears the wings as well as six legs in three pairs.
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What are some examples of hexapods?
Many of the common insects frequently encountered are examples of hexapods, such as ants, cockroaches, butterflies, and flies. It also includes insects that are winged (like fruit flies) and wingless (like fleas).
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What is the largest class within the hexapods?
Amongst the hexapods, the insects are the largest class in terms of species diversity as well as biomass in terrestrial habitats.
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What are some characteristics of the tagmata of hexapods?
Typically, the head bears one pair of sensory antennae, mandibles as mouthparts, a pair of compound eyes, and some ocelli (simple eyes) along with numerous sensory hairs. The thorax bears three pairs of legs (one pair per segment) and two pairs of wings, with one pair each on the second and third thoracic segments. The abdomen usually has eleven segments and bears reproductive apertures.
276
How many legs do myriapods have?
Subphylum Myriapoda includes arthropods with numerous legs. Although the name is hyperbolic in suggesting that myriad legs are present in these invertebrates, the number of legs may vary from 10 to 750.
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How many species of myriapods are there?
The Myriapoda subphylum includes 13,000 species; the most commonly found examples are millipedes and centipedes.
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Where are myriapods found?
All myriapods are terrestrial animals and prefer a humid environment. Myriapods are typically found in moist soils, decaying biological material, and leaf litter.
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What are the classes of the subphylum Myriapoda?
Chilopoda, Symphyla, Diplopoda, and Pauropoda.
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What are some characteristics of centipedes?
Centipedes like *Scutigera coleoptrata* are classified as chilopods. These animals bear one pair of legs per segment, mandibles as mouthparts, and are somewhat dorsoventrally flattened. The legs in the first segment are modified to form forcipules (poison claws) that deliver poison to prey like spiders and cockroaches, as these animals are all predatory.
281
What are some characteristics of millipedes?
Millipedes bear two pairs of legs per diplosegment, a feature that results from embryonic fusion of adjacent pairs of body segments, are usually rounder in cross-section, and are herbivores or detritivores. Millipedes have visibly more numbers of legs as compared to centipedes, although they do not bear a thousand legs.
282
How many species of crustaceans are there?
The total number of marine crustacean species stands at 67,000.
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Where are crustaceans found?
Crustaceans are the most dominant marine arthropods, but there are also freshwater and terrestrial crustacean species.
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What are some examples of crustaceans?
Krill, shrimp, lobsters, crabs, and crayfish are examples of crustaceans. Terrestrial species like the wood lice (*Armadillidium* spp.) (also called pill bugs, rolly pollies, potato bugs, or isopods) are also crustaceans, although the number of non-aquatic species in this subphylum is relatively low.
285
What are some characteristics of the appendages of crustaceans?
Crustaceans possess two pairs of antennae, mandibles as mouthparts, and biramous ("two branched") appendages.
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What are some characteristics of the bodies of crustaceans?
Unlike that of the Hexapoda, the head and thorax of most crustaceans is fused to form a cephalothorax, which is covered by a plate called the carapace, thus producing a body structure of two tagma. Crustaceans have a chitinous exoskeleton that is shed by molting whenever the animal increases in size. The exoskeletons of many species are also infused with calcium carbonate, which makes them even stronger than in other arthropods.
287
What are some characteristics of the circulatory systems of crustaceans?
Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the dorsally located heart. Hemocyanin and hemoglobin are the respiratory pigments present in these animals.
288
What are some reproductive characteristics of crustaceans?
Most crustaceans are dioecious, with separate sexes. Some species like barnacles may be hermaphrodites. Serial hermaphroditism, where the gonad can switch from producing sperm to ova, may also be seen in some species. Fertilized eggs may be held within the female of the species or may be released in the water. Terrestrial crustaceans seek out damp spaces in their habitats to lay eggs.
289
Which larval stages are found in crustaceans?
Larval stages - nauplius and zoea - are seen in the early development of crustaceans. A cypris larva is also seen in the early development of barnacles.
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What are some characteristics of the brain and eyes of crustaceans?
Crustaceans possess a tripartite brain and two compound eyes.
291
What are some crustacean food sources?
Most crustaceans are carnivorous, but herbivorous and detritivorous species are also known. Crustaceans may also be cannibalistic when extremely high populations of these organisms are present.
292
What are some examples of chelicerates?
The subphylum Chelicerata includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders.
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Where are chelicerates found?
The subphylum Chelicerata is predominantly terrestrial, although some marine species also exist. Chelicerates are found in almost all habitats.
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How many species of chelicerates are there?
An estimated 77,000 species are included in subphylum Chelicerata.
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What are some morphological characteristics of chelicerates?
The body of chelicerates may be divided into two parts: prosoma and opisthosoma, which are basically the equivalents of cephalothorax (usually smaller) and abdomen (usually larger). A “head” tagmum is not usually discernible. The phylum derives its name from the first pair of appendages: the chelicerae, which are specialized, claw-like or fang-like mouthparts. These animals do not possess antennae. The second pair of appendages is known as pedipalps. In some species, like sea spiders, an additional pair of appendages, called ovigers, is present between the chelicerae and pedipalps.
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What is the purpose of chelicerae?
Chelicerae are mostly used for feeding, but in spiders, these are often modified into fangs that inject venom into their prey before feeding.
297
What are some characteristics of the circulatory systems of chelicerates?
Members of the Chelicerata subphylum have an open circulatory system with a heart that pumps blood into the hemocoel. Aquatic species have gills, whereas terrestrial species have either trachea or book lungs for gaseous exchange.
298
What are some characteristics of the digestive systems of chelicerates?
Most chelicerates ingest food using a preoral cavity formed by the chelicerae and pedipalps. Some chelicerates may secrete digestive enzymes to pre-digest food before ingesting it. Parasitic chelicerates like ticks and mites have evolved blood-sucking apparatuses.
299
What are some characteristics of the nervous systems of chelicerates?
The nervous system in chelicerates consists of a brain and two ventral nerve cords.
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What are some reproductive characteristics of chelicerates?
Chelicerates use external fertilization as well as internal fertilization strategies for reproduction, depending upon the species and its habitat. Parental care for the young ranges from absolutely none to relatively prolonged care.
301
What is an archenteron?
A primitive gut cavity within the gastrula that opens outwards via the blastopore.
302
What is Chordata?
A phylum of animals distinguished by their possession of a notochord, a dorsal, hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development.
303
What is Echinodermata?
A phylum of deuterostomes with spiny skin; exclusively marine organisms.
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What is an enterocoelom?
A coelom formed by fusion of coelomic pouches budded from the endodermal lining of the archenteron.
305
What is a madreporite?
A pore for regulating entry and exit of water into the water vascular system.
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What is a water vascular system?
The system in echinoderms where water is the circulatory fluid.
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Which phyla belong to the superphylum Deuterostomia?
The phyla Echinodermata and Chordata (the phylum in which humans are placed).
308
How does the coelom develop in deuterostomes?
In deuterostomes, internal pockets of the endodermal lining called the archenteron fuse to form the coelom. The endodermal lining of the archenteron (or the primitive gut) forms membrane protrusions that bud off and become the mesodermal layer. These buds, known as coelomic pouches, fuse to form the coelomic cavity, as they eventually separate from the endodermal layer. The resultant coelom is termed an enterocoelom.
309
How does the alimentary system form during embryogenesis in deuterostomes?
The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult forms.
310
What are some experimental effects of indeterminant cleavage?
The fates of embryonic cells in deuterostomes can be altered if they are experimentally moved to a different location in the embryo due to indeterminate cleavage in early embryogenesis.
311
Where do echinoderms get their name?
Echinodermata are so named owing to their spiny skin (from the Greek “echinos” meaning “spiny” and “dermos” meaning “skin”).
312
How many species of echinoderms are there?
The phylum Echinodermata is a collection of about 7000 described living species.
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Where are echinoderms found?
Echinodermata are exclusively marine organisms. To date, no freshwater or terrestrial echinoderms are known.
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What are some examples of echinoderms?
Sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms.
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What are some morphological characteristics of echinoderms?
Adult echinoderms exhibit pentaradial symmetry and have a calcareous endoskeleton made of ossicles, although the early larval stages of all echinoderms have bilateral symmetry. The endoskeleton is developed by epidermal cells and may possess pigment cells, giving vivid colors to these animals, as well as cells laden with toxins. Gonads are present in each arm. In echinoderms like sea stars, every arm bears two rows of tube feet on the oral side. These tube feet help in attachment to the substratum. These animals possess a true coelom that is modified into a unique circulatory system called a water vascular system. An interesting feature of these animals is their power to regenerate, even when over 75% of their body mass is lost.
316
What are some characteristics of the water vascular system?
Echinoderms possess a unique ambulacral or water vascular system, consisting of a central ring canal and radial canals that extend along each arm. Water circulates through these structures and facilitates gaseous exchange as well as nutrition, predation, and locomotion. The water vascular system also projects from holes in the skeleton in the form of tube feet. These tube feet can expand or contract based on the volume of water present in the system of that arm. By using hydrostatic pressure, the animal can either protrude or retract the tube feet. Water enters the madreporite on the aboral side of the echinoderm. From there, it passes into the stone canal, which moves water into the ring canal. The ring canal connects the radial canals (there are five in a pentaradial animal), and the radial canals move water into the ampullae, which have tube feet through which the water moves. By moving water through the unique water vascular system, the echinoderm can move and force open mollusk shells during feeding.
317
What are some characteristics of the nervous systems of echinoderms?
The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Structures analogous to a brain or derived from fusion of ganglia are not present in these animals.
318
What are some characteristics of the excretory system of echinoderms?
Podocytes, cells specialized for ultrafiltration of bodily fluids, are present near the center of echinoderms. These podocytes are connected by an internal system of canals to an opening called the madreporite.
319
What are some characteristics of reproduction in echinoderms?
Echinoderms are sexually dimorphic and release their eggs and sperm cells into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, as well as regenerate body parts lost in trauma.
320
What are the classes of Echinodermata?
The phylum is divided into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers).
321
What are some characteristics of sea stars?
The most well-known echinoderms are members of class Asteroidea, or sea stars. They come in a large variety of shapes, colors, and sizes, with more than 1800 species known so far. The key characteristic of sea stars that distinguishes them from other echinoderm classes include thick arms (ambulacra) that extend from a central disk where organs penetrate into the arms. Sea stars use their tube feet not only for gripping surfaces but also for grasping prey. Sea stars have two stomachs, one of which can protrude through their mouths and secrete digestive juices into or onto prey, even before ingestion. This process can essentially liquify the prey and make digestion easier.
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What are some characteristics of brittle stars?
Brittle stars belong to the class Ophiuroidea. Unlike sea stars, which have plump arms, brittle stars have long, thin arms that are sharply demarcated from the central disk. Brittle stars move by lashing out their arms or wrapping them around objects and pulling themselves forward.
323
What are some characteristics of echinoids?
Sea urchins and sand dollars are examples of Echinoidea. These echinoderms do not have arms, but are hemispherical or flattened with five rows of tube feet that help them in slow movement; tube feet are extruded through pores of a continuous internal shell called a test.
324
What are some characteristics of crinoids?
Sea lilies and feather stars are examples of Crinoidea. Both of these species are suspension feeders.
325
Which invertebrate clades are found in the phylum Chordata?
Urochordata (tunicates) and Cephalochordata (lancelets). Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.
326
What are some characteristics of sea cucumbers?
Sea cucumbers of class Holothuroidea are extended in the oral-aboral axis and have five rows of tube feet. These are the only echinoderms that demonstrate “functional” bilateral symmetry as adults, because the uniquely extended oral-aboral axis compels the animal to lie horizontally rather than stand vertically.
