Ch. 12 Flashcards
(126 cards)
1
Q
Phylum Mollusca
Defining Characteristics:
A
Means soft
1) Dorsal epithelium forming a mantle, which secretes calcareous spicules or one or more shells;
2) cuticular band of teeth (radula) in the esophagus, used for feeding (not present-lost?—in bivalves);
3) ventral body wall muscles develop into a locomotory or clinging foot
2
Q
Phylum Mollusca
shells of most molluscs 3 layers
A
have a thin, outer organic layer (the periostracum);
a thin, innermost calcareous layer (the nacreous layer);
and a thick, calcareous middle layer (the prismatic layer)
shells consisting primarily of calcium carbonate set in a protein matrix.
Shell microstructure can differ dramatically among the members of different molluscan groups.
3
Q
Phylum Mollusca
Basic info
A
enormous phylum
100,000 described living species
body plan probably the most malleable in the animal kingdom.
1-2 mm (millimeters) in some marine gastropods, to body lengths exceeding 12 m (meters) for some squid.
Extant molluscs are distributed among 7 classes.
6 of these 7 classes are represented by fossils.
1 extinct class (Rostroconchia)
4
Q
Phylum Mollusca
mantle
A
Both the organic and inorganic components of the shell are secreted by specialized tissue known as the mantle.
its role varies substantially in different molluscan groups.
5
Q
Phylum Mollusca
Foot
A
molluscan foot is also highly modified for a variety of functions in different groups.
6
Q
Phylum Mollusca
mantle cavity
A
Most molluscs have a characteristic cavity lying between the mantle and the viscera.
This mantle cavity usually houses the comb-like molluscan gills, known as ctenidia (ctenidi = Greek: comb),
7
Q
Phylum Mollusca
ctenidia
A
comb-like molluscan gills (ctenidi = Greek: comb), and also generally
serves as the exit site for the excretory, digestive, and reproductive systems.
A ctenidium (the singular form of
“ctenidia”), when present, may have a purely respiratory function or may also function in the collection and sorting of food particles.
8
Q
Phylum Mollusca
osphradium
A
A chemoreceptor/tactile receptor known as the osphradium (osphra = G: a smell) is generally located adjacent to the ctenidium
9
Q
Phylum Mollusca
countercurrent exchange
A
a system that greatly increases the efficiency of gas exchange between blood flowing within the ctenidial filaments and the water flowing over them.
In this system, blood and water flow in opposite directions.
As the water moves from left to right in the diagram, it is always in contact with blood of lower oxygen concentration, maintaining a large concentration gradient for oxygen.
Conversely, following the path of blood flow from the blood— even though it is continually acquiring oxygen from the water— is always coming into contact with water of higher oxygen concentration as it moves from right to left in the figure.
Thus, oxygen will diffuse from water to blood as the blood moves over the entire length of the gill filament
10
Q
Phylum Mollusca
molluscan coelom
A
is very small, being restricted
largely to the area surrounding the heart and gonads.
has no locomotory role.
Current molecular data, shows that molluscs have descended from some coelomate ancestor and that the body cavity experienced a substantial reduction in size in the course of subsequent evolution.
11
Q
Phylum Mollusca
hemocoel
A
blood sinuses comprising a hemocoel (“blood cavity”) are well developed.
This hemocoel serves as a hydrostatic skeleton in the locomotion of some molluscs.
12
Q
Phylum Mollusca
radula.
A
a feeding structure.
consists of a firm ribbon, composed of chitin and protein, along which are found numerous rows of sharp, chitinous teeth
13
Q
Phylum Mollusca
radular sac
A
The ribbon is produced from a radular sac and is underlain by a supportive cartilage-like structure called the odontophore (literally, G: tooth bearer).
14
Q
Phylum Mollusca
Odontophore
A
a supportive cartilage-like structure called the odontophore (literally, G: tooth bearer).
Under the redular sac
15
Q
Phylum Mollusca
buccal mass or odontophore complex
A
The odontophore-radular assembly, together with its complex musculature, is known as the buccal mass (bucca = L: cheek), or the odontophore complex.
For feeding, the buccal mass is protracted so that the odontophore extends just beyond the mouth.
The radular ribbon is then moved forward over the leading edge of the supporting odontophore and then pulled back.
As each row of teeth passes back over the edge of the odontophore, the teeth automatically stand upright and rotate laterally, rasping food particles from the substrate and bringing them into the mouth as the radula is with-drawn.
As old teeth are worn down or broken off at the anterior end of the radular ribbon, new teeth are continually being formed and added onto the ribbon’s posterior end in the radular sac.
16
Q
Class Polyplacophora
Defining Characteristic:
A
Means many plate bearing
Shell forms as a series of seven to eight separate plates.
This series of eight overlapping and articulating plates covering the dorsal surface.
These plates are partially or largely embedded in the mantle tissue that secretes them.
Because the shell is multisectioned, the body can bend and conform to a wide variety of underlying substrate shapes.
17
Q
Class Polyplacophora
Basic info
A
800 species
known as “chitons”
3-10 cm (centimeters) long.
generally found close to shore, particularly in the intertidal zone; they live only on hard substrates, especially rocks.
18
Q
Class Polyplacophora
girdle
A
A chiton’s thick lateral mantle is called the girdle.
In most species, the girdle bears numerous calcareous spicules, secreted independently of the shell plates.
19
Q
Class Polyplacophora
Mantle cavity
A
The mantle cavity of chitons takes the form of two lateral grooves, one on each side of the body.
Up to about 80 bipectinate (L: double-combed, i.e., two-branched) ctenidia hang down from the roof of each groove, dividing each elongated mantle cavity into incurrent and excurrent chambers.
20
Q
Class Polyplacophora
countercurrent exchange system
A
Water is drawn into the incurrent chamber by the action of the gill cilia.
The flow of water is anterior to posterior, so waste products are discharged posteriorly in the excurrent stream.
The flow of blood through the individual gill lamellae is opposite in direction to the flow of water, forming a countercurrent exchange system that facilitates gas exchange.
21
Q
Class Polyplacophora
Foot
A
The foot extends along the animal’s entire ventral surface and is completely covered by the overlying shell and girdle.
22
Q
Class Polyplacophora
“pedal waves”
A
Locomotion is accomplished by subtle waves of muscular activity called “pedal waves,” (also in gastropods).
When disturbed, the chiton can press the girdle tightly against the substrate.
By then lifting up the central portion of the foot (and the inner margin of the mantle tissue as well, if required), while retaining a tight seal against the substrate along the entire outer margin of the foot (and girdle), the chiton can generate a suction, aided by mucus secretions, that holds the animal tightly against the substratum.
This ability to cling tightly to the substrate is a particularly effective adaptation for life in areas of heavy wave action.
23
Q
Class Polyplacophora
nervous and sensory systems
A
is simple and ladder-like.
Ganglia are lacking in many species, and only poorly developed in others.
Sensory systems are also reduced: Adult chitons lack statocysts, tentacles, and eyes on the head.
24
Q
Class Polyplacophora
Aesthetes
A
abundant organs derived from mantle tissue and extending through holes in the shell plates-function as light receptors in at least some species
but may function as chemoreceptors or mechanoreceptors,
or even assecretors of periostracum, replacing material that is naturally abraded away in the highly turbulent environment in which most chitons live.
25
Class Polyplacophora
digestive tract
linear digestive tract, with the mouth and anus at opposite ends of the body.
Food particles, usually algae, are typically scraped from the substrate by a radula/odontophore complex, although a few species are carnivores.
Many of the radular teeth are capped with iron-oxide.
26
Class Polyplacophora
sugar glands
A pair of pharyngeal glands, often called sugar glands, release amylase-containing secretions into the stomach.
most other herbivorous molluscs process food using something called a crystalline style
27
Class Aplacophora
Defining Characteristic:
Means not shell bearing
Cylindrical,
vermiform body with the foot forming a narrow keel
Vermiform = worm like
28
Class Aplacophora
Basic info
found in all oceans, mostly in deep water.
Usually only a couple mm, rarely more than a few cm.
Aplacophora is the only class of molluscs to have left no fossil record
Like the cephalopods, scaphopods, monoplacophorans, and chitons, aplacophorans are entirely marine.
Many aplacophorans burrow into or meander about on mud; other species live on cnidarians— primarily soft corals— on which they prey.
A few species are interstitial, living in the spaces between sand grains and feeding on interstitial hydrozoans (phylum Cnidaria).
29
Class Aplacophora
Body/shell
The body is unsegmented.
Bears numerous calcareous spines or scales embedded in an outer cuticle.
The spines or scales are secreted by individual cells in the underlying epidermis; there is no true shell.
30
Class Aplacophora
Style sac
some species possess a style sac (often complete with style and gastric shield), a small posterior mantle cavity with ctenidia, and a radula, although in most species, the radula is apparently used for grasping rather than rasping.
31
Class Aplacophora
Foot
have no conspicuous foot.
32
Class Aplacophora
nervous system
consists of paired cerebral ganglia, giving rise to four linear, ganglionated nerve cords-two lateral, two ventral-interconnected in a ladder-like arrangement as in chitons.
In addition, members of both groups form calcareous spicules in apparently identical fashion, through extracellular secretions from single cells.
33
Class Monoplacophora
Defining Characteristics:
Means one shell bearing
1) three to six pairs of ctenidia, six to seven pairs of nephridia;
2) multiple (usually eight) pairs of foot (pedal) retractor muscles
34
Class Monoplacophora
Basic info
31 living species have been described
all marine
35
Class Monoplacophora
Shell
produce sin-gle, unhinged, cap-shaped shells.
The shell of adult monoplacoph-orans is flattened rather than spirally wound, although the larval shell is spiral.
36
Class Monoplacophora
Foot
foot is flattened, as in gastropods and polyplacophorans
37
Class Monoplacophora
Mantle cavity
mantle cavity takes the form of two lateral grooves, as in polyplacophorans, and three, five, or six pairs of gills hang down within the mantle grooves.
In addition to the gills, the pedal retractor muscles, auricles and ventricles of the heart, gonads, and nephridia occur in multiple copies.
38
Class Monoplacophora
Radula, style, and digestive system
Both a radula and a crystalline style are present, and the gut is linear, with the mouth being anterior and the anus posterior.
39
Class Monoplacophora
nervous system
As in the polyplacophorans and aplacophorans, the nervous system includes both lateral and pedal nerve cords
40
Class Gastropoda
Defining Characteristics:
Means stomach foot
1) Visceral mass and nervous system become twisted 90-180° (exhibiting torsion) during embryonic development;
2) proteinaceous shield on the foot (operculum)
41
Class Gastropoda
Basic info
largest molluscan class, containing at least 60,000 species.
Many different habitats.
They exhibit a striking diversity of lifestyles, including suspension-feeding, carnivorous, herbivorous, deposit-feeding, and ectoparasitic species.
42
Class Gastropoda
visceral mass
visceral mass (i.e., all of the internal organs) sitting atop a muscular foot.
The visceral mass is commonly protected by a univalved shell that is typically coiled, probably as an adaptation for efficient packaging of the visceral mass.
Shell morphology differs considerably among species.
43
Class Gastropoda
columellar muscle and columella
For shelled species, the snail is attached to the inside of its shell by a columellar muscle, which extends from within the animal's foot to the central axis of the shell; this central axis is known as the columella.
The columellar muscle is important in most major body movements: protraction from the shell, retraction into the shell, twisting, raising the shell above the substratum, and lowering it back down.
44
Class Gastropoda
Shell
The shell is typically carried so that it leans to the left side of the body.
The shells of most gastropod species coil clockwise, to the right.
That is, the shells are "right-handed" or "dextral" (dextro = L: the right-hand side).
Probably as a consequence of space limitations within the coiled shell, the ctenidium, osphradium, kidney (nephridium), and heart auricle on the right side of the body tend to be reduced or absent.
"left-handed," or sinistral in their coiling
(sinister = L: the left-hand side).
45
Class Gastropoda
behavioral or chemical defenses against predators
These adaptations commonly take one of the following forms:
(1) the gastropod senses the presence of potential predators, either chemically or by touch, and initiates appropriate escape, avoidance, or deterrent behavior;
(2) the gastropod chemically senses the presence of injured individuals of its own species (i.e., conspecific individuals) and initiates appropriate escape behavior; or
(3) the gastropod accumulates noxious organic compounds in its tissues, thereby becoming distasteuful to potential predators.
46
Class Gastropoda
Torsion
a counterclockwise 180° twisting of the head and foot relative to the shell, mantle, and the rest of the body (the visceral mass) during early development.
As a consequence of torsion, the nervous and digestive systems become obviously twisted, and the mantle cavity moves from the rear of the animal to become positioned over the head.
The main contributors seem to be periodic contractions of the larval retractor muscles that connect the animal's foot to its shell, along with more rapid cell proliferation on the right-hand side of the mantle epithelium?
Torsion has no direct relationship to shell coiling.
In some species, an apparent detorsion occurs subsequent to torsion, probably through a process of differential growth.
47
Class Gastropoda
Pedal waves
Small gastropod species may move largely through the action of cilia located on the ventral surface of the foot, but most species move by means of pedal waves of muscle contraction along layers of adhesive mucus secreted by the mouth or foot.
Unlike peristaltic waves, pedal waves generally do not involve circular muscles or muscular contractions of great magnitude, and they are restricted to the central portion of the foot's ventral surface.
48
Class Gastropoda
Foot
The musculature of the foot is predominantly vertical (dorsoventral) and transverse.
49
Class Gastropoda
retrograde waves
(retro = [: back; grad = L; step);
Means = the wave of muscular
contraction travels in the direction opposite that in which the snail is moving.
At the start of a pedal wave, the dorsoventral musculature contracts at the anterior portion of the foot.
Apparently, the transverse muscles do not relax, which means that the foot cannot widen; instead, the foot is squeezed forward.
A wave of contraction of the dorsoventral musculature then moves posteriorly, allowing the rest of the foot to catch up with the anterior.
The edges of the foot are temporarily sealed against the substrate with mucus, so a small negative pressure (suction) is generated in the space between the substrate and the raised portion of the foot.
The dorsoventral muscles are reextended when they relax, at least in part, by this negative external pressure; this small space thus acts as a hydrostatic skeleton, even though it is external to the body, allowing the musculature.
At the forward edge of the wave to antagonize that at the trailing edge.
In this case, the hydrostatic skeleton operates through a temporary pressure decrease, essentially sucking the raised portion of the foot downward and extending the associated muscles, rather than through a temporary pressure increase.
50
Class Gastropoda
Direct waves
Pedal waves may also be direct (i.e., moving in the same direction as the animal).
Note that regardless of whether the pedal wave is retrograde or direct, the snails themselves can only move forward.
51
The Prosobranchs
Defining Characteristic:
Means anterior gill
Mantle cavity generally anterior, due to torsion
52
The Prosobranchs
Basic info
Most marine,
35,000 species
Generally free-living and mobile, although some species have evolved sessile or even parasitic lifestyles.
Free-living can be herbivores, deposit feeders, omnivores, suspension feeders, or carnivores.
Carnivore species can produce venoms.
These snails contain symbiotic bacteria living in the gill tissue; the bacteria may provide the snail with nutrients.
Are the most primitive of gastropods.
53
The Prosobranchs
Shell, mantle cavity, radula, foot
a well-developed shell, mantle cavity, osphradium, and radula, and the foot usually bears a rigid disc of protein (sometimes strengthened with calcium carbonate) called the operculum.
54
The Prosobranchs
operculum
When the foot is withdrawn into the shell, the operculum may completely seal the shell aperture, thus protecting the snail from predators and from such physical stresses as dehydration and low salinity
55
The Prosobranchs
ctenidium, hemocoel, flow of blood
The typical prosobranch gill is a ctenidium, consisting of a series of flattened, triangular sheets (filaments) lying one adjacent to the next.
Deoxygenated blood enters an afferent blood vessel from the animal's open system of blood sinuses (the hemocoel).
Once distributed to the individual sheets of the ctenidium, the blood moves through the sheet, where it becomes oxygenated, and then on to the auricle of the heart through an efferent blood vessel.
From the aurice, the blood is pumped into the single associated ventricle and is then distributed to the tissues through a single aorta leading to the blood sinuses of the hemocoel.
56
The Prosobranchs
Siphon, osphradium, and flow of water
Water is drawn into the mantle cavity and across the gill sheets by the movements of gill cilia.
In many prosobranch species, a portion of the mantle is drawn out into a cylindrical extension called the siphon water is drawn through this siphon, by the action of the gill cilia, into the mantle cavity and across the osphradium (a chemical and tactile receptor organ.
The snail moves the muscular siphon back and forth, sampling the water from different directions.
and water exits at the right side of the head. (Came in from left side)
siphon is especially well developed in carnivores and scavengers-which often hunt their prey by chemical sensing-and generally reduced or absent in suspension feeders, herbivores, and deposit feeders.
In all species, water movement across the gill is unidirectional and counter to the direction of blood flow, so that the principle of countercurrent exchange always applies
57
The Prosobranchs
gill numbers
In the primitive (ancestral) bipectinate condition, gill filaments extend from both sides of the ctenidial axis,
while in the relatively advanced (derived) monopectinate condition, the filaments project from only one side (the "downstream" side) of the supporting axis.
Regardless of the number of gills and the placement of gill filaments in different prosobranch species, however, the principle of countercurrent exchange applies to all.
58
The Prosobranchs
heteropods
The members of one group, the heteropods, show especially striking modifications of the basic prosobranch body plan and lifestyle.
Are planktonic, voracious carnivores, whose shell is reduced or absent and whose foot is a thin, undulating paddle that propels the animal through the water.
Except for the viscera, the body is nearly transparent, an excellent adaptation for inconspicuous water travel.
59
The Opisthobranchia
Defining Characteristic:
Means posterior gill
Mantle cavity lateral or posterior due to detorsion, or lost
60
The Opisthobranchia
Basic info
includes the sea hares, sea slugs, and bubble shells, are almost all marine.
About 5,000 species have been described.
Most species that have lost the ctenidia have evolved other respiratory structures that are developmentally unrelated to the ancestral gill.
61
The Opisthobranchia
The characteristics that distinguish adults of this group from those of the prosobranchs are
(1) a trend toward reduction, internalization, or loss of the shell,
(2) reduction or loss of the operculum (although it is present in all larvae),
(3) limited torsion during embryogenesis,
(4) reduction or loss of the mantle cavity, and
(5) reduction or loss of the ctenidia.
62
The Opisthobranchia
cerata
In many sea slugs (the nudibranchs-order Nudibranchia), gas exchange occurs across brightly colored dorsal projections called cerata which also contain extensions of the digestive system.
In at least one species, the cerata exhibit rhythmic muscular contractions, apparently serving to move blood through the hemocoelic sinuses for gas exchange.
Instead of cerata, many other nudibranchs possess feathery gills arising from the dorsal surface
63
The Opisthobranchia
nematocysts
Shell reduction or loss potentially increases vulnerability to predators, and it is reasonable to expect that pressures selecting for alternate means of defense have been quite strong.
In particular, the cerata of many opisthobranch species house unfired defensive organelles (nematocysts) usurped from cnidarian prey; these nematocysts can then function in defending the nudibranch.
64
The Opisthobranchia
rhinophores.
head typically bears in addition to a pair of tentacles adjacent to the mouth as in prosobranchs, a second pair of tentacle-like structures located dorsally, called rhinophores.
The rhinophores are believed to be chemosensory, making them analogous to the osphradium of prosobranch gastropods and to the osphradium of those opisthobranchs bearing a mantle cavity.
65
Opisthobranchs show various degrees of departure from the ancestral, prosobranch-like condition.
Adult sea slugs:
- have no mantle cavity, ctenidia, osphradium, shell, or operculum, and some species show no evidence of torsion as adults.
Sea slug larvae:
- have a pronounced mantle cavity, shell, and operculum, indicating a clear affinity with the prosobranchs.
Adult sea hares:
- Have a mantle cavity (with gill and osphradium) and a shell.
- However, the mantle cavity is very small and on the animal's right side, and the shell is reduced and internal.
Mantle cavity in such species (containing a gill and osphradium) is well developed, and the snails often show little sign of detorsion.
66
The Opisthobranchia
parapodia and Pteropods
locomotion is generally by means of cilia and pedal waves along the ventral surface of the foot.
Some can swim in short spurts by flapping lateral folds of the foot called parapodia.
In other members of this subclass, the entire foot is drawn out into two thin lobes, also called parapodia, which are used for swimming.
These animals are known as pteropods ("wing-footed"), or sea butterflies.
Pteropods may or may not have shells, depending on the species, but all are permanent members of the plankton.
Pteropods typically have no specialized respiratory organs, so gas exchange is accomplished across the general body surface.
67
The Pulmonata
Defining Characteristic:
Means lung
Mantle cavity highly vascular-ized and otherwise modified to form a lung
68
The Pulmonata
Basic info
19,000 sps are marine
Most pulmonate species are found in terrestrial or freshwater environments; slugs and "escargot" are terrestrial members of this subclass.
A major feature distinguishing the pulmonates from other gastropods is that the mantle cavity is highly vascularized and functions as a lung.
69
The Pulmonata
Shell, operculum, foot, radula, Torsion, mantle cavity
Coiled shell is present in most but the shell is reduced, internalized, or completely lost in others (the slugs)
Only a few species have an operculum on the foot.
Most pulmonates possess a long radula, in keeping with their generally herbivorous diet, and the head commonly bears two pairs of tentacles.
Torsion is limited to about 90°, so the nervous system is not so greatly twisted.
and the mantle cavity opens on the right side of the body, as in many opisthobranchs.
70
The Pulmonata
pneumostome
Downward movement of the floor of the mantle cavity increases the cavity's volume so that air, or in some cases water, is drawn into the mantle cavity for respiration.
The fluid is then expelled by decreasing the volume of the mantle cavity.
Air or water flows into and out of the lung through a single small opening called the pneumostome (pneumo = G: lung; stoma = G: mouth).
Although pulmonates lack ctenidia, a gill has secondarily evolved in some freshwater species.
This gill takes the form of folds of mantle tissue near the pneumostome.
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Class Bivalvia (= Pelecypoda)
Defining Characteristics:
Means two valved
1) Two-valved shell;
2) body flattened laterally
72
Class Bivalvia (= Pelecypoda)
Basic info
9,000 sps
clams, scallops, mussels, and oysters.
primarily marine, but about 10-15% of all species occur in freshwater.
No terrestrial.
Most species are suspension-feeders, using their gill cilia to drive water through the mantle cavity and capture phytoplankton and other microscopic particles from the seawater.
The hinged portion of a bivalved shell is dorsal.
The shell valves, then, are on the animal's left and right sides. The shell opens ventrally.
For many years, bivalve classification was based mainly on gill structure.
Its members have been distributed among four new subclasses.
For simplicity, discuss these bivalves as "lamellibranchs."
The most primitive bivalve species are found within a fourth sub-class, the Protobranchia.
The most bizarre bivalves are now placed within a fifth subclass, the Anomalodesmata.
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Class Bivalvia (= Pelecypoda)
Major bivalve characteristics include
(1) a hinged shell, the two sides (left and right "valves" —valva = L: a folding door) of which are closed by one or two adductor muscles; a springy ligament springs the shell valves apart when the adductor muscles relax;
(2) lateral compression of the body and foot;
(3) lack of cephalization: virtual absence of a head and associated sensory structures;
(4) a spacious mantle cavity, relative to that found in other molluscan classes;
(5) a sedentary lifestyle; and
(6) the absence of a radula/odontophore complex.
74
Class Bivalvia (= Pelecypoda)
Umbo, growth lines, and foot
A conspicuous bulge in the shell is frequently seen on the dorsal surface, adjacent to the hinge.
This bulge, termed the umbo, is comprised of the earliest shell material deposited by the animal.
Distinct growth lines typically run parallel to the shell's outer margins.
The foot projects ventrally and anteriorly, in the direction of movement, and the siphons, when present, project posteriorly.
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Subclass Protobranchia
Defining Characteristics:
Means first gill
1) Gills small and resembling those of gastropods, functioning primarily as gas exchange surfaces; 2) food collected by long, thin, muscular extensions of tissue surrounding the mouth (palp proboscides)
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Subclass Protobranchia
Basic info
entirely marine
all species live in soft substrates
Protobranch bivalves' mode of feeding clearly restricts them to soft sediments.
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Subclass Protobranchia
Gills, demibranch, bioectinate,
One pair of gills is present in the mantle cavity, with one gill on either side of the body.
The two gills extend far enough posteriorly (beyond the foot) to join together by means of ciliary tufts on the gill filaments.
Each gill consists of two parts, called demibranchs (Middle English: half-gills), extending from opposite sides of a central gill axis.
Thus, the gills of protobranch bivalves are always bipectinate (L: double-combed).
The many units making up each demibranch may be flat, rounded sheets as in the typical gastropod ctenidium or be filaments
78
Subclass Protobranchia
Filament and gills
they may be finger-like structures that have a more circular cross section; in both cases, each unit is called a filament.
The gill filaments hang down into the mantle cavity, dividing it into an incurrent (ventral) chamber and an excurrent (dorsal) chamber.
Water enters the mantle cavity ventrally, generally passes between gill filaments, and then exits dorsally.
The protobranch gill functions primarily in gas exchange (as in most prosobranch gastropods), although it may also play some role in food collection by filtering unicellular algae from the water.
79
Subclass Protobranchia
lateral cilia and frontal cilia
Gill cilia are confined to the area near the edges of the gill filaments, the lateral cilia always playing the primary role in driving water through the mantle cavity.
The frontal cilia function primarily in cleaning the gill of sediment and debris.
The ciliated discs thus fasten together adjacent filaments; that is, they form interfilamental junctions-stabilizing junctions between individual gill filaments
80
Subclass Protobranchia
palp proboscides
long, thin, muscular extensions of the tissue surrounding the mouth.
The palp proboscides protrude between the shell valves and probe the surrounding mud substrate, entangling particles in mucus.
The sediment-laden mucus is then transported into the mantle cavity by cilia along the ventral surface of the proboscides.
81
Subclass Protobranchia
labial palps
Attached to the palp proboscides at their bases, within the mantle cavity, are flattened structures called labial palps.
these have conspicuous ridges on their inner surfaces that sort particles by ciliary action,
transporting small and nutritious particles to the mouth for ingestion and transferring large and less nutritious (and possibly toxic) particles to the margins of the labial palps, where they are ejected into the mantle cavity and expelled.
This rejected material is termed pseudofeces, since it is material that has never been ingested.
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Subclass Protobranchia
deposit feeding
type of feeding, in which sediment is taken in and the organic fraction is digested, is called deposit feeding, and is quite common among invertebrates.
83
The Lamellibranchs
Defining Characteristics:
Means plate gill
1) Gills modified to collect suspended food particles, in addition to serving as gas exchange surfaces;
2) secretion of proteinaceous attachment material (usually in the form of threads) by a specialized gland (the byssus gland) in the foot
84
The Lamellibranchs
Basic info
Most bivalves have lamellibranch characteristics.
Although most lamellibranchs are marine, all freshwater bivalve species also belong to this subclass.
often play important ecological roles, especially in shallow water.
Commercially important for many years as food
85
The Lamellibranchs
incurrent and excurrent siphons
In bivalves of all subclasses,
water typically enters and exits the mantle cavity posteriorly;
the water generally enters through an incurrent siphon,
passes dorsally between adjacent gill filaments,
and then exits through a more dorsally located excurrent siphon.
The siphons, where present, are tubular extensions of mantle tissue that often can be protruded far beyond the posterior shell margins,
permitting the rest of the animal to live safely, deep within the surrounding substrate
86
The Lamellibranchs
Water current
As in the prosobranch gastropods and the protobranch bivalves, water currents are generated by the action of gill cilia-not by muscular activity of the siphons-and the gill is the primary gas exchange surface.
But lamellibranch gills are, in addition, modified to collect food particles from the surrounding water.
And it is largely the capacity of individuals to process water at high rates, in combination with what are often large numbers of individuals per square meter of substratum, that accounts for their substantial ecological impact.
87
The Lamellibranchs
ctenidium
is often much larger than that of protobranchs and is variously modified to provide an enormous surface area for collecting, sorting, and transporting suspended particles.
88
The Lamellibranchs
V and W Filaments, descending limb, ascending limb, interfilamental ciliary junctions, = filibranch gills
The individual filaments of the lamellibranch gill are thin and greatly elongated, typically bending to form a V shape so that the entire ctenidium is generally shaped like a W.
A ciliated ventral food groove lies between the two limbs of each filament, at the base of each V and the cilia within this groove pass food particles from one filament to the next, toward the mouth.
The two arms of each V are named in reference to the central axis from which each filament hangs.
A descending limb descends from the central axis, and an ascending limb bends upward from the bottom of the descending limb.
When two V-shaped filaments unite to form a W on each side of the foot, adjacent Ws in a gill are always attached to each other.
Sometimes the Ws are attached by interfilamental (i.e., "between filaments") ciliary junctions, as in the protobranch gill.
These lamellibranch gills— consisting of individual filaments linked together solely by ciliary disc junctions-are termed filibranch gills.
89
The Lamellibranchs
eulamellibranch gills
In the most highly modified bivalve gills, called eulamellibranch gills,
the junctions between adjacent filaments are made of tissue rather than cilia; these interfilamental tissue junctions completely and firmly attach adjacent filaments together.
90
The Lamellibranchs
lamellae
In both filibranch and eulamelli-branch gills,
the series of ascending and descending limbs form continuous sheets of tissue, or lamellae.
Water passes through the gill lamellae, between adjacent gill filaments-before leaving the mantle cavity.
91
The Lamellibranchs
Bipectinate, demibranch
As with protobranch gills, lamellibranch gills are bipectinate, with one demibranch ("half-gill") extending from each side of the central gill axis;
each demibranch typically consists of an ascending and a descending lamella, formed from the ascending and descending limbs of the numerous V-shaped filaments.
The demibranch nearest the foot is termed the inner demibranch; that on the side nearest the mantle is termed the outer demibranch.
92
The Lamellibranchs
interlamellar junctions, Ostia,
Commonly, the ascending and descending limbs of a single filament are connected by crosspieces of tissue, forming interlamellar (i.e., "between lamellae") junctions.
In eulamellibranch gills, the interlamellar junctions between the ascending and descending limbs of each filament may be so extensive as to form a complete, solid sheet of tissue across the space between the two limbs.
the interlamellar and inter-filamental junctions in such gills essentially turn pairs of adjacent filaments into lidless, rectangular boxes.
Water must now pass between gill filaments, from the incurrent chamber of the mantle cavity to the excurrent chamber, through minute holes, called ostia, located in the sides of each box.
93
The Lamellibranchs
Tips of ascending lamellae attaching to bivalves foot and foot mantle
In addition, the ascending lamellae of the inner demibranchs (e.g., the two demi-branchs closest to the foot) often attach, at their tips, to the bivalves foot,
and the ascending lamellae of the two outer demibranchs often attach, at their tips, to the mantle.
With the tips of the ascending lamellae firmly attached to other tissues of the bivalve,
all water must pass between adjacent gill filaments before exiting the mantle cavity;
this must increase the gill's filtering effectiveness considerably, since no particles can now escape the clutches of the food-gathering cilia.
94
The Lamellibranchs
Lateral cilia
The surfaces of lamellibranch gill filaments that face the incurrent chamber of the mantle cavity show complex patterns of ciliation.
Lateral cilia along the sides of each filament create the water currents responsible for moving water into, and out of, the mantle cavity, as in protobranch bivalves;
remember that even though water enters the mantle cavity through an incurrent siphon, the water is pulled in through the actions of these lateral gill cilia, not through any direct muscular action of the siphon itself.
Individual bivalves can reduce water flow through the mantle cavity by contracting muscles to decrease siphon diameter, or by contracting smooth muscle within the gills themselves, or possibly by varying ciliary beat frequency.
95
The Lamellibranchs
laterofrontal cilia, frontal cilia
The traditional model has food particles being captured by compound laterofrontal cilia, which then pass the captured particles to the nearby frontal cilia.
More recent work indicates that particles are captured instead largely by hydrodynamic forces, without any physical contact between the particles and the laterofrontal cilia; in addition, some particles are intercepted directly by the gill filaments.
In both cases, frontal cilia then move the captured food particles to specialized food grooves located at the ventral and dorsal margins of each demibranch.
In all cases, the particles are transported by gill cilia to the labial palps, where they are sorted according to particle size and nutritional value and then carried to the mouth, as in protobranchs.
96
The Lamellibranchs
crystalline style, style sac, gastric shield,
After food particles are ingested at the mouth and passed down the esophagus, entangled in strings of mucus, they are drawn into the stomach, stirred, and, in part, digested by the action of a rotating translucent rod known as the crystalline style.
The crystalline style is composed of structural protein and several digestive enzymes.
One end of the rod lies in a style sac, a pouch of the intestine lined by cilia.
The activity of these cilia causes the rod to rotate.
The end of the style protruding into the stomach abrades against a chitinous gastric shield as the rod rotates, breaking food into smaller pieces.
The abrasion also causes the rod to slowly degrade at the end, releasing digestive enzymes into the stomach.
Additions to the crystalline style are made in the style sac.
97
The Lamellibranchs
digestive glands (digestive diverticula),
As in most other molluscs, the bivalve stomach connects with larger digestive glands (digestive diverticula), which serve as the major sites of digestion and absorption.
Whether or not particles are sent to the digestive glands may depend on their nutritional value, so that the stomach may serve as an additional site of particle sorting.
98
The Lamellibranchs
Bacterial symbionts
Many bivalves hosting gill bacteria have a greatly reduced digestive system, and in some species the digestive system has been lost com-pletely; symbiotic bacteria surely contribute substantially to host nutrition in such animals.
The relationship with bacterial symbionts typically correlates with a reduction of the labial palps and digestive system, the loss of the outer gill demibranchs,
99
The Lamellibranchs
byssal threads.
Some lamellibranchs, such as mussels, live attached to hard substrates by means of proteinaceous secretions called byssal threads.
A proteinaceous liquid is secreted by a byssal gland at the base of the foot, within the mantle cavity, and is quickly transported to the substrate along a groove in the foot.
No protobranch bivalves secrete a byssus.
100
The Lamellibranchs
Burrowing, adductor muscles, pedal retractor muscles
by living within a substrate-sediment, or burrows of their own making.
Burrowing into a soft sub-strate, such as sand or mud, is accomplished by the muscular foot.
The foot is initially extended into the substratum by hydraulic or hydrostatic means, through contractions of the appropriate musculature in the foot.
The adductor muscles, which attach the animal to the shell in all bivalve species, are relaxed at this time.
This relaxation of the adductor muscles permits the release of energy that was stored in the compressed hinge ligament, so that the shell valves now move apart and press tightly against the surrounding substrate.
This laterally directed force of the shell valves provides anchorage for the shell as the foot extends down into the substratum.
Thus, downward extension of the foot need not eject the animal from the burrow.
The adductor muscles then contract, drawing the shell valves toward each other.
This action releases the shell anchor.
Contracting the adductor muscles also pumps blood into the foot, which then dilates at the tip to form another anchor.
Abruptly closing the shell valves, in addition to swelling the foot, forces water out of the mantle cavity, blowing away and loosening some of the sediment adjacent to the opening between the shell valves.
With the foot firmly anchored in the substrate, the shell now can be pulled downward by contracting the pedal retractor muscles that extend from the foot to the shell.
The pedal retractor muscles do not contract simultaneously, but rather sequentially, so the bivalve shell "rocks" forward and then backward as it progresses, slicing into the substrate.
101
Subclass Anomalodesmata
Defining Characteristic:
Means irregular ligament
Hinge lacks true teeth (although secondary teeth are present in some species)
102
Subclass Anomalodesmata
Basic info
These unusual bivalves occur in marine and estuarine environments worldwide, from intertidal to abyssal depths.
Some species have slightly modified lamellibranch gills and are either suspension-feeders or deposit-feeders.
The most bizarre members of the group have a highly modified ("septibranch") ctenidium that enables them to act as bona fide carnivores, feeding especially on polychaetes and crustaceans up to several mm long.
The stomach is lined with hardened chitin, serving to grind up ingested food.
Labial palps, although present, are quite small and do not have any sorting function.
A very reduced style sac is found but never a crystalline style.
103
Subclass Anomalodesmata
("septibranch") ctenidium,
The septibranch ctenidium lacks filaments and instead forms a muscular septum.
The septum divides the mantle cavity into ventral and dorsal chambers, as in other bivalves, but it is perforated by a series of ciliated openings.
One-way valves regulate water flow through these openings in some species.
The septum can be moved forcefully upward within the mantle cavity, drawing water in through the incurrent siphon and simultaneously expelling water through the excurrent siphon.
These animals thus feed as organic vacuum cleaners, commonly sucking in small crustaceans and annelids.
104
Class Scaphopoda
Defining Characteristics:
Means spade foot
1) Tusk-shaped, conical shell, open at both ends;
2) development of anterior, threadlike, adhesive feeding tentacles
105
Class Scaphopoda
Basic info
300-400 sps
All marine
Sedentary Lifestyle
possess these "typical" molluscan features: foot, mantle tissue, mantle cavity, radula, and shell.
Scaphopod morphology has long suggested a close relationship with bivalves.
Recent molecular studies, how-ever, have linked them more closely either to cephalopods or to gastropods.
106
Class Scaphopoda
Shell
shell is never spirally wound, but rather grows linearly as a hollow, curved tube; hence, the common names "tooth shell" and "tusk shell."
The shell has an opening at each end.
107
Class Scaphopoda
Waterflow
Water enters at the narrower end of shell, which protrudes above the substrate. Inflow of water is due to the action of ciliated cells restricted to ridges of mantle tissue.
Periodically, water is expelled through this same opening by a sudden contraction of the foot musculature.
108
Class Scaphopoda
Ctenidia, circulatory system, hemocoel, osphradium,
possess no ctenidia.
They also lack a heart and circulatory system; instead, the blood circulates through the various large sinuses of the hemocoel as a consequence of the foot's rhythmic movements.
Neither do any scaphopods possess an osphradium; specialized sensory receptors in the posterior region of the mantle cavity, where respiratory currents enter and leave, may provide the same water-sampling function provided by osphradia in other molluscs.
109
Class Scaphopoda
Burrowing, Captacula
Burrowing into soft substratum is accomplished by the foot, essentially as described previously for the bivalves.
Scaphopods capture small food particles from the surrounding sediment and water using specialized, thin tentacles known as captacula.
A typical individual possesses 100 to 200 such tentacles.
Each captaculum terminates in a ciliated, sticky bulb; for feeding, the tentacles are extended by the creeping action of the cilia until the bulb contacts food.
The food is then transported to the mouth by captacular cilia or, in the case of large food particles such as large foraminiferans and small bivalves, by muscular contractions of the tentacles themselves.
110
Class Cephalopoda
Defining Characteristics:
Means head foot.
1) Shell divided by septa, with chambers connected by the siphuncle: a vascularized strand of tissue contained within a tube of calcium carbonate (shell reduced or lost in many species);
2) closed circulatory system;
3) foot modified to form flexible arms and siphon;
4) ganglia fused to form a large brain encased in a cartilaginous cranium
111
Class Cephalopoda
Basic info
Unlike most molluscs, cephalopods are often fast-moving, active carnivores capable of remarkably complex behaviors.
Of all living cephalopods, only the chambered nautilus forms a true external shell.
Reduction or loss of the shell has clearly been a major theme in cephalopod evolution.
112
Class Cephalopoda
Radula, ctenidia, mantle cavity, foot, head
All have radula, ctenidia
Mantle cavity and foot are present as well, but they do not generally function in typical molluscan fashion.
The head and associated sensory organs of cephalopod molluscs are extremely well developed.
113
Class Cephalopoda
Nautilus Shell, septa, siphuncle
only the 5 or 6 species in the genus Nautilus possess a true external shell.
The shell of Nautilus is spiral, but unlike that of gastropods, it is divided by septa into a series of compartments.
The living animal is found only in the largest, outermost chamber.
The septa are penetrated by the siphuncle—a calcified tube and its enclosed strand of vascularized tissue that spirals through the shell from the visceral mass, traversing all shell chambers.
114
Class Cephalopoda
Water transport, osmotic concentration, cameral fluid
Liquid may be slowly transported to and from the shell chambers through the siphuncle, gas diffusing into or out of the chambers as the fluid volumes are altered.
This liquid transport is apparently made possible by enzymes in the siphuncular tissue; these enzymes actively concentrate solutes-probably ions-either inside or outside the siphuncular tissue, thereby establishing local osmotic gradients.
If the osmotic concentration is higher within the siphuncular tissue, water will diffuse along the osmotic gradient from the liquid in the chamber-termed the cameral fluid-into the siphuncle and thence into the blood, to be discharged by the kidney.
115
Class Cephalopoda
Gas diffusion
Gas probably diffuses from the blood-remember, the siphuncle is well vascularized-into the shell chamber as cameral fluid is removed.
As the gas content of each chamber is changed by this mechanism, the buoyancy of the shell, and therefore of the animal, also changes.
In this way, the nautilus can maintain neutral buoyancy as it grows, exactly compensating for increased weight of shell and tissue through appropriate discharge of cameral fluid.
Cameral fluid flux also may be involved in buoyancy regulation during the chambered nautilus's vertical migrations, in which the nautilus may ascend and then descend hundreds of meters each day.
116
Class Cephalopoda
Locomotion, siphon, funnel
The nautilus locomotes by jet propulsion, expelling water from the mantle cavity through a flexible, hollow tube called the siphon or funnel.
Water is expelled through the contraction of the head retractor muscles.
The muscular funnel, which is derived from the foot of the "typical" mollusc, may be turned in various ways to move the animal in different directions.
117
Class Cephalopoda
Shell of non-nauitloids, mantle, water flow for locomotion
lack an external shell, and the mantle can thus play a major role in movement.
The mantle tissue is thick and replete with both circular and radial musculature.
Contracting the radial musculature of the mantle tissue while the circular muscle fibers are relaxed increases the volume of the mantle cavity.
Water then enters the mantle cavity along the anterior mantle margin, often through one-way valves.
This influx of water causes the animal to be drawn forward slightly.
When the radial muscles relax and the circular muscles then contract, the margins of the mantle tissue form a tight seal against the neck, and a large volume of water is forcefully expelled entirely through the flexible,
hollow funnel.
Since thrust = mass of expelled fluid x
velocity, the expulsion of so much water through the funnel at great velocity enables these cephalopods to move at high speeds; for brief periods.
In such rapidly moving squid, the mantle cavity refills partly by radial muscle contractions, partly by elastic recoil of the compressed mantle, and partly (or largely) by pressure gradients set up along the sides of the body as the squid moves through the water.
Jet propulsion is used most commonly in escaping from predators and capturing prey.
For more leisurely locomotion, many species use the arms or muscular lateral fins
118
Class Cephalopoda
pen
The squid shell is also internal, but it is little more than a thin, stiff, proteinaceous sheet called the pen.
This nonchambered "shell" plays no role in buoyancy regulation; instead, squid compensate for their own body weight by accumulating high concentrations of ammonium ions in the coelomic fluid.
119
Class Cephalopoda
chromatophores, iridocytes
As external shells became reduced or lost in evolution, cephalopods must have become increasingly vulnerable to predation.
Leading to other forms of protection.
skin of most cephalopods contains several layers of tiny colored cells called chromatophores, which overlay reflective cells called iridocytes.
Typically, the skin of a single individual contains hundreds of thousands, or even millions of chromatophores.
Expansions and contractions of the chromatophores are mediated by muscle elements in the skin and are under direct nervous control from the brain.
Thus, the coloration of cephalopod skin may change extremely quickly.
Defensive, camou-faging, and courtship-related changes of color have been described."
Not surprisingly, the chambered Nautils lacks chromatophores.
120
Class Cephalopoda
photophores, bioluminescence
Light organs.
These are distributed on the body in species-specific patterns, particularly on the ventral surface.
Light is produced in the photophores by biochemical reactions that parallel to a remarkable degree those demonstrated by many arthropods (including fireflies) and fishes.
In at least 3 dozen squid species, the bioluminescence is produced by symbiotic bacteria living within the photophores.
Although such bioluminescence-the biochemical production of light with minimal heat—may play a role in attracting or recognizing mates and in luring potential prey, it almost certainly also protects against predation.
121
Class Cephalopoda
Ink sac
ink sac associated with the digestive system.
The dark-pigmented fluid secreted by the ink sac may be discharged deliberately through the anus, forming a cloud that presumably confuses potential predators and that also may act as a mild narcotic.
Nautilus lack ink sac.
122
Class Cephalopoda
Mantle cavity, ctenidia, countercurrent exchange system
mantle cavity generally contains a pair of ctenidia, but unlike the situation in other molluscs, blood and water do not flow continuously in opposite directions:
There is no countercurrent exchange system.
Moreover, the ctenidia of cephalopods are not ciliated.
Instead, water circulation is maintained by the continual emptying and refilling of the mantle cavity, accomblished through contraction of the mantle musculature.
123
Class Cephalopoda
closed circulatory system, systematic heart, branchial heart,
Have completely closed circulatory system, in which blood flows entirely through a system of arteries, veins, and capillaries.
Blood sinuses found in other molluscs not present in cephalopods.
In addition to a single systemic heart, which receives oxygenated blood from the gills and sends it back to the tissues, an accessory (branchial) heart is associated with each gill.
The two branchial hearts increase blood pressure, helping to push blood through the gill capillaries.
Concentrations of oxygen-binding blood pigments, are also unusually high in cephalopod blood.
The cephalopod circulatory system is thus more efficient than that of other molluscs, supporting a far more active lifestyle.
124
Class Cephalopoda
Foot, arms, tentacles
In addition to forming the funnel, derivatives of the molluscan foot form the cephalopods' muscular arms and extensible tentacles.
The mouth thus lies in the center of the "foot."
The total number of arms and tentacles is usually either 8 or 10, depending on the species, although some nautiloids possess 80-90.
The tentacles of most cephalopods—all except those of the nautiloids-have small suction cups that are used for clinging to the substrate or to objects, including potential prey.
The arms are also studded with receptors sensitive to touch and taste; all cephalopods tested to date have well-developed chemical sensory capabilities.
125
Class Cephalopoda
cephalization
The degree of cephalization (concentration of sensory and nervous tissue at the anterior end of an animal) found among the Cephalopoda exceeds that found in any other invertebrate.
Cephalopods possess a large, complex, highly differentiated brain.
Capacity for learning and memory.
Octopus perceives shape, color intensity, and texture but, surprisingly, that it cannot distinguish between objects differing only in weight.
Even more surprising, cephalopods show no behavioral response to sound except at very low frequencies.
While direct electrical recordings clearly indicate that the statocysts are sensitive to sound stimuli, the information apparently is not suitably processed in the brain.
Cephalopod deafness may be an adaptive response to millions of years of predation by toothed whales and dolphins, which may stun their prey with sound?
126
Class Cephalopoda
Eyes
Each cephalopod has two eyes.
In Nautilus spp., the eyes are simple and function on the pinhole camera principle; there is no lens.
The eyes form an image, but visual acuity is not great.
In contrast, all other cephalopods have image-forming eyes that are incredibly similar to those of mammals.
Shows convergent evolution.
