Ch. 6

Question 1

Phylum Cnidaria

Answer 1

Phylum Cnidaria (= Coelenterata)
(Greek: a stinging thread [G: hollow gut)

Defining Characteristics:
1) Secretion of complex intra-cellular organelles called cnidae (nematocysts)
2) planula larvae in the life cycle

• contains over 11,000 species
• sea anemones, corals, jellyfish, freshwater Hydra, and the Portuguese man-of-war.
• 99% marine, 0.2% freshwater
• basic radial symmetry
• Medusa or polyp body plan
• possess only two layers of living tissue (the epidermis and the gastrodermis).
• All possess a gelatinous layer, the mesoglea, located between the epidermis and the gastrodermis.
  — mesoglea nonliving but may contain living cells derived from embryonic ectoderm
• amoebocytes in the mesoglea probably play roles in digestion, nutrient transport and storage, wound repair, and antibacterial defense.
• All have tentacles surrounding the mouth and only a single opening to the digestive system.
• All cnidarians secrete cnidae.
• absence of an anus.
  — All undigested food material passes out through the same opening through which the food enters: the mouth.
  — must expel the undigested remains of one meal before it can ingest more food.
• Movements generally accompanied by physical distortion of the digestive cavity, including partial or complete expulsion of the contained fluid.
  — Extensive movement is therefore not conducive to leisurely, thorough digestion.
• Gonadal development takes place within the digestive cavity
  — gametes or embryos must be released into this cavity before being expelled to the exterior through the mouth.
• primarily carnivorous.
• Some can photosynthesize
• Possess bona fide nerves and muscles.
• Have no central nervous system.
• Diploblastic animals
  — their musculature cannot have its origins in embryonic mesoderm.
• Muscle layers are composed of numerous ectodermal and endodermal cells that possess elongated, contractile bases anchored in the mesoglea; these cells are termed epitheliomuscular cells or nutritive-muscular cells, respectively
• Depending upon the orientation of the contractile bases, the cells form layers of either longitudinal musculature or circular musculature.
• nutritive-muscular cells also capture small food particles and then digest them intracellularly.
• epitheliomuscular cells are not a uniquely cnidarian feature.
• Cnidarians lack gills and other specialized respiratory structures; gases diffuse across all exposed epidermal and gastrodermal surfaces.

Question 2

Body plans in Cindaria

Answer 2

• medusa form (referred to as a “jellyfish”), which resembles a gelatinous saucer or upside-down cup with tentacles, and generally swims.
• polyp form, which has a tubular body and is generally stationary.

In many species, each body form is present in a different part of the life cycle, and in some species both are represented simultaneously in one individual.

Question 3

Cnidae’ of Phylum Cnidaria

Answer 3

(G: a nettle, a stinging thread), unique to the members of this phylum, are remarkable organelles secreted within cells called cnidoblasts (or nematoblasts) and discharged with explosive force for a variety of functions.

_{Of the three major categories of cnidae, nematocysts (literally, “thread-bags”) are the most widespread and the best studied.}

• Cnidae are among the most complex intracellular secretion products known.
• Each cnida consists of a rounded, proteinaceous capsule, with an opening at one end that is often occluded by a hinged operculum.
• Within the capsule is a long, hollow, coiled tube.
• During discharge, the hollow tube shoots out explosively from the sac, turning inside out as it goes.
• Entire process requires only about 3 ms (milliseconds).
• Discharge triggered by combination of chemical and tactile stimulation
  — generally perceived through a cluster of modified cilia (the cnidocil) that projects from the cnidoblast and by surface chemoreceptors on specialized nearby cells.
• Each cnida can be discharged only once.
• The primary force behind the actual expulsion of the cnidal filament is osmotic pressure.
• Especially abundant on the feeding tentacles of all species and within the digestive cavity of some species.
• Functional significance of cnidae lies in their great number per square millimeter of body surface, rather than in their individual size.

Question 4

Phylum Cindaria

Nervous System

Answer 4

• Nervous system consists of a network of nerve cells (neurons) and their processes (neurites), which generally synapse on one another repeatedly before terminating at a neuromuscular junction.
• Nerve impulses may cross certain synapses in one direction only, many synapses permit impulses to pass in both directions.
• Moreover, a given cell body may give rise to two or more neurites, radiating in different directions.
  — Thus, a nerve impulse received by one neuron may proceed in several directions at once.
• With such a nerve network, stimulation of a given sensory cell in the epithelium results in an outward spread of excitation over the animal’s entire body.
• The amount of surface area of the cnidarian that is affected by stimulating a given nerve cell increases in proportion to the frequency of stimulation.
• In addition to this slow-conducting nerve network, a second, fast-conducting nerve network generally underlies the epithelium.
• These cells are less branched (bipolar as opposed to multipolar) than are the cells in the slow-conducting network, so that signal transmission is more directed.
• Neurites in the two nets differ in size: Nerves in the fast-conducting network are of greater diameter, allowing more rapid conduction of nerve im-pulses.

Question 5

Phylum Cindaria

Nervous System

Answer 5

• Nervous system consists of a network of nerve cells (neurons) and their processes (neurites), which generally synapse on one another repeatedly before terminating at a neuromuscular junction.
• Nerve impulses may cross certain synapses in one direction only, many synapses permit impulses to pass in both directions.
• Moreover, a given cell body may give rise to two or more neurites, radiating in different directions.
  — Thus, a nerve impulse received by one neuron may proceed in several directions at once.
• With such a nerve network, stimulation of a given sensory cell in the epithelium results in an outward spread of excitation over the animal’s entire body.
• The amount of surface area of the cnidarian that is affected by stimulating a given nerve cell increases in proportion to the frequency of stimulation.
• In addition to this slow-conducting nerve network, a second, fast-conducting nerve network generally underlies the epithelium.
• These cells are less branched (bipolar as opposed to multipolar) than are the cells in the slow-conducting network, so that signal transmission is more directed.
• Neurites in the two nets differ in size: Nerves in the fast-conducting network are of greater diameter, allowing more rapid conduction of nerve im-pulses.

Question 6

Subphylum Medusozoa

Answer 6

This recently established but well-accepted grouping contains all cnidarians except for the sea anemones and corals and possibly the parasitic myxozoans.

Unlike that of any other metazoan so far studied, the medusozoan mitochondrial genome is linear rather than circular.

Question 7

Subphylum Medusozoa

Answer 7

This recently established but well-accepted grouping contains all cnidarians except for the sea anemones and corals and possibly the parasitic myxozoans.

Unlike that of any other metazoan so far studied, the medusozoan mitochondrial genome is linear rather than circular.

Question 8

Class Scyphozoa

Answer 8

Class Scypho • zoa
(G: cup animals)

Defining Characteristic:
Asexual replication by strobilation

• fewer than 200 species
• marine
• mesoglea layer of scyphozoans is thick and has the consistency of firm gelatin; thus, scyphozoans are known collectively as jellyfish.
• Medusoid. The body is in the form of an inverted cup, with nematocyst studded tentacles extending downward from the cup, or bell.
• The mouth is borne at the end of a muscular cylinder known as the manubrium.
• can swim actively, by contracting muscles and exploiting the mechanical properties of the mesoglea. (Mesoglea functions as a skeletal system)

_{- When the muscle fibers of the swimming bell contract, the volume of fluid enclosed under the bell decreases.}

• Water is forcefully expelled from under the bell as a consequence, and the animal is propelled in the opposite direction.
• The muscular contraction deforms the elastic mesoglea so that when the musculature is relaxed, the mesoglea “pops” back to its normal shape.
• This, of course, pulls the jellyfish downward as the volume enclosed by the swimming bell increases.
• Net forward movement of the animal occurs primarily because the speed with which the bell contracts exceeds the speed with which the bell recoils to its resting state.</sub>
• well-developed system of fluid-filled gastrovascular canals, ultimately connecting to the mouth through the manubrium.
• periodic vertical migrations from surface waters to deeper waters and back again, and the temporary formation of breeding aggregations.
• medusae are equipped with fairly sophisticated sensory receptors, implying that the nervous system is able to process and integrate a variety of sensory inputs.

Question 9

Class Scyphozoa

Feeding

Answer 9

• Food particles captured by nematocysts on the tentacles and/or oral arms are ingested at the mouth and conveyed to the stomach through the manubrium.
• Food is then distributed among four gastric pouches, which contain short, nematocyst-bearing tentacles (gastric filaments) that secrete an array of digestive enzymes.
• The partially digested food particles are then phagocytosed, and digestion is completed intracellularly, a typical feature of cnidarian biology.
• Fluid within the gut is circulated by cilia lining the walls of the gastrovascular canals.
• The gastrovascular canals are believed to function in circulating oxygen and carbon dioxide (the “vascular” part of gastrovascular), as well as in distributing nutrients (the “gastro” part of the term).
• Also obtain nutrients from certain unicellular algae (zooxanthellae) that live symbiotically in the jellyfish tissues.

Question 10

Class Scyphozoa

Sensory systems

Answer 10

• Sensory systems include balance organs (statocysts), simple light receptors (ocelli), and, in some species, touch receptors (sensory lappets).
• The statocysts and ocelli are contained within club-shaped structures called rhopalia, which are distributed along the margins of the swimming bell.
• Dense aggregations of nerve tissue are found associated with the rhopalia.
  — These ganglia act as pacemakers, triggering the rhythmic contraction of the swimming bell
• Statocysts operate on a simple principle.
  — Tubular pieces of tissue (the rhopalia) hang freely at several locations around the margins of the swimming bell.
  — Each of these rhopalia is adjacent to (but not in continuous contact with) sensory cilia.
  — Also, each tube is weighted at the free end with a spherical calcareous mass (the statolith).
• rhopalium/statocyst system thus provides a mechanism through which the animal can be informed of its physical orientation.
• non-image-forming ocelli (light receptors) are also found along the bell margin.
• An ocellus is simply a small area, often cup shaped, backed by light-sensitive pigment.

Question 11

Class Scyphozoa

Life cycle

Answer 11

• diagnostic feature of their biology.
• Gonads develop within gastrodermal tissue and are closely associated with the gastric pouches.
• gonochoristic (gono = G: reproductive organs; chorist = G: separate), or dioecious (G: two houses).
  — individual medusae are either male or female; that is, the sexes are generally separate.
• planula larva eventually results from the union between sperm and egg.
  — typically has the form of a heavily ciliated, microscopic sausage.
• nonfeeding larva attaches to a substrate and transforms into a small polypoid individual called a scyphistoma.
• This polyp form has the same two-layered construction (plus mesoglea layer) as the medusa, but the mesoglea layer is substantially thinner in the polyp than in the medusa morph.
• The scyphistoma is sessile and lacks ocelli and statocysts.
• It is a feeding individual, with the mouth oriented away from the substrate.
• As the scyphistoma grows, it may produce additional scyphistomae asexually by budding.
• Eventually, a process called strobilation takes place in most species.

Question 12

Class Scyphozoa

strobilation

Answer 12

• During strobilation, the body column of a scyphistoma subdivides transversely, forming numerous modules that are stacked on top of each other like hotel ashtrays.
• Each module eventually breaks away from the stack as a swimming ephyra.
• As it swims, each ephyra gradually grows and changes in physical appearance, becoming an adult scyphozoan.
• From a single fertilized egg producing a unique genotype (or genet), a large number of genetically identical, sexually reproducing medusae are generated.
• Each of these independent but genetically identical units is termed a ramet.

Question 13

Class Cubozoa

Answer 13

Class Cubo • zoa
(G: cube animals)

Defining Characteristics:
1) Medusa with boxlike body;
2) rhopalia bear complex, lensed eyes

• ~25 species
• called cubomedusae.
• most derived cnidarians.
• restricted to tropical and subtropical areas.
• have a cuboidal swimming bell: The bell is square in transverse section.
• Each individual bears four tentacles, or four clusters of tentacles, emerging from the four corners of the bell, near the four rhopalia.
• tentacles boast highly virulent nematocysts that have earned cubomedusae the well-deserved epithet
  “sea wasp.”
• possess an unusually well developed nervous system and remarkably complex eyes
• swimming bell of sea wasps curves inward at the lower edge, restricting the size of the opening through which water is expelled when the bell contracts.
  — This increases the force with which the water exits the bell, and thus the amount of jet propulsion obtained.
• polyp stage does not strobilate.
• the polyp resulting from a single planula larva buds off more polyps, each of which develops into a single medusa.
• The asexual production of genetically identical individuals (ramets) characterizing scyphozoans is again achieved in the cubozoan life cycle, but through a different vehicle.

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Question 14

Class Hydrozoa

Answer 14

Class Hydro • zoa
(G: water animals)

• characterized by generally greater representation of the polyp morph in the life cycle than is the case for scyphozoans.
• the gastrodermal tissue of hydrozoans lacks nematocysts
• no cells are found within the mesoglea; nematocysts are restricted to the epidermis.
• Includes more than 3,000 species divided among two subclasses.
• At least 95% of the described species belong in the subclass Hydroidolina.
• Most are marine.

Question 15

Class Hydrozoa

Subclass Hydroidolina

Answer 15

• marine or freshwater
• medusoid as adults; that is, the sexual stage of the life cycle resembles that found among the Scyphozoa, and like scyphozoan medusae, those of hydrozoans are commonly called jellyfish.
• mesoglea layer of the hydrozoan medusa is thick
• mouth is borne at the end of a manubrium
• ocelli and statocysts are present.
• The sense organs may be found at the base of the tentacles, as in scyphozoans, or between the tentacles.
• All medusae are gonochoristic, a given individual being either male or female but never both.
• possess a shelf of tissue (the velum) extending inward from the edge of the swimming bell toward the manubrium.
  — velum causes water to be ejected from under the swimming bell through a narrower opening, and thus with greater velocity, when the musculature contracts.
• planula larvae develop from fertilized eggs, and the planula typically metamorphoses into a sessile polypoid individual lacking both statocysts and ocelli.
• Hydrozoan polyps are structurally and functionally more complex than are the scyphistomae of the scyphozoan life cycle.
• each polyp is a separate, distinct being, completely responsible for its own welfare.
• some species of hydra harbor unicellular green algae, called zoochlorellae, in their tissues. Both the host and the algae benefit from the relationship.

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Question 16

Class Hydrozoa

Subclass Hydroidolina

Life cycle

Answer 16

• lacks a medusa stage.
• most other hydrozoans are colonial in the polyp stage of the life cycle; that is, a single planula usually gives rise to a large number of polyps, called zooids (or modules of the colony), all of which are interconnected and share a continuous gastrovascular cavity.
• The zooids are often connected to each other, or to a substrate, by means of a rootlike stolon.
• The oral end of a polyp (i.e., the end bearing the mouth and tentacles) is called the hydranth.
• The stolon and stalks of the colony are commonly encased in a transparent protective tube, known as the perisarc, composed of polysaccharide, protein, and chitin.
• The perisarc may or may not extend upward to encase the hydranth of a polyp, depending on the species.
• The perisarc surrounding the hydranth is known as a hydrotheca and the hydroid is said to be thecate (as opposed to athecate, the Greek prefix a meaning “not,” or “without”)

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Question 17

Class Hydrozoa

Subclass Hydroidolina

Several structurally and functionally distinct modules are often present in a single hydroid colony;

Answer 17

such colonies are dimorphic (consisting of two types of mod-ules) or polymorphic (consisting of more than two types of modules).

• Modules specialized for feeding are called gastrozooids.
• Gastrozooids collect small animals using the tentacles (which are densely clothed in nematocysts) and ingest the prey through the single opening into the gastrovascular cavity.
• Digestion is extracellular in the gastrovascular cavity, and then becomes intracellular as the partially digested food is distributed throughout the colony, largely through rhythmic contractions of the muscular polyps.
• Medusoids are produced asexually by budding from various regions of the polyp colony.
• Often, the medusoids derive from a particular type of module called a gonozooid.
• Some gonozooids lack tentacles and so are incapable of feeding; they are specialized for producing medusoids and must depend upon other members of the colony for nutrition.
• Hydrozoan colonies commonly contain fingerlike modules specialized for defense (dactylozooids; dactylus = G: finger), as well as modules specialized for feeding and reproduction
• dactylozooids are heavily studded with nematocysts.
• Dactylozooids never possess mouths and thus, like some of the highly specialized gonozooids, they depend upon the gastrozooids for food collection.

Question 18

Order Siphonophora

Answer 18

• free-floating hydrozoan colonies in which medusoid and polypoid morphs are present simultaneously in a number of different incarnations.
• Modified medusae serve as modules modified to propel the colony through the water by jet propulsion (nectophores), or as leaflike defensive modules (bracts, or phyllozooids; phyllo = G: leaf).
• Some species exhibit gas-filled (carbon monoxide!) floats called pneu-matophores, which may also derive from basic medusoid architecture.
• The mesoglea layer is much reduced or entirely absent in pneumatophores. Nectophores lack both mouth and tentacles.
• Bracts are well endowed with nematocysts.
• The polyp morph is represented by gastro-zooids, gonozooids, and dactylozooids.
• Each gastrozooid has a single tentacle associated with it; elongated, nematocyst-bearing structures (tentillae) may project from these tentacles.
• Dactylozooids also may have associated tentacles.
• All siphonophore tentacles are highly retractile, and the nematocysts are often very toxic.
• Modules within a colony often occur in clusters, called cormidia, arranged upon a long stem.
• Each cormidium typically contains gonozooids, dactylozooids, phyllozooids (bracts), and gastrozooids.
• voracious carnivores.

Question 19

The Hydrocorals

Answer 19

• small number of species, all of which are colonial and secrete a substantial calcareous skeleton, and most of which are restricted to warm waters.
• The dactylozooids are especially abundant and potent in many species; the common name “fire coral” is well deserved.
• These animals are not true corals, however.
• The true corals are contained within a different class of cnidarians, the Anthozoa,

Question 20

An Unusual Group of Likely Medusozoans: the Myxozoa

Answer 20

• 2,200 species of extracellular, spore-forming parasites
• infective spore serves to disperse the parasite to a new host.
• Myxozoans mostly infect aquatic annelids and freshwater bryozoans, with fish typically serving as intermediate hosts in the life cycle.
• possess distinctive organelles called polar filaments; individual myxozoans usually bear several of these filaments, and each is coiled tightly within specialized polar capsules; these capsules may occupy much of the space within the spore.
• each spore is often multicellular, containing two or more amoeboid sporo-plasms.
• The sporoplasms are released by rupture of the spore, rather than by discharge through an everted polar filament.

Question 21

A Medusozoan Sister Group?

Class Staurozoa

Answer 21

• cold shallow waters
• live attached to seaweeds, rocks, and gravel by a stalk
• feed on mostly on small crustaceans that pass by, catching their prey with the eight sets of tentacles found at their distal end; the tentacles end in great clusters of nematocysts.
• live the lives of polyps; they are commonly referred to as “stalked jellyfish.”
• There is no swimming medusa stage in the life cycle.
• Instead, males and females release their gametes into the seawater, and the embryos develop into elongated planulae.
• The planulae are not ciliated, which sets them apart from those of other medusozoans; instead they creep over the substrate by muscular contractions.
• The crawling larvae eventually attach to a substrate and soon develop feeding tentacles and gonads.

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Question 22

Subphylum Anthozoa

Answer 22

Subphylum Antho • zoa
(G: flower animals)

Defining Characteristics:

1) Absence (loss?) of a medusa stage (or any trace of one);

2) absence (loss?) of operculum and cnidocil;

3) mitochondrial DNA is circular (as in most eukaryotes) rather than linear (as in other cnidarians);

4) presence of ciliated groove (siphonoglyph) in the pharyngeal wall leading from the mouth;

5) coelenteron partitioned by distinct sheets of tissue (mesenteries/septa)

• 6,000 species
• sister group to the Medusozoa.
• marine
• polyp body form and lifestyle exclusively; no trace of the medusa morph appears in the life cycle.
• Lacking a medusa, gametes are produced directly by the anthozoan polyp.
• A planula larva develops from the fertilized egg and metamorphoses to form another polyp.
• The planula larvae of some anthozoan species can feed on phytoplankton and other microscopic food particles; no feeding planulae have been found too.
• some secrete buoyant, calcified skeletons that enable them to float for weeks at the water’s surface.
• eventually, at least some of those polyps abandon the buoyant skeleton, sink to the bottom of the sea, and secrete a new, normal skeleton for benthic life.
• reproduce asexually, often through longitudinal or transverse fission or through a process of pedal laceration, in which parts of the pedal disc (foot) detach from the rest of the animal and gradually differentiate to form a new single-module ramet.
• carnivorous; they capture food using nematocyst-studded tentacles and transfer it to a central mouth opening.
• Internally, near the base of an anthozoan, thin filaments called acontia extend in some species from the middle lobe of the mesenteries they are loaded with nematocysts and secretory cells and can be extended outside the body through small pores in the body wall.
• Acontia are used both offensively and defensively, and they may also function in digestion.
• These hollow acrorhagi can be extended a substantial distance from the body column, possibly by forcing fluid into them from the gastrovascular cavity, with which they are continu-ous.
• Acrorhagi are covered with very potent nematocysts; they are used in defending a territory against invasion by other anemones.
• catch tentacles are analogous to acrorhagi, functioning in aggressive encounters among individuals.
• Anthozoan tissues contain both circular and longitudinal muscle fibers
• Provided that the animal keeps its mouth closed by contracting appropriate sphincter muscles, the seawater in the gastrovascular cavity can serve as a hydrostatic skeleton.

Question 23

anthozoan polyps differ from those of hydrozoans in several respects.

Answer 23

• mouth opens into a tubular pharynx rather than directly into the gastrovascular cavity, and one or two discrete, cili-ated grooves, called siphonoglyphs, typically extend down the pharynx from the mouth.
• The anthozoan gastrovascular cavity is partitioned by numerous sheets of tissue called mesenteries or septa, whereas no mesenteries are found in the gastrovascular cavity of hydrozoan polyps.
• These infoldings of gastroderm and mesoglea greatly increase the surface area available for secreting digestive enzymes and absorbing nutrients.
• Mesenteries that extend far enough from the body wall into the gastrovascular cavity to actually attach to the pharynx are called primary or complete mesenteries.
• Those extending only partway into the gastrovascular cavity are termed incomplete mesenteries.
• The free edge of the incomplete mesenteries is trilobed, ciliated, and studded with nematocysts, cells that secrete digestive enzymes, and cells that phagocytose bacteria
• the arrangement of the mesenteries shows a basic bilateral, rather than radial, symmetry.
• Additionally, the mesenteries contain thick, longitudinal retractor muscles and bear the gonads.
• Anthozoans are usually gonochoristic, but some species are sequential hermaphrodites.

Question 24

Subclass Hexacorallia (= Zoantharia)

Answer 24

Subclass Hexa • corallia (= Zoantharia)
(G: six)

• possess many tentacles around the mouth opening (usually in some multiple of 6; hexa = G: six) and six pairs of primary mesenteries.
• One pair of siphonoglyphs is associated with the pharynx.
• Many species in this subclass are solitary (i.e., they are independent ramets rather than colonies of connected modules) and lack any specialized protective covering.
• The other species in this subclass tend to be colonial; unlike the colonial hydrozoans, however, these anthozoans are never polymorphic.
• The best known of these colonial species are the true (or stony) corals, which secrete substantial external calcium carbonate skeletons. These species are also called scleractinian corals (sclero = G: hard).
• Scleractinian corals may be reef-building (hermatypic) or not (ahermatypic).

Question 25

Subclass Hexacorallia (= Zoantharia) coral growth

Answer 25

- coral growth is aided in such places by a complex relationship with endosymbiotic, unicellular photosynthesizers (modified dinoflagellates actually, usually members of the genus Symbiodinium) called zooxanthellae. - Through photosynthesis, the zooxanthellae provide the anthozoan host with energy-rich, small-molecular-weight organic compounds; recent measurements show 20% to 95% of the material fixed through photosynthesis being released to the host, principally as fatty acids, lipid drop-lets, amino acids, glucose, or glycerol. - In return, the zoo-xanthellae have intimate access to the metabolic wastes of the coral, including carbon dioxide for photosynthesis and nitrogenous wastes essential for algal growth.

Question 26

Subclass Hexacorallia (= Zoantharia) coral growth mechanism through which this effect is mediated remains uncertain.

Answer 26

1. through an effect on availability of bicarbonate ion (HCO,*), an essential constituent of the calcification process; 2. through a contribution by the algae of a critical component of the organic matrix serving as the nucleating site for calcium carbonate deposition; 3. through localized removal of soluble phosphate (PO,*) by the algae during photosynthesis (phosphates are known inhibitors of carbonate calcification); and 4. indirectly, through an influence of elevated dissolved oxygen concentrations on rates of coral metabolism.

Question 27

Subclass Octocorallia (= Alcyonaria)

Answer 27

Subclass Octo • corallia (= Alcyonaria) (G: eight) Defining Characteristic: Polyps bear eight tentacles and are subdivided by eight complete mesenteries - Members of the subclass Octocorallia possess 8 tentacles (and eight primary septa) and generally have only a single siphonoglyph. - The tentacles of octocorallians are pinnate; that is, they bear numerous lateral out-foldings called pinnules. - All octocorallian species form modular colonies, which are often polymorphic. - In species with polymorphic polyps, some individuals cannot feed and function solely in driving water through the gastrovascular spaces of the colony. - The polyps of octocorals may be embedded in a thick matrix of mesoglea; these are the soft corals. - In other octocoral species (colonial species with skeletal supports), the polyps are supported by proteinaceous or calcareous internal skeletons secreted by cells in the mesoglea. - This latter group of octocorals includes the sea fans and sea whips (known collectively as gorgonians, or horny corals) and the pipe corals. - The tissues of octocorallians have long been known to accumulate a variety of unusual biochemicals derived from fatty-acid metabolism. - These compounds are not directly involved in the metabolic processes of the corals, but seem instead to protect the coral from predation and overgrowth by other organisms.