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Phylum Mollusca

compose the large phylum of invertebrate animals known as the

phylum Mollusca. Around 85,000 extant species of molluscs are recognized.

Molluscs are the largest marine phylum, comprising about 23% of all the named

marine organisms. Numerous molluscs also live in freshwater and terrestrialhabitats. They are highly diverse, not only in size and in anatomical structure, but

also in behaviour and in habitat.

The phylum is typically divided into 9 or 10 taxonomic classes, of which two are

entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are

among the most neurologically advanced of all invertebrates—and either 

the giant squidor the colossal squid is the largest known invertebrate species.

The gastropods (snails and slugs) are by far the most numerous molluscs in

terms of classified species, and account for 80% of the total.

Molluscan Characteristics

1.  Body of two parts: head-foot and visceral mass

2.  Mantle that secretes a calcareous shell and covers the visceral mass.

3.  Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, and

release of reproductive products

4.  Bilateral symmetry

5.  Trochophore larvae, spiral cleavage, and schizocoelous coelom formation

6.  Coelom reduced to cavities surrounding the heart, nephridia and gonads

7.  Open circulatory system in all but one class (Cephalopoda)

8.  Radula usually present and used in scraping food

Three main regions of the Mollusc Body

  Head-foot- elongate with an anterior head, containing the mouth and certain nervous

and sensory structures, and an elongate foot, used for attachment and locomotion.

  Visceral mass- contains the organs of digestion, circulation, reproduction, and excretion

and is positioned dorsal to the head-foot

  Mantle- attaches to the visceral mass, enfolds most of the body, and may secrete a shell

that overlies the mantle

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Classification of the Mollusca

Class Caudofoveata Wormlike molluscs with a cylindrical, shell-

less body and scale like, calcareous

spicules; lack eyes, tentacles, statocysts,

crystalline style, foot and nephridia. Deep

water marine burrowers

Class Aplacophora Shell, mantle, and foot lacking; wormlike;

head poorly developed; burrowing mollusc

Class Polyplacophora Elongate,dorsoventrally flattened; head

reduced in size; shell consisting of eight

dorsal plates. Marine, on rocky intertidal

substrates

Class Monoplacophora Molluscs with a single arched shell; foot

broad and flat; certain structures seriallyrepeated. Marine.

Class Scaphopoda Body enclosed in a tubular shell that is

open at both ends; tentacles used for

deposit feeding; no head. Marine.

Class Bivalvia Body enclosed in shell consisting of two

valves, hinged dorsally; no head or radula;

wedge-shaped foot. Marine and freshwater

Class Gastropoda Shell, when present, usually coiled; body

symmetry distorted by torsion; somemonoecious species. Marine, freshwater,

terrestrial

Class Cephalopoda Foot modified into a circle of tentacles and

a siphon; shell reduced or absent; head in

line with the elongate visceral mass.

Marine.

Phylum Echinodermata

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are a phylum of marine animals. The adults are recognizable by their 

(usually five-point) radial symmetry, and include such well-known animals

as starfish, sea urchins, sand dollars, and sea cucumbers. 

Echinoderms are found at every ocean depth, from the intertidal zone to

the abyssal zone. 

The phylum contains about 7000 living species, making it the second-

largest grouping of deuterostomes (a superphylum), after 

the chordates (which include the vertebrates, such

as humans, sharksand frogs).

Echinoderms are also the largest phylum that has no freshwater or 

terrestrial (land-based) representatives.

The echinoderms are important both biologically and geologically.

Biologically, there are few other groupings so abundant in the biotic

desert of the deep sea, as well as shallower oceans. 

The more notably distinct trait, which most echinoderms have, is their remarkable powers of regeneration of tissue, organs, limbs, and of 

asexual reproduction, and in some cases, complete regeneration from a

single limb.

Geologically, the value of echinoderms is in their ossified skeletons, which

are major contributors to many limestone formations, and can provide

valuable clues as to the geological environment.

Further, it is held by some scientists that the radiation of echinoderms was

responsible for the Mesozoic revolution of marine life 

Characteristics of Echinoderms:

1.  Pentaradial symmetry- body parts are arranged in fives, or a multiple of five, around an

oral- aboral axis

2.  The echinoderm skeleton consists of a series of calcium carbonate plates called ossicles.

3.  The water vascular system of echinoderms is a series of water-filled canals, and their

extensions are called tube feet.

4.  A hemal system consists of strands of tissue that encircle an echinoderm near the ring

canal of the water vascular system and run into each arm near the radial canals.

Classification of Echinodermata

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Class Crinoidea Free living or attached by an aboral stalk of 

ossicles, flourished in the Paleozoic era.

Approximately 230 living species. Sea lilies,

feather stars

Class Asteroidea Rays not sharply set off from central disk;

ambulactal grooves with tube feet; suctiondisks on tube feet; pedicellariae present. Sea

stars. About 1500 species

Class Ophiuroidea Arms sharply marked off from the central disk;

tube feet without suction disks. Brittle stars.

More than 2, 000 species.

Class Echinoidea Globular or disk shaped ; no rays; movable

spines; skeleton of closely fitting plates. Sea

urchins, sand dollars. Approximately 1, 000

species

Class Holothuroidea No rays, elongate along the oral-aboral axis;microscopic ossicles embedded in a muscular

body wall; circumoral tentacles. Sea

cucumbers. Approximately 1,500 species

Molluscs

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Echinoderms

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