Mollusca

MOLLUSCA

(Latin.mollis = soft)
This phylum is one of the largest marine groups with over 80 000 species. All comprise of a soft, unsegmented body, consisting of an anterior head, a dorsal visceral mass and a ventral foot.The body is more or less surrounded by a fleshy mantle (an outgrowth of the body wall) and nearly all species in the group secrete a lime shell that covers and protects the body. All, except the class Bivalvia, have a ribbon-like rasping tongue (radula - unique to this phylum) with small chitinous teeth that processes the food. Most mollusks are free living, but slow moving creatures, showing a close association with the substrate. Some attach to rocks or shells, others burrow, others float, octopuses and squids swim freely.

Characteristics:

1. Body usually short and partially or wholy enclosed by a fleshy outgrowth of the body wall called the mantle, which may be variously modified. Between the mantle and the visceral mass is a mantle cavity containing components of several systems (secondarily lost in a few groups).

2. A shell (if present) is secreted by the mantle and consists of one, two or eight parts. the head and the ventral muscular foot are closely allied (the foot being variously modified for burrowing, crawling, swimming, or food capture).

3. The digestive canals are complete and intricate with ciliary canals for the sorting of particles. The mouth with a rudula bearing traverse rows of minute chitinous teeth to rasp food , except in Bivalvia. The anus opening in the mantle cavity. A large digestive gland and often salivary glands are present.

4. The circulatory system is open, except in Cephalopoda and usually includes a dorsal heart with one or two atrias and one ventricle. This is situated in a pericardial cavity. An anterior aorta and other vessels and many blood spaces (hemocoels) exist in the tissues.

5. Respiration occurs via one to many uniquely structured ctenidia (gills) in the mantle cavity (secondarily lost in some), by the mantle cavity, or by the mantle.

6. Excretion by kidneys (nephridia), one or two or six pairs, or only a single one. They usually connect to the pericardial cavity and they exit in the mantle cavity. The coelom is reduced to the cavities of the nephridia, gonads and pericardium.

7. The nervous system is typically a circumesophageal nerve ring with multiple pairs of ganglia and two pairs of nerve cords (one pair innervating the foot and another the visceral mass). Many poses organs for smell, or touch, or taste. Eyespots or complex eyes present. A statocyst for equilibration present.

8. The sexes are usually seperate(some are monoecious, a few are protandric). Gonads add up to four, two or one, all with ducts. Fertilization occurs externally or internally. Most species are oviparous. Egg cleavage determinate, spiral, unequal and total (meroblastic in Cephalopoda). Trochophores and veliger larvae form, or a parasitic stage occurs(Unionidae), or the development is direct (Plumonata, Cephalopoda).

9. Unsegmented (except Monoplasophora). Symmetry bilateral or asymmetrical.

This phylum has seven different classes, five of which are often encountered.

POLYPLACOPHORA

Chitons

This class, Polyplacophora ("many plate bearers"), contains the Chitons, easily recognizable because of their shells that are split into eight dorsal plates that cover the centre of their bodies.

Dorsal view of a chiton with a spike bearing mantle.

A gridle surrounds the plates and beneath the body lies a broad foot that provides tenacious attachment to rocks.

Vental view of a chiton with a spike bearing mantle.

Respiration occurs through 6 to 80 pairs of gills in a groove around the foot. The Chitons are herbivores that have strongly toothed radulae. They are chiefly found in shallow coastal waters. Around 600 known species.

Chiton Olivaceous : West Indian Chiton.

BIVALVIA

Bivalve molluscs

Members of this class (including mussels, clams and oysters) has a shell that consists of two parts, but unlike the Brachiopoda (Lamp shells) the two parts are hinged together dorsally and then extends downwards, laterally on either side of the body, thereby encasing it.

Interior view of right valve showing the muscel scars.

Most of the bivalves poses large gills for the purposes of respiration and filtering out of small food particles.

Bivalve with left valve and mantle removed.
(Heavy arrows - path of water current,
Dashed arrow - path of filtered particles).

Section through the visceral mass showing the internal organs.

Because of this diet and process of feeding, Bivalves lack a radula. They poses one or two pairs of gills(ctenida) or branchia for respiration. Many species burrow in the sand or mud and poses large, wedge-shaped feet for this. Mussels attach to rocks by means of a beard-like byssus, while oysters and their allies cement one of their valves to the rock face. Around 20 000 known species.

SCAPHOPODA

Tusk shells

Tusk or tooth shells. The shell and the mantle are slenderly tubular, slightly curved (shaped like an elephant tusk) and open at both ends. The foot conical foot protrudes from the larger ventral end of the shell and is used for burrowing. Delicate ciliated contractile tentacles are found around the mouth to capture food. No ctenidia are present and the large mantle cavity serves for respiration.

A scaphopod, or tooth shell, burried in the sand.

Living at depths of up to 4570m. Around 350 known species.

GASTROPODA

The class Gastropoda is the largest in the phylum and includes the snails, whelks, winkles and sea slugs.

Gastropod anatomy.

Most of these species belong to the subclass, Prosobranchia, and have a spiral shaped shell, a well developed head that poses tentacles and a radula, and a large flat foot for motion. The primitive members are herbivores that rasp seaweeds and micro algae. The more advanced forms are predators and poses a long proboscis and cylindrical siphon. To house the siphon the shell has an anterior canal, or groove betraying the animals predatory habits. Most of these gastropods are shelled, but many of the subclass Opisthobranchia (sea slugs, sea hares, nudibranches etc.) have forsaken their gills and shells. The reason for this is unknown, but it has been speculated that their ancestors were sand-burrowers, for whom these would have been a hindrance. With out the assistance of a shell the modern forms protect themselves in various other ways. Some produce toxins which make them poisonous. others prey on anemones or bluebottles and then engineer the stinging cells of their victims into their own tissues. To advertise their non edible nature, most are wildly coloured and exquisitely patterned. Around 40 000 known species.

PROSOBRANCHIA

Snails, limpets

OPISTHOBRANCHIA

Sea slugs

CEPHALOPODA

Octopus, squid

The final large class includes the octopus, cuttlefish and squid among the most highly evolved of all invertebrates.

Dorsal view of a squid in swimming position.

Dorsal view of a squid (loligo) in swimming position.
The tentacles and arms are held together and
functions as a rudder.

Cuttlefish (sepia) seizing a shrimp with the use of
its tentacles.

Instead of a foot they poses eight or ten long tentacles armed with suckers.

Anatomy of a squid - loligo (ventral view with the
mantle cut open).

Most of the cephalopods are active and predatory swimmers posesing jaws and radula. This class includes the deep-sea giant squids (20m long) which were the inspiration for many a mythical story about sea monsters. The Cephalopods poses eyes as complex as those of humans, and a greater capacity for learning than any other invertabrates.Around 650 known species.

Sagital section of nautilus.

Nautilus : the only shelled cephalopod.