General Topics of Phylum Mollusca

Neopilina

Only living representative of a palaeozoic group of molluscs. Neopilina is, therefore, called living fossil.

It is also a connecting link between annelids and molluscs.

It has a cup-like shell and disc-like foot with creeping sole.

The animal shows metameric segmentation. Larva is trochophore.

It bears dorsally a shell formed by articulation of 8 plates. The foot helps in attaching.

Chiton

Nocturnal and herbivorous. Trochophore larva occurs in the life cycle.

It has a slightly curved tubular shell open at both the ends. Foot is pointed.

Respiration occurs through mantle. Excretory organs are nephridia.

Aquatic fresh water form; can also live on land.

Body covered by single piece shell (univalve).

Mouth of the shell guarded by operculum attached to foot; if shell coils towards right (clockwise) shell is dextral, if towards left (anticlockwise) then sinistral.

Body divided into head, foot and visceral mass.

Head distinct with a pair of stalked eyes, a pair of labial palps, a pair of true tentacle.

Foot large, thick and muscular. Visceral mass contains main organs, mantle encloses the body; true coelom but reduced to haemocoel.

Digestive system includes an alimentary canal, pair of salivary glands and a large digestive gland, buccal mass possesses jaws, radula and odontophore.

Radula possesses seven teeth in each transverse row; serves to cut the food into finer particles.

Respiration aquatic by ctenidia, aerial by pulmonary sac, nuchal lobes also help in aerial respiration.

Circulatory system is open type; respiratory pigment haemocyanin.

Nervous system includes ganglia, connectives and commissures. Ganglia are cerebral, buccal, pleuropedal, supra-intestinal, visceral.

Sense organs-osphradium, eyes-photoreceptors; statocyst balancing organs; tentacles-tactile organs.

Reproduction-Dioecious, male reproductive organs-single; sperms of two types eupyrene and oligopyrene; copulatory organs present in male; female reproductive organs-ovary single, oviduct, receptaculum seminis, uterus, vagina, and rudimentary penis (copulatory organ).

Unio

Body soft, unsegmented; bilaterally symmetrical; foot plough-shaped.

Shell made of two pieces (bivalve) attached by hinge joint; having an umbo and lines of growth; protects the body and serves for respiration also

A pair of gills on either side of the body. Body cavity is haemocoel. Respiration by a pair of gills, also by mantle.

Digestive System includes alimentary canal and digestive gland, radula absent, stomach surrounded by digestive gland, rectum passes through the pericardium traversing the ventricle food consists of diatoms, protozoa, filter-feeder; digestion intracellular by phagocytes or partly intercellular.

Blood Vascular System open type; blood has colourless plasma with haemocyanin. Excretory organs-a pair of kidneys or Organs of Bojanus and pericardial gland or Keber’s organ. Keber’s organ opens into pericardium.

Sense organs poorly developed; tentacles absent; receptors are statocyst for balancing, ospharadium to test quality of water, epithelial sensory cells-tactile and photoreceptors.

Reproductive System-Dioecious, indirect development; in fresh water forms larva is glochidium; in marine forms larva is velliger.

Limax (Grey Slug)

Terrestrial greyish-black gastropod with reduced internal shell. It is commonly
found creeping in gardens. Anterior end bears a pair of sensory tentacles and a pair of stalked eyes.
Mantle cavity serves as a lung.

Pearl Oyster (Pinctada vulgaris)

Marine, sedentary, bivalved mollusc. The two valves are unequal.

Pearl is an ‘inside out’ tiny shell which is secreted by the mollusc internally around any irritating substance like sand grain, parasite or place of injury. In culture, mouth of oyster is opened and a fine silica particle is introduced between mantle and shell. The oyster secretes an iridescent pearly material or nacre over the particle, layer after layer. Star fish is harmful to pearl industry.

Shipworm (Teredo) :

The shell is used for burrowing in wooden structures. This mollusc can digest cellulose. It is destructive since it causes holes in hulls of boats and ships.

Sepia (Cuttle fish)

A marine form living in shallow water of the warm seas.

The body is differentiated into head, neck and trunk.

The head has a pair of large eyes and five pairs of arms surrounding the mouth.

Each arm bears four rows of suckers on the inner surface, the fourth pair of arms is known as tentacles which are longer and bear suckers only at their distal end. In males, 4th ventral arm is modified to serve as intromittent organ.

The neck is small and constricted.

The trunk has a lateral fin on either side which helps in swimming. There is present a large tubular funnel or siphon lying below the head and opening on the exterior behind the neck which is the main outlet of the mantle cavity.

The shell is internal. A pear-shaped ink-sac containing the ink-like fluid is present. When the animal is attacked ink-like fluid is ejected through the funnel to form a smoke cloud. Sepia is edible.

Loligo (Squid)

Found in the warm seas. Loligo is fast swimmer and carnivorous. It also ejects a dark ink to form a smoke cloud to escape from the enemy.

Octopus (Devil Fish)

A bottom dweller, noctural cephalopod which is devoid of lateral fins and internal shell found in Sepia and  Loligo.

Oral arms are 8 in number. Suckers occur in two rows.

Swimming is by jet propulsion through the use of siphon. Crawling is done by arms and their suckers. Ink is expelled for camouflage.