- BS Media
- Published: 04, Jun, 2017 | Updated: June 04, 2017
Introduction: Mollusca includes soft bodied animals. It was first applied by Ariistotle to the cuttle-fish of the Aegean Se. The Moiluscans are soft bodied, triploblastic coelomate & bilaterally symmetrical animals. There are about 10,000 living species. It is the second biggest phylum of the animal kingdom.
- They are mostly aquatic ani mals, mostly marine and a few are fresh water. Some are terrestrial.
- The body is soft and unsegmented.
- The body shows 4 parts. 1) visceral mass 2) the head 3) the foot and 4) mantle. Head is absent in Pelecypoda.
- They are bilaterally symmetrical, but some gastropod are assymmetrical.
- Head bears a terminal mouth, eyes, tentacles and other sensory organs. It is absent in the Pelecypoda and the Scaphopoda.
- The foot is ventral, muscular and locomotory organ. It is absent in the oysters.
- The visceral mass, includes the vital organs of the body.
- The mantle or pallium is muscular fold of the body wall which covers the visceral mass dorsally and laterally.
- The space present between the mantle and the visceral mass is called pallial cavity.
- In the pallial cavity gills and the openings of the digestive, and reproductive systems are present.
- The mantle secretes a hard shell, which covers the body. It may be absent in some molluscs Ex: Doris.
- The body cavity is haemecoel.
- The true coelomic cavities are restricted.
- Open type of circulatory system is seen except cephalopoda. The ciorsai hear t Having one to fcuf auricles and one ventricle, is enclosed within the pen ardial cavity.
- Tht olood is colourless. Respiratory pigment, if present is haemocyanin.
- The alimentary canal is straight, or U-shapedor coiled.
- The buccal cavity contains a raduia, except in bivalve.
- A large digestive gland or hepatopancreas and salivary glands are present.
- The espiratory organs are ctenidia. Pulmonary chamber is developed for their respiration in the terrestrial forms.
- The excretory system contains one to six pairs of kidney.
- The nervous system shows paired cerebral, pleural pedal and visceral ganglia.
- The sexes are usuaUy separate. Few are hermaphrodite.
- Cleavage is spiral, and holoblastic. The development is either direct or indirect. Glochidium larva is seen in Unio. Veliger larva is seen in Gastropods.
Class 3: SCAPHOPODA: These are burrowing molluscs called tusk shells. Body is bilaterally symmetrical. Distinct head is absent. Eyes and gills are absent. They are marine. Shell has single valve and open at both ends. Foot is conical and used for burrowing, Sexes are separate. Ex: Dentalium.