Anatomy of finfish and shellfish (2+1)

Unit 13 - Mollusca

13.1. General characters

• Molluscs are essentially aquatic, mostly marine, few freshwater and some terrestrial forms.
• The body is soft, unsegmented, bilaterally symmetrical and consists of head, foot, mantle and visceral mass.
• The body is clothed with a one layered often ciliated epidermis.
• Body is commonly protected by an exoskeleton calcareous shell of one or more pieces, secreted by the mantle.
• Head is distinct, bearing the mouth and provided with eyes, tentacles and other sense-organs excel: the Pelecypoda and Scaphopoda.
• Ventral body wall is modified into a muscular flat or plough-like surface, the foot which is variously modified for creeping, burrowing and swimming.
• Mantle or pallium is a fold of body wall that leaves between itself and the main body mass, the mantle cavity.
• Visceral mass contains the vital organs of the body in a compact form taking the form of a dorsal hump or dome.
• Body cavity is haemocoel. The true coelom is generally limited to the pericardial cavity and the lumen of the gonads and nephritic.
• Digestive tract is simple with an anterior mouth and posterior anus but in gastropods, scaphopods , and cephalopods the intestine becomes U-shaped brin
• ging the anus to an anterior position.
• Pharynx contains a rasping organ the radula except in Pelecypoda.
• Circulatory system is open except in cephalopods which show some tendency towards a closed system.
• Respiratory system consists of numerous gills or ctenidia usually provided with osphradium at the base. Lung is developed in terrestrial forms.
• Excretory system consists of a pair of metanephridia which are true coelomoducts and communicate from pericardial cavity to the exterior by nephridiopore.
• Nervous system consists of paired cerebral, pleural, pedal and visceral ganglia joined by longitudinal and transverse connectives and nerves.
• Sexes usually separate (dioecious) but some are hermaphroditic. Fertilisation is external or internal.
• Development is either direct or with metamorphosis through the trochophore stage called veliger larva.