Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.6.Mantle cavity
The space enclosed between the two lobes of the mantle is the mantle or pallial cavity. On removing one mantle lobe, the mantle cavity and its organs (visceral mass, gills, foot, etc) are exposed.
Visceral mass
The soft body or visceral mass occupies the dorsal parts of the mantle cavity. It is dark in colour and contains various organs including the digestive, circulatory, excretory, and reproductive systems. In a freshly-killed animal, a greenish-brown digestive gland is visible in the antero-dorsal region, a pericardial cavity containing the heart in the mid-dorsal region and dark-coloured paired kidneys below the pericardium.
Head
The freshwater mussel lacks a distinct head as it would not be of much use to an animal that lives with its anterior end buried in mud. The eyes and tentacles are absent. The large mouth opens beneath the anterior adductor muscle, bordered by a pair of broad, lamellar labial palps on each side, while the anus' lies above the posterior adductor muscle.
Gills
A pair of long, double plate-like gills hangs freely in the mantle cavity, one on each side from the visceral mass. The gills have a sieve-like structure, perforated by minute pores and covered by cilia. Their line of attachment to the visceral mass forms a continuous horizontal partition, dividing the mantle cavity into a large ventral infra-branchial chamber, and a small dorsal supra-branchial chamber.
Foot or podium
The antero-ventral hatchet-like prolon¬gation of the visceral mass, hanging down in the mantle cavity forms a large, muscular, extensile foot or podium which is adapted for burrowing. It is laterally compressed and terminates below into an elongated keel. The thick basal part of the foot contains a portion of alimenty canal, the digestive gland and the gonad. The foot can be extended by blood pressure and by the muscular action of a pair of pedal pro¬tractor muscles. It can be withdrawn into the shell by the action of anterior and posterior retractor muscles.