Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.9. Body Cavity
The general body cavity is a haemocoel filled with blood. The true coelom is schizocoelic and greatly obliterated by the connective tissue, unstriped muscle fibres and blood sinuses. It is represented only by three small cavities—(l) a single ovoidal chamber, the verlcardium, which lies dorsally, containing the heart and a part of the intestine, and lined by the coetomic epithelium; (2) the gonocoels or the cavities of the gonads; and (3) the urocoels or the cavities of the excretory organs.
Musculature
The muscles are mainly of the slow contracting, unstriped type and arranged in distinct bands or sheets. The two shell valves are closed by the contraction of two large, strong, cylindrical transverse muscles, situated one close to either end dorsally and passing across the body from one valve to another. They are called anterior and posterior adductor muscles. When these muscles relax, the elastics hinge ligament opens the valves. Near these muscles, are two smaller muscles, the anterior and posterior retractor muscles, which run from the foot to the shell and serve to withdraw the foot, during locomotion. A small protractor muscle, close behind the anterior adductor, serves to compress the visceral mass, thus causing the protru¬sion of the foot. The complex intrinsic muscle of the foot also serves as a protractor of that organ. The delicate palliàl muscles, inserted upon the shell all along the pallial line, serve to retract the edge of the mantle.