PELECYPODA 



565 



The siplional muscle impression is present only in shells possessing elon- 

 gated, retractile siphons, and is situated beneath or behind the posterior 

 adductor impression, where it forms a more or less prominent forward 

 bend or angle in the pallial line, which is called the pallial sinus (10). 



The two valves of the shell are closed by the two powerful adductor 

 muscles above mentioned, which are attached to their inner surface; in 

 a large number of pelecypods but one muscle is present, the anterior 

 adductor being w^anting. When the adductors are relaxed the elasticity 

 of the hinge ligament causes the shell to open. Other muscles which 

 are attached to the shell extend the foot and the siphons. 



The mantle cavity contains the ctenidia or gills, which are usually 

 plate-like structures attached along their dorsal margins, two gills lying 

 usually on each side (Fig. 890, C). In the simplest pelecypods (Fig. 

 890, A), however, a pair of bipinnate ctenidia similar to those of gas- 

 tropods is present. At the foi-ward end of the visceral mass in the 

 median line is the mouth 

 (Fig. 740, C). It is with- 

 out jaws or radula, and 

 on each side of it are two 

 ciliated flaps called oral 

 palps (Fig. 904), the cilia 

 of which cause small food 

 particles, more or less 

 immersed in mucus, to 

 pass into the mouth. The 

 anus is at the hinder end and opens into the dorsal or cloacal portion 

 of the mantle cavity. The foot is wanting in some pelecypods, but in 

 most of them it is a wedge or tongue-shaped structure ; the most primi- 

 tive forms have a disc-like sole like the gastropods. Very many pelecy- 

 pods secrete a tough, stringy substance from a gland in the foot called 

 the byssus (Fig. 898, 5), by means of which they attach themselves to 

 stationary objects, becoming thus fixed in one place. In many forms 

 which are without it as adults; the byssus is present in the larva. Many 

 bivalves attach themselves permanently by cementing one of the valves 

 to some other object or by boring cavities into clay, rock, or wood, from 

 which they cannot escape. 



Internal Structure (Fig. 904). — The mouth is without salivary glands 

 and is joined by a short oesophagus with the large stomach. Surround- 

 ing this organ is the voluminous liver. The intestine (Fig. 740, C) is a 

 long tube which winds about in the visceral mass, to the dorsal side of 

 which it finally ascends, where it passes backwards through the ventricle 

 of the heart (except in the oyster, and certain others) to the anus. At 



Fig. 890 — Diagrams of pelecypod gills. A, Proto- 

 hranchiataj B, Filihranchiata ; C, Eulamellihran^ 

 chiata; D, Septihranchiata (Lang). 1, mantle; 

 2, visceral mass ; 3, ascending lamella of gill ; 4,- 

 descending lamella of gill. 



