VENUS MERCENARIA LINN^US 211 



In some lamellibranchs they attain a comparatively enormous 

 length, while in others they are quite absent. The dorsal 

 siphon is called the cloacal or excurrent siphon, the ventral the 

 branchial or incurrent siphon. These are the only channels 

 through which an exchange of water takes place. By the action 

 of the ciliated epithelium of the gills and of the mantle water 

 is drawn in through the branchial and expelled through the 

 cloacal siphon. 



Inclosed in the mantle cavity is the body proper, together 

 with all its organs and the gills. The latter, together with the 

 osphradium, oral papillae and external openings of various or- 

 gans, are often united under the name of the pallial organ com- 

 plex. The lower extremity of the body forms a muscular foot 

 which is laterally so strongly compressed that it has been likened 

 to the keel of a boat. The foot is the chief organ of locomotion. 

 The body cavity or ccelome is reduced to the pericardium. 



Digestive system. The mouth is situated at the anterior end 

 of the body above the anterior retractor of the foot. It is pro- 

 vided with an upper and an under lip, each formed by the fusion 

 of two oral lappets or labial palps, of triangular shape. A 

 pharynx is lacking and with it naturally is lacking the grating 

 plate or radula so characteristic of other classes of molluscs. 

 Salivary glands are also absent. The mouth leads directly into 

 the oesophagus, which is followed by the stomach. A large liver 

 opens into the stomach near its anterior end. The stomach is a 

 rather thick tube running obliquely backward and downward and 

 has at its pyloric end two openings. The one leads into the intes- 

 tine, the other into the crystal rod-sheath, a blind diverticle of the 

 stomach secreting a crystal rod of unknown function. The 

 intestine is a coiled tube lying between the pyloric end of the 

 stomach and the liver, under the stomach. The rectum is a very 

 long and thin tube. At first it runs backward to the right of the 

 stomach and at an almost right angle to it. It rises then at 

 more than a right angle until it reaches the pericard. Here it 

 turns again backward, perforates the ventricle of the heart and 



