260 Introduction to Animal Morphology. 



A solid diverticulum passes from the upper surface of 

 the intestine behind the muscle in Pinna (the trachea 

 of Polt). The cloacal may be quite separate from the 

 branchial chamber. The liver is many-lobed, acinose, 

 often mixed with the sex gland, and opening by two, 

 three, four, or many ducts, into the stomach and up- 

 per end of the intestine. 



The foot or ventral surface of the body is usually 

 laterally compressed, sharp-edged, never trifid, nor 

 provided with an epipodium ; it may be straight (cor- 

 mopoda) or bent on itself (pelecypoda), ploughshare, 

 or club-like, or rudimental, in sessile forms (Ostrea, 

 Anomia),* ormay be provided with a gland secreting a 

 silky chitinoid material (byssus), whereby the animal 

 is anchor 



There are two pair of retractor muscles for the foot (one 

 pair in Dreissensia), the anterior arising from the inner sur- 

 face of the shell close to the umbo, the posterior brim 

 the attachment of the hinder adductor. (An additional pair 

 of retractors, behind the anterior adductor, exist in some 

 Unionidx.) There is one protractor of the foot arising be- 

 hind the anterior adductor and passing in front of the foot. 

 The muscles are smooth and longitudinally striped. The 

 base of the foot receives the liver, digestive canal, and sex 

 gland.f In boring forms it is cylindrical, and has siliceous 

 particles in its dermis. The foot may be pierced by an open- 

 ing (Unio), leading into a long canal which ends in lacunae ; 

 from these the water flows when the foot is suddenly retracted. 

 Galeomma has a double canal ; Cyclas has many sieve-like 

 openings ; Mactra one large aperture. :f The byssus gland is 



* u^Etheria, the freshwater oyster of the African rivers, has a large foot. 



t Cryptodon has a long cylindrical tentacle-like foot, with no viscera 

 extending into it, and the liver and sex organs in branched processes lying 

 between the gills. The foot is pillar-like in Area. 



J Openings are found in Solen, Cardium, and many others. 



