Introduction to Animal Morphology. 259 



nal retracting fibres over the wall of both tubes. The 

 distal end of the siphon is often fringed by sensitive 

 processes (siphonal tentacles, as in My a). 



The mouth is directed forwards, and bordered by 

 two or more ciliated, sensitive, labial tentacles,* like 

 the base of the arms in Brachiopoda. There is no 

 odontophore, nor salivary gland (Teredo has two irre- 

 gularly-lobed, oral glands with a common duct). The 

 oesophagus is short (long in Teredo) ; the stomach 

 globular or oval, and the intestine has one, neural, or 

 2-12 flexuresf (most numerous in Cardium). On the 

 right of the stomach there opens a csecal pouch im- 

 bedded in the liver, or between it and the genital 

 gland, containing a curved crystal style, or transparent 

 rod, rounded at one end, which is often attached by a 

 triangular process, flattened or pointed at its free end, 

 and made of tesselated epithelium, sometimes lamel- 

 lar, or with calcareous particles in it : this is irregu- 

 larly periodic in development, being largest after the 

 winter. Its caecal pouch has a valvular opening, and 

 rarely is rudimental, the style then lying in the intes- 

 tinal tract ; it is only present in Anomia among the 

 Monomya, absent in Ostrea, &c., and is sometimes 

 absent in Cyclas, Pisidium, and Galeomma. It may 

 act as a gizzard. The intestine having traversed the 

 liver, next pierces through the ventricle of the heart 

 (except in Ostrea, Anomia, Teredo) ; then, passing 

 over the main (posterior) adductor, ends in the cloaca. 



* Small in Lucina, Corbis, &c. 



t Unio has three concentric coils, of which the first is marked by a pair 

 of longitudinal ridges on its mucous membrane ; the second, which pro- 

 jects into the foot, by transverse folds, and the third by longitudinal 

 ridges. 



S 2 



