FRESH-WATER MUSSEL. 



133 



intestine, may be seen just behind the spot where the ascending segment 

 of the intestine bends backwards. The lower wall of the ventricle is 

 particularly evident, and there is a well-marked space (ventricular cavity) 

 between it and the intestine. 



A portion of the pericardial space is visible as a triangular cavity 

 ventrally to, i. e. to the right in this preparation of, the ventricle. Still 

 more to the right is the organ of Bojanus or nephridium. The two 

 parts of its cavity may be seen in section. Next to the pericardium 

 is the non-glandular thin-walled duct, the walls of which are more closely 

 apposed than in nature, leaving a mere chink. The glandular portion 

 of the organ with its lamellae extends far forwards ; it surrounds the 

 tendon of the posterior retractor pedis muscle, which is seen just in front 

 of the posterior adductor muscle, and reaches to the anterior edge of this 

 muscle and even a little to its ventral surface. Behind the posterior 

 adductor the attachment of the left gill to the mantle is displayed. It 

 corresponds to the junction of the tentaculate and non-tentaculate portions 

 of the mantle which constitute the inferior or inhalent, and the superior or 

 exhalent, siphonal notches respectively. 



The muscular portion of the foot is well seen in partial section. In 

 locomotion it swells up and protrudes from the shell into the soft mud 

 of the streams and ponds inhabited by these creatures. The rate of motion 

 is slow, and the animals, as they move along, leave a deep furrow behind 

 them. 



The two adductor muscles, as seen in this preparation, are equal in size; 

 hence the name Isomya applied to the great group among Lamellibranchiata to 

 which Anodonta belongs. In others, e.g. the freshwater Dreissena, the anterior ad- 

 ductor is small compared to the posterior, hence Heteromya ; and in a third order, 

 the Monomya^ to which the Oyster and Scallop belong, the anterior adductor is absent 

 altogether, though it is present in the Oyster when the shell and muscles first 

 develope. The anterior adductor is formed in the mantle region above, and in 

 front of the mouth. The posterior adductor may perhaps correspond to the 

 columellar muscles of the Snail. 



The crystalline style occurs among Monomya only in Anomia, but is found 

 in most other Lamellibranchiata ', either in a special caecum or in the intestinal 

 tract, as in the Unionacea. It is transparent in most instances. Hazay has 

 recently made some researches on its formation and function in the Unionacea. 

 He finds that from spring to autumn the stomach is full of a gelatinous mass 

 in which the crystalline style is slowly differentiated, a process complete by 

 October. The remaining jelly, apparently superfluous food-material, passes into 

 the first section of the intestine, and by November the stomach is empty of it. In 

 the intestine the jelly becomes a thick, compact hyaline body, which is gradually con- 

 sumed during the winter months. The style appears to act as a stopper, closing the 

 entrance of the pylorus, but it is itself gradually consumed ; any remains serve 



