244 



COLLEGE ZOOLOGY 



4. 



External Features. - - Mussels usually lie almost entirely 

 buried in the muddy or sandy bottom of lakes or streams. They 

 burrow and move from place to place by means of the foot (Fig. 

 *73> 9\ which can be extended from the anterior end of the 

 shell. Water loaded with oxygen and food material is drawn 

 in through a slit-like opening at the posterior end, called the 



ventral sip/ion (8), 

 and excretory sub- 

 stances and faeces 

 along with deoxy- 

 genated water are 

 carried out through 

 a smaller dorsal 

 siphon (7). 



THE SHELL. The 

 shell consists of two 

 parts, called valves 

 (Fig. 173), which are 

 fastened together at 

 the dorsal surface by 

 an elastic ligamen- 

 tous hinge. In Unio 

 the valves articulate 

 with each other by 

 means of projections 

 called teeth, but 

 these are almost entirely atrophied in Anodonta. A number of 

 concentric ridges appear on the outside of each valve; these are 

 called lines of growth (Fig. 173, 10), and, as the name implies, rep- 

 resent the intervals of rest between successive periods of growth. 

 The small area situated dorsally toward the anterior end is 

 called the umbo (6) ; this is the part of the shell with which the 

 animal was provided at the beginning of its adult stage. The 

 umbo is usually eroded by the carbonic acid in the water. 

 The structure of the shell is easily determined. There are three 



FIG. 173. External features of a clam, Anodonta 

 mutabilis. Behind is the inner face of an empty 

 shell. /, points of insertion of anterior protractor 

 (above) and retractor muscles (below) of the shell; 

 2, of anterior adductor muscle; j, of posterior pro- 

 tractor of the shell; 4, of posterior adductor muscle; 



5, lines formed by successive attachment of mantle; 



6, umbo; 7, dorsal siphon; 8, ventral siphon; Q, foot 

 protruded; 10, lines of growth. (From Shipley and 

 MacBride.) 



