126 MOLLUSCA 



sinus. (Not true for Unio.) What is the meaning of this 

 sinus? 



4. Along the dorsal margin of the valve notice promi- 

 nences, the teeth. There are two kinds of teeth. The an- 

 terior, cardinal, consist of short elevations. The posterior, 

 lateral, are not very prominent, but are comparatively long 

 and extend along the dorsal margin. Notice that the teeth 

 on the two valves interlock. What is their function? 



Draw a valve as seen from the inside. 



5. By examining the inside of a shell of Unio or Mytilus 

 near its margin, the typical three layers of which it is com- 

 posed can be seen. How is it possible for all three layers to 

 be secreted by the mantle, which lines the inside of the shell? 

 Can you find any reason for more than one layer? 



Mantle. — This consists of a dorsal covering and two lateral 

 lobes (one of which is applied to the inner surface of each 

 valve of the shell). 



1. The free border of each lobe is thickened, and contains 

 muscles that were attached to the shell along the pallial line. 

 What function do these muscles perform? 



2. The posterior portions of the lobes of the mantle are 

 thickened and united to each other so as to form two tubes 

 (in Unio the ventral tube is formed by contact only), the 

 siphons, through which water passes into and out of the shell. 



3. See how the muscles of the siphons are arranged and 

 attached. Does the attachment bear any relation to the pal- 

 lia! sinus in Venus? 



Visceral Mass and Foot. — These portions form the large 

 median mass. The viscera are contained in the dorsal por- 

 tion. 



1. The ventral portion is hard and muscular, and forms 

 the foot. 



2. Besides the crossing muscle fibers of which the foot is 

 largely composed, it is supplied with two pairs of muscles 

 that are attached to the shell. The cut ends of these muscles, 

 the anterior and the posterior foot muscles, may be seen pro- 



