VENUS MERCENARIA. 113 



nal, 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 composed 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 at- 

 tached. Does the attachment bear any relation to the pallial 

 sinus in Venus? 



Visceral Mass and Foot. These portions form the large 

 median mass. The viscera are contained in the dorsal portion. 



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 an- 

 terior and the posterior foot muscles, may be seen protruding 

 through the lobe of the mantle. 1 They correspond in position 

 to the scars on the shell. 



1 The anterior foot muscles are sometimes called protractors, and the 

 posterior foot muscles retractors. Both are actually used to retract the foot. 

 The greatly retracted foot may be pulled slightly forward by the anterior 

 muscles, but the mechanism of protruding the foot is very different. 



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