VENUS MERCENARIA. 93 



applied to the inner surface of each valve of the shell) that are 

 united dorsally. 



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 w^ater 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 



smus 



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. They correspond in position 

 to the scars on the shell. 



Do you understand by what means the foot is protruded? 



Gills. — These consist of two pairs of thin, striated, some- 

 what brownish organs, a pair lying on each side of the visceral 

 mass, between it and the lobes of the mantle. 



1. Each gill extends from the wall that separates the two 

 siphons, anteriorly and dorsally to a point nearly opposite the 

 beaks of the shell, and is attached by its dorsal margin only. 



2. Each outer gill is attached along its dorsal border to the 

 corresponding mantle lobe on the outer side. The inner gills, 

 besides being attached to the dorsal margins of the outer gills, 

 are on their inner sides attached to each other and to the vis- 

 ceral mass. (For some distance the inner side of the inner gill 

 lies against the visceral mass, but is not attached to it.) 



By this arrangement the space between the lobes of the 



