THE FRESH-WATER CLAM. 51 



separate channel above the gills, into which the 

 upper siphon leads, while the lower siphon leads to 

 the lower cavity, outside of and below the gills. 

 9. With the thumb and all the fingers of the left hand 

 seize the left lobe of the mantle and pull it toward 

 the ventral margin, thus drawing the body away from 

 the dorsal margin. Just under the hinge a pale organ 

 may be seen, pulsating every few seconds ; this is the 

 heart. 



10. Holding the mantle stretched, again examine the 

 upper siphonal opening ; probe to see how it extends 

 forward above the united hinder portion of the gills. 

 In the upper part of this cavity find a tube running 

 back over the posterior adductor muscle, and ending 

 in a conical elevation ; this tube is the intestine, and 

 the opening at its end is the anus ; hence the siphon 

 leading from this cavity is called the anal siphon; the 

 lower siphon, which conducts water to the gills and 

 mouth, is called the branchial siphon or gill siphon. 

 Examine the gills from above, i.e., examine their 

 dorsal margins; observe that the two outer walls of 

 each gill are a short distance apart at this edge, while 

 below these walls unite, so that if the gill be cut 

 across, these walls, as seen at the cut, are like the 

 letter V. These diverging walls are connected by 

 cross partitions, thus forming a series of compart- 

 ments within the gill, whereas if these partitions 

 were absent, each gill would be a deep, narrow, undi- 

 vided trough. The lateral walls of the gills are sieve- 

 like, and the surface of the gill and the edges of the 

 holes are covered with cilia. The vibrations of these 

 cilia drive the water which is around the gill through 



