128 GENERAL BIOLOGY 



(c) Foot. This is the large dense median part of the body; 

 it forms a muscular wedge or keel by means of which the 

 animal moves. Above the foot is the softer visceral mass 

 of the body. 



(a) Muscles. The adductor muscles, already noted, close 

 and hold the shell shut. There are protractor muscles for 

 pulling the foot and body ventrally, and extending it from 

 the shell. The retractor muscles draw these same parts back 

 into the shell. Note their position and the manner in which 

 they work. (In Venus there is no protractor muscle.) 



(e) Labial Palps. Small, thin, leaf-like organs behind 

 the anterior adductor muscle. They aid in passing the 

 food to the mouth. 



(/) Mouth. Between the palps and below the anterior 

 adductor muscle. 



(</) Anus. Opens into the excurrent or cloacal chamber 

 it will be found against the posterior border of the posterior 

 adductor muscle. 



Make a drawing with the mantle removed. 



II. Internal Anatomy. 



1. Circulatory System. The heart lies dorsally between the 

 ligament and the bases of the gills in an oval sac, the pericardial 

 cavity. Dissect the pericardium from the dorsal side and 

 expose the cavity and the heart. The latter is made up 

 of a central muscular portion, the ventricle, and two tri- 

 angular lateral portions, the auricles. Compare the auricles 

 and ventricles in thickness of walls. If a live clam is at 

 hand observe the pulsations of the heart. The ventricle 

 surrounds the posterior portion of the intestine. There are 

 two arteries leaving the ventricle, the anterior aorta and 



