A FRESH-WATER CLAM 75 



to each other, two short tubular openings, which place these two 

 chambers in communication with the outside water and are 

 called the siphons. The ventral siphon is called the branchial or 

 incurrent siphon; through it water streams into the branchial 

 chamber bearing food and air for respiration. The dorsal siphon 

 is called the excurrent or cloacal siphon, and through it water 

 passes outward charged with fecal matter from the alimentary 

 tract and carbon dioxide of respiration. Note the sensory ten- 

 tacles on the branchial siphon. 



Probe the dorsal siphon. Carefully remove the left lobe of the 

 mantle after cutting it with fine scissors along its line of attach- 

 ment with the visceral mass. 



Through the transparent body wall observe the organs in the 

 dorsal portion of the visceral mass. Just back of the anterior ad- 

 ductor muscle is the liver, which can often be recognized by its 

 greenish color. Between the hinge ligament and the base of the 

 gills Hes the heart in its transparent pericardium, and beneath it 

 is the dark-colored kidney. The rectum may be seen passing 

 through the pericardium and the heart, then extending above the 

 posterior adductor muscle to the cloaca, where it ends with the 

 anus. Cut open the cloacal chamber by a slit in the side of its 

 siphon. Find the hinder end of the rectum and the anus. Note 

 just beneath the muscle a canal which accompanies the base of 

 the gill forward. This is the suprabranchial passage of the outer 

 gill ; it runs posteriorly to the cloacal chamber. Blow into it with 

 a blowpipe, also probe it from behind. 



Exercise 5. Draw a semidiagrammatic view of the animal lying in 

 the right-hand valve of the shell, representing the organs above 

 mentioned. Carefully label all. 



The Respiratory System. The gills have already been noticed. 

 The two gills on each side of the visceral mass are in origin 

 but a single organ, which is called the ctenidium. The clam is 

 thus provided with a single pair of ctenidia, which are homolo- 

 gous to those of the squid and of snails. Each gill consists of a 

 pair of plates, or lamellae, united at their lower edges and open 

 above, and further joined by vertical or dorsoventral cross 



