DISSECTION OF CLAM. 249 



mantle thickened ? Are the mantles of the right and left 

 sides united anywhere along the free margin of the shell ? 



Cut through the mantle near its ventral edge and fold 

 hack. Is it free back to the hinge line ? Cutting through ! 

 the mantle opens the mantle or branchial chamber. In 

 this several structures are to be noticed. Arising from the 

 side of the body are plaited folds (how many ?), the 

 branchiae or gills. Are there branchiae on the right side 

 as well ? Extending downward between the gills is the 

 soft abdomen, terminated at the anterior ventral angle by a 

 more solid foot. In front, just ventral to the anterior 

 adductor, are two pairs of fleshy flaps, the labial palpi, 

 and where they meet at their junction with the body is 

 the mouth. At the posterior end of the animal look for 

 two fleshy tubes (siphons) formed by the edge of the man- 

 tle. * Run a wire in each from the outer end, and see where 

 it appears inside the shell. The ventral siphon is the in- 

 current or branchial siphon; the dorsal is the excurrent or 

 cloacal siphon. Draw the parts so far made out. 



Just beneath and behind the hinge is the heart, its 

 position in the living animal being readily seen by its 

 pulsations. Carefully cut into the chamber in which it is 

 situated and make out a central ventricle, rather dense in 

 texture, and leading to it on either side a delicate tubular 

 auricle which brings the blood from the gills to the 

 ventricle. Notice the intestine passing through the ven- 

 tricle. Just in front of the posterior adductor is the dark 

 organ of Bojanus or kidney. Draw the parts made out. 



The alimentary canal and the nervous system are best 

 followed in specimens which have been in alcohol a few 

 days. In such a specimen insert a probe into the excur- 



* These are small in the fresh-water clams, but are greatly de- 

 veloped, and form the part commonly but erroneously called the 

 "head," in the long clam. 



