THE CLAM. 99 



valve. If properly done, this will leave the animal covered 

 with a thin membrane, the mantle. Projecting through 

 this, near the dorsal line, are the adductor muscles which 

 keep the shell closed, and which were cut in removing the 

 valve. According to their position, these are known as 

 the anterior and posterior adductors. Are the edges of the 

 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 

 back. 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 branchial, 

 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 mantle.* Run a 

 wire in each from the outer end and see where it appears 

 inside the shell. The ventral siphon is the incurrent 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 (pericardium) 

 in which it is situated and make out a central ventricle, 

 rather dense in texture, and leading to it on either side a 



* 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 lono; clam. 



