74 PHYLUM MOLLUSCA 



Exercise 3. Draw a view of the inner surface of the shell. 



Break the shell and examine the broken edge with a hand lens. 

 Study the structure of the shell. It is composed of three layers : 

 the inner mother-of-pearl layer (which is secreted by the entire 

 surface of the mantle) , the prismatic layer, and the organic layer 

 or periostracum on the outside. The two latter layers are secreted 

 by the edge of the mantle ; the periostracum is very thin and easily 

 peeled off. Place a piece of the shell in a solution of hydro- 

 chloric acid ; note the effervescence which results ; note also that 

 an organic remnant, even of the two inner layers, is left. 



Exercise 4. Draw a view of the broken edge of the shell on a scale 

 of 5. Show the prisms of the prismatic layer. 



Place the animal in water and study it as it lies in the right 

 shell.^ The two halves of the mantle will be seen to envelop en- 

 tirely the visceral mass and the foot. Over the entire dorsal por- 

 tion of the visceral mass the mantle is fused with it and cannot 

 be separated, but the lateral and ventral portions of the mantle 

 lobes hang free, inclosing an extensive space, which is called the 

 mantle cavity. In it, on each side of the visceral mass, lie the two 

 leaflike gills. In front of the gills are two pairs of triangular 

 flaps, the oral palps, between which, in the median line and just 

 back of the anterior adductor muscle, lies the mouth. Find it. 



Trace the irregular Hne of attachment of the mantle with the 

 visceral mass ; it follows the base of the gills and of the oral palps 

 and passes beneath both adductor muscles. Observe the edges 

 of the mantle and note that at the hinder end of the animal they 

 are darkly pigmented, and the middle point of the pigmented 

 line is joined with the base of the gills by a short septum. This 

 septum divides the posterior portion of the mantle cavity into 

 a dorsal and a ventral chamber. The latter is the very large 

 branchial chamber which contains the gills; the former is the 

 very small cloacal chamber. Identify these chambers. The pig- 

 mented edges of the mantle are at this place modified so as to 

 form, when the edges of the two sides of the mantle are applied 



1 For the study of the soft parts of the clam it is well to have at hand also a 

 specimen which has been deprived of both valves of the shell. 



