XXI. MOLLUSKS 



MoUusca. — The name Mollusca (Latin mollis = soft) gives 

 the character which chiefly aids us in identifying a mollusk. 

 The body is soft and unsegmented. It is usually covered with a 

 limy shell, formed by the agency of a delicate envelope called the 

 mantle. The animal usually possesses a single muscular foot, by 

 means of which locomotion takes place. There are several groups 

 of mollusks which are, as we shall see, quite unlike in appearance 

 and in habits. 



The Shell of the Fresh-water Mussel (Unio species) } — Notice that the 

 shell is made up of two parts or valves. Such a shell is called a bivalve. 

 Notice that the valves are joined together by a structure, somewhat elastic, 

 called the hinge ligament. Close the two shells; why do the shells spring 

 open again? The lines which run more or less parallel to the edge of the 

 shell are called lines of growth. If a line of growth once represented the 

 outer edge of the shell, then 

 find the oldest part of the 

 shell. This raised area is 

 called the umbo. It is always 

 possible to locate the anterior 

 end of the shell because the 

 umbo points toward that end. 

 The hinge ligament marks the 

 dorsal side of the animal. 



The shell is covered on the 

 outside by a thin layer of 

 horny material. This is called 

 the periostracum. Can you 

 explain why it does not 

 cover the entire shell? The 

 shell proper, if tested with acid, will be found to contain considerable 

 lime. 



Draw, natural size, a single valve, and locate the hinge ligament, umbo, 

 and lines of growth. Place the dorsal surface upmost in the drawing. 



Mussels may be opened by first placing the living animal in hot water 

 until the shell gapes. Then insert a knife, keeping the blade close to the 

 inner surface of one valve, cut through the tough muscles which hold the 

 shells together, and the shells will open. Notice the mother of pearl cover- 

 ing the inner surface. Notice in a freshly opened clam that a delicate 

 membrane, the mantle, adheres to the shell. 



Structure of Shell. — The shell, if examined in cross section with a good 

 lens, is seen to be made up of three layers : the outer periostracum, made 



* See Hunter and Valentine, Manual, page 138, for exercise on Venus Mercenaria. 



259 



Shell of fresh-water clam, the left half polished to 

 show the prismatic layer. 



