140 MANUAL OF ANIMAL BIOLOGY 



valves, in distinction to other types of Molluscs, like the Snails, 

 in which the shell is single and which, therefore, are termed uni- 

 valves. 



In the Clam, the two valves are hinged together along one edge 

 which is regarded as the dorsal surface of the animal. Definite 

 lines of growth in the form of concentric ridges can be noted on 

 the surface of the shell. These lines radiate from an elevated por- 

 tion, or beak, of each valve, known as the umbo, which typically 

 points toward the anterior end of the animal. From these lines of 

 growth the age of the animal at various stages during its develop- 

 ment can be definitely determined. Holding the Clam with the 

 hinged, or dorsal, side up and with the umbo, or anterior, end 

 pointing away, we are able definitely to orient the animal and 

 designate the right and left valves. 



The two valves of the shell are connected inside by two large 

 internal muscles, one of which is situated near the anterior end 

 (anterior adductor muscle) and the other in the same relative 

 position near the posterior end (posterior adductor muscle). 

 These two muscles are very strong and are attached directly to the 

 two valves, so that when the muscles are contracted the valves are 

 drawn tightly together. In opening a Clam, the point of a knife 

 blade can be inserted between the valves at some point along the 

 ventral edge, and the adductor muscles cut. When these muscles 

 have been cut the valves will spring apart somewhat, owing to the 

 action of certain ligaments in the hinges. (W. f. 49.) 



B. Organ Systems 



Both valves of the shell are lined, except at the extreme 

 edges, by a thin secreting tissue, known as the mantle. With 

 care the mantle can be separated from the inside of the shell, 

 detached along the dorsal surface, and thus all the internal organs 

 of the Clam, enclosed within the two folds of the mantle, can be 

 removed from the shell. Or, stated in another way, the internal 

 organs of the Clam may be said to be enclosed by, first, a heavy 

 external shell and, second, a thin internal layer of living tissue, 

 the mantle, which lies just within and attached to the shell. The 

 cells of the mantle tissue form the shell as a secretion, and the 

 concentric lines of growth indicate the successive positions of the 

 secreting surface of the mantle. 



The internal structure of the Clam can be studied to advantage 



