Pelecypoda. 



121 



burrow. The internal structure is essentially the same as in 

 the fresh-water clam. To accommodate the long siphon 

 tube when it is retracted, there is a deep indentation of the 

 mantle line in the posterior region. The shell of My a can- 

 not be snugly closed, there being a. gap both anteriorly and 

 posteriorly. Probably this may be accounted for by the 

 more protected position and the need of having the siphon 



FIG. 75. HARD CLAM; ROUND CLAM; QUAHOG. 



With foot, siphons, and edge of mantle extended. 



tube extended most of the time. The siphon tube, with its 

 black tip, is commonly called the " head," but this clam is 

 as headless as its fresh-water relative. % 



The Hard Clam. The hard clam, or quahog, is also an 

 important sea-coast food, especially where the soft clam 

 is not obtainable. It is oval, with a thick shell. It bur- 

 rows but a short distance, hence the siphons are not long, 

 and the two tubes are partly separated. The foot is well 

 developed, and the clam crawls more or less like the fresh- 



