34 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



is elastic and yet very tough, like that of the mussel, and arises from 

 the byssus gland which is situated on the ventral side of the foot 

 near the extreme posterior end. It is an entirely homogeneous 

 structure, and is usually single throughout its entire length, but 

 sometimes near the end it divides into two or more branches. The 

 clam has the ability to free itself from attachment and to form a 

 new byssus at will. 



The clam at this period ma}' be recognized readily by its shape, 

 yet it differs in the following manner from the adult. The outline 

 is more rounded, and the umbones are more prominent and farther 

 apart. The foot is very long and capable of quick movements, and 

 extends over the entire surface ventral to the visceral mass, while 

 in the adult clam it is hatchet-like and protrudes only from 

 the extreme anterior end of the visceral mass. The siphon (snout), 

 while quite like that of the adult in shape, is relatively much 

 smaller, and consequently the shell does not gape open so much pos- 

 teriorly. The siphon, like the foot, is more easily and rapidly 

 protracted and retracted than in the adult. 



The growth of the clam is still very rapid, and it is during this 

 period that it assumes the shape of the adult. One remarkable 

 fact in this change is the double shiftng of the position of the um- 

 bones, or hinge prominences. It will be recalled that, in the adult 

 clam, the position of the umbones is nearly central. This is also the 

 case with the very small clam, but, through an increased growth 

 posteriorly, the umbones become shifted more and more an- 

 teriorly till the clam is about 6 mm. long; then, by increased 

 anterior growth, the umbones again gradually assume a central 

 position. 



The duration of this attachment period seems to depend, to a cer- 

 tain degree, on the environment. Certainly it is not dependent en- 

 tirely on size, for frequently clams measuring half an inch in 

 length are found attached. The great majority, however, reacji the 

 ground and burrow long before attaining this size. The byssus is 

 retained some time after the clam has left its place of attachment 



