CLAM AND SCALLOP INDUSTRIES 619 



haps wholly covered, if the sand of the bottom be very fine. When 

 a length of 6 or 7 millimeters is reached, a clam is able to dig below 

 the surface on any bottom, and is able to cover itself with much 

 celerity. 



As soon as burrowing is possible, the young clam remains buried 

 most of the time, reaching up to the water with its siphons, by means 

 of which it obtains its microscopic food (chiefly diatoms) and the 

 oxygen used in breathing, both of which are suspended in the water. 

 A very interesting fact is discovered when these burrowed clams 

 are dug up and examined. It is that here also the byssus fiber is 

 thrown cut and attached firmly to the surrounding pebbles and 

 sand grains, in order that the clam may not be dislodged from its 

 shallow burrow by waves or currents and carried away, or perhaps 

 thrown up on the beach to perish. 



Even in its burrow, the small clam exhibits a strange restlessness. 

 It repeatedly casts off the byssus from its body, digs out to the 

 surface, and creeps away, only to go down and again attach itself. 



As the clam becomes larger, this wandering habit manifests itself 

 less frequently. It asserts itself, however, till the creature is 12 or 

 13 millimeters long (about half an inch). An individual of this 

 length still possesses the byssus, but at about this time digs down 

 to remain permanently, and the byssus gland is absorbed, and never 

 reappears. It is simply an organ of the young, and, while in func- 

 tional activity, performs a very important office, as has been 

 shown. The clam now, except from accident, always remains buried, 

 only reaching up to the surface by its siphons, which soon become 

 long and heavy. If it should be dug up, it is still able to bury itself 

 again, though the foot becomes relatively small. Large clams per- 

 form this act of, burrowing with very great difficulty. 



These, briefly stated, are the chief points of interest in the devel- 

 opmental history of M y a. We may now notice a few facts con- 

 cerning the " set " of young clams. 



The number produced varies greatly in different seasons, The 

 reasons for this are complex and at present not well understood. 

 Whenever a full " set " of clams occurs along the shor.g, thg young 

 are found to occupy certain restricted regipn,s m vast nuy*ibers. Thi§ 



