32 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



THE FREE SWIMMING PERIOD. 



The eggs of the clam, after being fertihzed in the water, develop 

 into free-swimming larvae which are carried by the tides and scat- 

 tered in all directions. In this stage they are barely visible to 

 the naked eye. When examined under the microscope they are seen 

 to resemble the adult clams in the possession of a pair of hinged 

 shells, but differ widely in shape and structure. This is very un- 

 fortunate for the investigator, because the oyster, quahaug, scallop, 

 and other bivalves breed about the same period, and these also have 

 free-swimming larvae, which in many cases closely resemble those of 

 the clam. A close study of these forms, however, enables one to 

 distinguish by one means or another, the oyster and the scallop from 

 the clam, and doubtless sufficient investigation would enable one 

 to identify, all the forms. For positive identification certain of 

 these free-swimming larvae, which were supposed to be clams, were 

 kept under observation until they lost their swimming organs and 

 took on the form which could be recognized readily. 



In collecting the clams of this stage a silk bolting-cloth tow-net, 

 which has an extremely fine mesh, is used, and from the first of June 

 until the middle of July these minute clams can be obtained every day 

 by dragging the net along the surface of the water. In order to 

 separate them from the many other small organisms, the dish of sea 

 water into which the contents of the net have been emptied is gently 

 tapped. The clams immediately close their shells and sink to the 

 bottom. The top water is then drawn off, some clear sea water is 

 added, and, whenever the clams are desired, they can be precipitated 

 to the bottom by tapping the dish, and then transferred by means 

 of a pipette to a watch glass under the microscope. As one watches 

 these specimens under the microscope, very frequently he will see one 

 of them open its shell, thrust out and expand a circular swimming 

 organ (velum), and go whirling about through the water; then sud- 

 denly draw in the velum, close the shell, and fall to the bottom. 



Some specimens larger than the average, have the foot and siphon 



