CLAM AND SCALLOP INDUSTRIES 617 



The breeding season extends through the months of May, June 

 and July, though in this last month comparatively few young are 

 produced. The sexes are separate, and the ova, or egg cells of the 

 female, as well as the spermatozoa, or sexual cells of the male, are 

 extruded directly into the water. Here a single spermatozoon 

 unites with an ovum, the two becoming a single new cell. This cell 

 multiplies and eventually builds the body of an embryo. By means 

 of delicate, hairlike projections from the body, which lash very 

 r?vpidly in a definite direction, the creature is enabled to swim. The 

 details of these early stages in the clam are not known. 



We do know, however, that eventually the swimming embryo 

 develops a minute shell similar to that in the adult, ' in that it is 

 made up of two pieces covering the right and left sides of the body. 



During- the continuance of thg swimming period the creature 

 may have been carried, not only by its own efiforts in swimming, but 

 also by tide currents, to some point far removed from that where 

 its life began. A single pair of clams may give rise to millions of 

 embryos during one season. The great majority of these are lost, 

 but a few, losing their swimming organs, happen to settle in some 

 locality which is favorable for their future growth and development. 



From this time, their history has been followed in some detail. 

 The small clams, when they cease swimming, are still minute in 

 size. Many individuals only .4 of a millimeter in length have been 

 observed, and their bodies are so small as to be indistinguishable 

 to the unaided eye from the smallest grains of sand on which they 

 may have fallen. 



In its general outline, this small clam is very different from the 

 adult, in that its body is very much rounded, instead of being elon- 

 gated (fig. i). As it grows, however, its shell elongates, but, at the 

 same time, the two prominent points of the shell, or umbones, are 

 shifted relatively farther forward, as in fig. 2; and then, eventually, 

 they come to lie nearly in the middle of the shell on its upper side, 

 as in the adult clam. 



A glance at fig. i, drawn from an individual .4 of a millimeter in 

 length, will show two conspicuous structures projecting from the 

 margins of the shell. One of these (s) is the pair of filmy siphons — 



