REPORT OF COMMISSIONERS OF INLAND FISHERIES. 33 



well developed, and, when watched under the microscope, are seen 

 to crawl about rapidly, by means of the foot, instead of swimming. 

 The swimming organ gradually disappears while the foot grows, and, 

 at a certain stage, they alternately swim and crawl. The foot soon 

 becomes proportionately very large, and it is not impossible that 

 the clams may sometimes use it in swimming, though this has not 

 been observed.* 



The clams often swm about very freely for a week or two after 

 they are captured. Then they lose their swimming organs, the 

 siphon and foot are developed, the shell changes in shape, and the 

 larvae assume the unmistakable characteristics of the species. Up to 

 this time the transparent shell allows the internal organs to be 

 plainly visible. 



The growth at this period is very remarkable. Exactly how fast 

 they grow under the natural conditions i^ not known, but those 

 which were kept in a dish of w^ater, where the supply of food was 

 meagre as compared mth that in the tide water, and where other 

 conditions were necessarily artificial, grew to ten or twelve times 

 their original length in less than two weeks. 



The principal changes which may easily be observed in the clam 

 are the elongation of the shell (the outline at first being very much 

 rounded), the increase in the number of gill plates, and the relative 

 lengthening of the foot. 



THE ATTACHMENT PERIOD. 



The free-swimming period usually ends late in June or early 

 in July. If the eel-grass and sea-weed are carefully examined at this 

 time, large numbers of these little clams may be found hanging to 

 the stalks by the slender gelatinous threads. They attach them- 

 selves also to driftwood, spiles, stones, etc. This thread (the byssus) 



*Tlfe razor clam (Ensatella Americana) has this ability even after it has attained the length 

 of three or four inches, although its foot is aided by forcing backward of water through the 

 mantle opening. On one occasion a large number of specimens about one-quarter of an inch 

 long were caught at the surface. 

 5 



