278 CIVIC BIOLOGY 



makes it scarce in the markets except along the Maine coast. This 

 scallop reaches a diameter of 7 inches, and the sexes are distinct, while 

 the southern pecten is hermaphroditic and seldom grows over half this 

 diameter. The life history of P. irradians has been studied carefully 

 and has a direct bearing on its practical utilization. These pectens are 

 spawned in midsummer, grow rapidly, and spawn when a year old. They 

 continue to grow, but rarely survive to spawn a second time, most of 

 them dying in the early spring of their second year. It is thus clear 



FIG. 125. Digging soft clams 

 United States Bureau of Fisheries 



gain to utilize all of these pectens over one year old. This will not 

 cause any decrease in the species if all those under a year old are left 

 on the breeding grounds. 



The fine Pacific clams, the geoduck, or giant clam (Glycimeris gen- 

 erosa), that grows to weigh 6 pounds, the gaper clam (Schizothcerus 

 nuttalli), now becoming rare in the western markets, the western little- 

 neck (Tapes staminea), and the butter clam (Saxidomus nuttalll), along 

 with the western species of Mytilus and the closely similar, and equally 

 edible, Modiolus, are all fine subjects for study in western-coast schools. 

 The Pacific has also two valuable scallops. 



Life history. In general outline, the life histories of all the marine 

 bivalves described above are similar. Eggs are produced by the millions, 

 and hatch within a few hours into free-swimming embryos entirely 



