642 EVOLUTION OF ANIMALS Part V 



snaps its shells permanently into the skin of the fish and is gradually enclosed 

 in a fleshy case. Through the next weeks or months the glochidium is a parasite 

 receiving nourishment and protection from the fish. Finally it breaks out of the 

 case and falls to the bottom, now formed like its parents but still small. During 

 its fife in the fish, it may have traveled many miles; after that it becomes inde- 

 pendent and for a time at least a local resident. 



Other Bivalves 



The bivalves are all aquatic, mainly dwellers on the bottom, most of them 

 marine, and commonest between the tide Hnes. Among the rare climbing ones 

 are the little fingernail clams (Family Sphaeridae), many of them less than half 

 an inch long. A fingernail clam forages over the bottom of ponds. It also 

 curves its supple foot around the stems of water weeds like a pole climber with 

 one leg. Meanwhile, its split siphon is extended and apparently it is drawing 

 in some of the minute organisms which it must disturb as it climbs. 



The razor-shells (Ensis), 4 to 7 inches long, are both agile and strong 

 burrowers that can outspeed a human shoveler. They also jump with a steel- 

 spring action of the foot. The common scallop {Pecten irradians) is another 

 lively bivalve that makes zigzag jumps by opening and forcibly closing its 

 valves (Fig. 31.9). One clap expels the water from the mantle cavity and drives 

 the scallop, hinge first, a yard or more in a straight line through the water — 

 sometimes out of it like a flying fish. Another clap drives it in a different direc- 

 tion. It is as difficult to catch as a clothes moth when it performs the familiar 

 zigzag trick in the air. The scallop closes its valves by its one powerful ad- 

 ductor or cross muscle, and the springy hinge-ligament opens them. The adduc- 

 tor muscles are the tasty fried scallops. Tons of scallops are harvested annually 

 along the Atlantic Coast and only one muscle from each animal is used. Deep 

 sea scallops {Pecten grandis), five inches or more wide, are most abundant 

 off the coast of Maine and most expensive in restaurants. 



Oysters undergo rhythmical changes of sex during the individual's life- 

 time. There are two similar types of these changes; one type occurs in the 

 European oyster {Ostrea edulis), and in the Pacific oyster (O. lurida), a 

 species native to Japan; the second type occurs in the American oyster, Cras- 

 sostrea virginica (formerly Ostrea virginica), and others. In the American 

 oyster, the majority of the young are males and during the first spawning 

 season they function as males and produce sperm cells that are extruded into 

 the water. Before they become sexually mature however, these young oysters 

 may present all gradations from true males in which there are developing 

 sperm cells to other individuals that have complete ovaries. After the second 

 spawning season, the number of individuals of each sex is almost equal. The 

 adults usually function permanently as one or the other sex. American oysters 

 begin to spawn soon after the temperature of the water passes 63° F., usually 



