MOLLUSKS — SPECIALISTS IN SECURITY 



641 



Chap. 31 



water from whence they are usually drawn into the incurrent siphons. Sperm 

 and eggs ripen at the same time and the latter are shed into the mantle cavity. 

 Both sperm and eggs are drawn through the microscopic holes into the water 

 tubes of the outer gills where fertilization occurs. There millions of embryos 

 develop. The outer gills become swollen brood pouches, and the young clams 

 thrive until they are easily visible to the naked eye. They are then definitely 

 clam-shaped animals called glochidia. They are discharged from the excurrent 

 siphon and scattered on the bottom with their valves open and a sticky thread 

 trailing out between them. For a time they are gamblers for their existence, 

 and then for several months they are parasites (Fig. 31.10). The edges of the 

 shells are smooth in some species; armed with hooks in others. From time to 

 time glochidia snap their valves together, bounce upward, then drop back with 

 valves open. If there is any disturbing motion of the water their snaps and 

 bounces increase. Fish or anything savoring of fish creates the wildest excite- 

 ment. All of this can be seen with glochidia in a glass of water and a bit of 

 fish meat or blood. It is easy to remove glochidia from a ripe brood pouch — a 

 slight cut in it and they pour out like sand. 



All of this happens in nature when fishes are near except that the glochidium 



Fig. 31.10. Life history of a fresh-water clam. The embryos develop in the outer 

 gill. Later they are shed through the excurrent siphon (nearer the hinge), as 

 minute clams with one strong muscle connecting the valves and a sticky thread 

 dangling from them. They clap their shells at every fish that approaches and some 

 among the millions are able to hook themselves into the fins and gills where they 

 live for weeks as parasites. Finally they drop off into the mud. 



