ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 261 



Fig. 12-11. The knobbed whelk (Busycon corico) laying its string of egg capsules. This large whelk inhabits the 

 eastern coast of the United States. This one was taken at Woods Hole, Mass. 



float through the currents or sink to the 

 bottom. There is no free movement of 

 the glochidium at this time, other than the 

 opening and closing of its valves. Hooked 

 forms try to attach themselves to any fish 

 with which they may come in contact, 

 whereas the hookless forms grasp the gills 

 of fishes by means of their valves. In time, 

 the epithelium of the fish encases the 

 glochidium in a cyst-like case. During this 

 period the young clam is entirely para- 

 sitic, receiving nourishment from its host 

 through absorption. After the adult organs 

 have developed, the glochidium bursts out 

 of the cyst and sinks to the bottom as an 

 independent free-living animal. 



Most pelecypods (from the class name 

 — Pelecypoda) are bottom dwellers, some 

 species of clams even burrow far down into 

 the sand and push their siphons up into the 

 water. Other forms, such as the oyster, are 

 permanently attached to rocks or similar 

 objects beneath the water. The shipworm 

 (Toredo navalis) has such a slender shell 



that it can burrow into the wood of ships 

 and wharves, where it does extensive dam- 

 age. The scallop can swim freely in the 

 water by flapping its shells together. 



Other mollusks 



Members of the class Amphineura are 

 the most primitive forms among the mol- 

 lusks. In this group are the chitons, which 

 most nearly resemble the probable worm- 

 like ancestors of the phylum (Fig. 12-1). 

 Their dorsal covering of eight calcareous 

 plates has led some biologists to believe 

 that this may be the remnant of segmenta- 

 tion. The ocean-inhabiting chitons attach 

 themselves so securely to rocks that it is 

 almost impossible to pry them loose. If they 

 are dislodged they promptly curl up into a 

 ball. Much like the annelids, chitons live 

 under rocks. They are principally "vegetar- 

 ians," feeding on various kinds of marine 

 algae. 



In most forms the sexes are separate. 

 One investigator, Grave, found that the 



