104 



BEGINNERS' ZOOLOGY 



Fig. 199. — Oyster. 



C, mouth; a, vent; g,^, 

 ganglia; ;«/, mantle; b, 

 gill. 



When the mussel first hatches from the egg, it has a tri- 

 angular shell. It soon attaches itself to some fish and thus 



travels about. After two months it 

 drops to the bottom again. 



Other Mollusca. — The oysters shells 

 are not an exact pair, the shell which 

 lies upon the bottom being hollowed 

 out to contain the body, and the upper 

 shell being flat. Can you tell by ex- 

 amining an oyster shell which was the 

 lower valve? Does it show signs of 

 having been attached to the bottom? 

 The young oyster, like the young mus- 

 sel, is free-swimming. Like the arthropoda, most molluscs 

 undergo a metamorphosis to reach 

 the adult stage (Fig. 199). 



Examine the shells of clams, 

 snails, scallops, and cockles. Make 

 drawings of their shells. The slug 

 is very similar to the snail except 

 that it has no shell. If the shell of the snail shown in 

 Fig. 202 were removed, there would be left a very good 



representation of a slug. 



Economic Importance of 



Mollusca. — Several species 



^^^ !?»^ of clams are eaten. One of 



them is the hard-shell clam 

 (quahog) found on the At- 

 lantic coast from Cape Cod 

 to Texas. Its shell is white. It often burrows slightly 

 beneath the surface. The soft-shell clam is better liked as 

 food. It lives along the shores of all northern seas. It 

 burrows a foot beneath the surface and extends its siphons 



Fig. 200. — Trc-chus. 



Fig. 201. — Cypr^ea. (Univalve, 

 with a long opening to shell.) 



