ARTHROPODS AND MOLLUSCS 171 



piece, univalved, as in the usual spiral snail and sea shell 

 type. The eggs are usually laid in a mass held together by 

 a gelatinous substance. In most species the young mollusc 

 on hatching from the egg does not resemble its parents, 

 but is a free-swimming larva called a veliger. It is pro- 

 vided with cilia for organs of locomotion. It must undergo a 

 radical change in order to reach the adult stage. Thus 

 metamorphosis occurs in this branch as well as among the 

 Arthropods and Echinoderms. In the development of some 

 molluscs, however, there is little or no metamorphosis, the 

 young being hatched in a condition much resembling, except 

 in size, the parent. 



The branch Mollusca is divided into five classes, three of 

 \vhich include the more familiar kinds. These three classes 

 are the Pelecypoda, including the mussels, cockles, clams, 

 scallops, oysters, etc., molluscs with a shell composed of 

 two pieces, one on each side of the body and hinged to- 

 gether; the Gastropoda, including the snails, slugs, peri- 

 winkles, whelks, and a host of other univalved shell-fish, 

 that is, molluscs which have a shell composed of a single 

 piece; and the Cephalopoda, including the squids, cuttle- 

 fishes, octopi, and the pearly nautilus. 



Clams show a range in size from the little fresh-water 

 Cydas about i cm. long to the giant clam of the Indian and 

 Pacific islands "which is sometimes 60 cm. (2 feet) in length 

 and 500 pounds in weight." They show also some variety in 

 the form and appearance of the shell, but not anything like 

 the degree of variety shown by the shells of the Gastropods. 



The edible clams are of several different species. The 

 hard-shell clam (Venus mercenaria) , or "quohog" as it 

 is often called, is found along the Atlantic Coast from Texas 

 to Cape Cod. It is "common on sandy shores, living chiefly 

 on the sandy and muddy plots, just beyond low- water 

 mark. ... It also inhabits estuaries, where it most 

 abounds. It burrows a short distance below the surface, 



