OCEAN ANIMALS: SPONGES, SEA-ANEMONES, ETC. I39 



short spines. They lie buried in the sand, and are often 

 very brightly colored. 



Their hard bleached tests with the spines all rubbed off 

 are common on the sands of both the Atlantic and 

 Pacific coasts. 



Oysters, clams, and sea-shells. — Very different from 

 the sea-anemones, jellyfishes and starfishes are those 

 inhabitants of the ocean commonly called "shell-fish." 

 These include the oysters, clams, pectens, shipworms, 

 and the host of snail-like creatures whose lime " houses " 

 of manifold shapes and colors we know under the ex- 



FlG. lOO. 



Fig. ioo. — The eastern oyster, Ostrea i>irginiana. (One-third natural size; 



after pliotograph by W. H. C. Pynclion.) 

 Fig. ioi. — Young oyster. (Greatly magnified; after Krooks.) 



pressive name of sea-shells. All these animals which 

 with the fresh-water mussels and the pond and land 

 snails and slugs make up the branch of Alollusca, have a 

 soft, sac-like body, not built on the radiate plan like the 

 starfishes nor on the segmented plan like the insects, but 

 on the plan well shown by the snail. The body is pro- 

 tected by a firm shell of carbonate of lime, which may 

 be in two pieces, bivalved, as in the oyster and clams, or 

 in one piece, univalved, as in the usual spiral sea-shell 

 type. 



The oyster (fig. lOo) is carefully cultivated by man in 

 many countries. It has two shells, or two dissimilar 



