CHAPTER XX 



OYSTERS, CLAMS, MUSSELS, OTHER MOLLUSCS, 

 AND THE SHELL-FISH INDUSTRIES 



The oysters, clams, mussels and snails, the molluscs (branch 

 Mollusca) with which we are most familiar, have their soft 

 bodies protected by a firm outer shell which is formed of car- 

 bonate of lime. But the slugs, which are common in the garden 

 and in moist places, the beautiful sea-slugs or nudibranchs, 

 which are found in salt water, and the cuttlefish and octopi, 

 which are also marine, all of which also belong to this branch, 

 are not protected by such a shell. In habits and distribution 

 the members of this branch vary as much as they do in structure 

 and general appearance. Many of the snails and slugs live 

 on land, feeding on live or dead and decaying vegetable tissue. 

 Most of the mussels live in fresh water, but all the other 

 members of the branch live in the sea, some at the surface, 

 others at moderate or great depths, many in the sand or mud of 

 shores and shallow bottoms. 



The Fresh-water Mussels. A study of the fresh-water 

 mussel will give one a general idea of the structure of the typical 

 members of this branch. The two valves of the shell are held 

 together along the dorsal edge by the horny hinge-ligaments. 

 Toward the rounded anterior end there is, on each valve, a 

 prominent elevation, the umbo, which marks the oldest part of 

 the shell, and from which extends a series of concentric lines 

 of growth. When the mussel is feeding in the bed of the stream 

 the anterior end is buried deep in the mud and only a little of 

 the posterior end is exposed. The valves are opened slightly 

 at the posterior end, and between them may be seen the edges 

 of the mantle that covers the soft body and lines the inside of 

 the shell. When one of the valves is removed it will be seen 

 that the mantle is attached to the inner surface of the shell a 



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