MOLLUSCA 45 



the shell consisting of two more or less similar valves and the mantle 

 of symmetrical lobes. 



2. Gastropoda, Fig. 43. These molluscs have usually a single, 

 spirally coiled shell, and are not bilaterally symmetrical. They are 

 the snails, slugs, etc. 



3. Cephalopoda, Figs. 46 and 460. Bilaterally symmetrical animals 

 with the foot divided into arms which are provided with sucking disks. 

 Well-developed nervous system concentrated in' the head. Usually 

 with a spiral external shell or with an elongated internal bone or pen. 

 Ink sac usually present. The squids, cuttlefishes, octopi, and nautili. 



4. Amphineura, Fig. 47. Bilaterally symmetrical forms often cov- 

 ered with eight transverse calcareous plates. Typically represented 

 by the chitons. 



5. Scaphopoda, Fig. 48. A small group of small marine forms 

 whose tubular mantle secretes a slightly curved, conical shell open at 

 both ends. From the shape of the white shell they are called the 

 elephants' tusk shells or tooth shells. 



Shell-fish have been used as food by man from prehistoric times. 

 At various places along the sea-coast huge shell mounds or " kitchen - 

 middens" are found as relics of the shell-fish devoured by past genera- 

 tions, the antiquity of which is indicated by the primitive implements 

 sometimes found in these mounds. 



The Oyster. Of all molluscs, certainly of those found in America, 

 the oyster, Ostrea mrginiana (virginica), is the most important eco- 

 nomically. A generation or more ago there were dredged in Chesapeake 

 Bay, one of the most famous regions for the production of this bivalve, 

 each year, about 17,000,000 bushels of oysters. These oysters, which 

 were the "wild crop" obtained without cultivation, were shipped alive 

 and in cans all over the country. The supply was considered to be 

 "inexhaustible." Other extensive beds are found in Long Island 

 Sound, in Louisiana, at other places on both coasts, in Japan, and in 

 various European countries. 



After this maximum dredging of the wild crop had continued for 

 some years it began to be noticed that the supposedly inexhaustible 

 supply was failing and, in 1882, Dr. W. K. Brooks, of Johns Hopkins 

 University, was appointed by the Governor of Maryland, a commis- 

 sioner to investigate the oyster business and to make recommendations 

 for its improvement. The results of these and his previous investiga- 



