30 



SHELL GALLEEY. 



the food to the mouth. The mantle secretes the substance out of 

 which the shell is formed. The two valves are always in contact at 

 the hinffe, which is generally formed by small interlocking projections 

 or hinge-teeth, and they are closed by large adductor muscles, which 

 are attached to impressions in the interior of the shell. When these 

 muscles cease to act, as after death, the valves of the shell open in 

 consequence of the elasticity of the ligament on the dorsal margin. 

 The majority of species have two principal adductors, one at each end, 

 like the Venus-shells, Cockles, Eazor-shells, &c. ; but in Oysters, 

 Scallops, and a few others, there is but a single central muscle. All 

 Bivalves are aquatic, and the majority marine. They are found 

 mostly burrowing in sand or mud or attached to rocks. Some 

 perforate stones and corals, others wood and other substances, and 

 Lima constructs a sort of nest of fragments of shells, stones, &c. 



Many schemes of classification have from. time to time been pro- 

 pounded, based upon the presence or absence of respiratory siphons, 

 the number and position of the adductor shell-muscles, the character 

 of the shell-hinge, &c. The most recent arrangement is founded 

 principally upon the structure of the gills. The value of such a 

 classification has yet to be fully tested. Dr. Paul Pelseneer has 

 suggested four orders of Pelecypods : Protobranchia, Filibranchia, 

 Eulamellibranchia, Septibranchia ; but Dr. W. G. Ridewood has 

 proposed to limit the main divisions to three, namely, Protobranchia, 

 Eleutherorhabda, and Synaptorhabda. 



Fig. 25. 



(From the ' Cambridge Natural History.' Messrs. Macmillan & Co.) 



A. Protobranchia. B. Filibranchia. C. Eulamellibranchia. D. Septibranchia. 

 m. Mantle, v. Body. /. Foot. e. Outer gill-lamella; i. Inner gill-lamella; 



e'. Keflected portion of outer lamella; i'. Eeflected portion of inner lamella. 



s. Septum-like gill. 



