30 
SHELL GALLEKY. 
soft thin flaps, or labial palps, which have the function of conveying 
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 hinge, 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, Razor-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 
burrowing in sand or attached to rocks. Some perforate stones and 
corals, others wood, and a few construct 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. Mr. Paul Pilseneer has 
suggested five orders of Lamellibranchs : Protobranchia, Fili- 
branchia, Pseudolamellibranchia, Eulamellibranchia, Septibranchia. 
Fig. 
(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 ; %'. Beflected portion of inner lamella 
s. Septum-like gill. 
