Many schemes of classification have from time to time been propounded, 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, Filibranchia, Pseudolamellibranchia, Eulamellibranchia, Septibranchia.
Fig. 25.
(From the ‘Cambridge Natural History.’ Messrs. Macmillan & Co.)
A. Protobranchia. B. Filibranchia. C. Eulamellibranchia. D. Septibranchia.
m. Mantle, v. Body. f′. Foot. e. Outer gill-lamella; i. Inner gill-lamella; e′. Reflected portion of outer lamella; i′. Reflected portion of inner lamella; s. Septum-like gill.
Fig. 26.
Gill of Mytilus edulis.[[7]]
A. Part of four filaments showing ciliated interfilamentar junctions (cj).
B. Diagram of a single filament showing the two lamellæ connected at intervals by interlamellar junctions (ilj) and the position of the interfilamentar ciliated junctions (ep).
Protobranchia. (Fig. 25, A.)
Case 137.
In this order the filaments of the gills are not reflected, but arranged in two divergent rows, the foot being expanded and flattened beneath with crenulated margins and with the byssal gland very slightly developed. The Nuculidæ and Solenomyidæ are the only families belonging to this order. The shells of the former are remarkable for the numerous fine interlocking hinge-teeth, and those of the latter on account of the strong fringed periostracum.
Filibranchia. (Fig. 25, B.)
Cases 137–145.