HOMARUS AND NEPHROPS.

259

Moreover, the bran

fully developed pleurobranchiæ. chial filaments of these gills are much stiffer and more closely set than in most crayfishes. But the most important distinction is presented by the podobranchiæ, in which the stem is, as it were, completely split into two parts longitudinally (as in fig. 68, B); one half (ep)

FIG. 68. Podobranchiæ of A, Parastacus; B, Nephrops; C, Palamon. A', C', transverse sections of A and C respectively. a, point of attachment; al, wing-like expansion of the stem; b, base; br, branchial filaments; ep, epipodite; 7, branchial lamina; pl, plume; st, stem.

corresponding with the lamina of the crayfish gill, and the other (pl) with its plume. Hence the base (b) of the podobranchia bears the gill in front; while, behind, it is continued into a broad epipoditic plate (ep) slightly folded upon itself longitudinally but not plaited, as in the crayfish.

The Norway Lobster (Nephrops norvegicus, fig. 69)

are susceptible of being grouped into six genera-Astacoides (fig. 65), Astacopsis, Chæraps, Parastacus (fig. 64),

FIG. 64.-Parastacus brasiliensis (nat. size). From southern Brazil. Engaus, and Paranephrops-on the same principle as that which has led to the grouping of the Northern forms into two genera. But the same convenience which has

led to the association of groups of similar species into genera, has given rise to the combination of allied genera into higher groups, which are termed Families. It is obvious that the definition of a family, as a statement of the characters in which a certain number of genera agree, is another morphological abstraction, which stands in the same relation to generic, as generic do to specific abstractions. Moreover, the definition of the family is a statement of the plan of all the genera comprised in that family.

The family of the Northern crayfishes is termed Potamobiide; that of the Southern crayfishes, Parastacide. But these two families have in common all those structural characters which are special to neither; and, carrying out the metaphorical nomenclature of the zoologist a stage further, we may say that the two form a Tribe-the definition of which describes the plan which is common to both families.

It may conduce to intelligibility if these results are put into a graphic form. In fig. 66, A. is a diagram representing the plan of an animal in which all the externally visible parts which are found, more or less modified, in the natural objects which we call individual crayfishes are roughly sketched. It represents the plan of the tribe. B. is a diagram exhibiting such a modification of A. as converts it into the plan common to the whole family of the Parastacidæ. C. stands in the same relation to the Potamobiida. If the scheme were thoroughly worked out, diagrams representing the peculiarities of