In their fundamental structure the rock lobsters agree with the crayfishes; hence the plans of the two may be regarded as modifications of a plan common to both. To this end, the only considerable changes needful in the tribal plan of the crayfishes, are the substitution of simple for chelate terminations to the middle thoracic limbs and the suppression of the appendages of the first somite of the abdomen. Thus not only all the crayfishes, but all the lobsters and rock lobsters, different as they are in appearance, size, and habits of life, reveal to the morphologist unmistakable signs of a fundamental unity of organization; each is a comparatively simple variation of the general theme—the common plan. Even the branchiæ, which vary so much in number in different members of these groups, are constructed upon a uniform principle, and the differences which they present are readily intelligible as the result of various modifications of one and the same primitive arrangement. In all, the gills are _trichobranchiæ_; that is, each gill is somewhat like a bottle-brush, and presents a stem beset, more or less closely, with many series of branchial filaments. The largest number of complete branchiæ possessed by any of the _Potamobiidæ_, _Parastacidæ_, _Homaridæ_, or _Palinuridæ_, is twenty-one on each side; {264} and when this number is present, the total is made up of the same numbers of podobranchiæ, arthrobranchiæ, and pleurobranchiæ attached to corresponding somites. In _Palinurus_ and in the genus _Astacopsis_ (which is one of the _Parastacidæ_), for example, there are six podobranchiæ attached to the thoracic limbs from the second to the seventh inclusively; five pairs of arthrobranchiæ are attached to the interarticular membranes of the thoracic limbs from the third to the seventh inclusively, and one to that of the second, making eleven in all; while four pleurobranchiæ are fixed to the epimera of the four hindmost thoracic somites. Moreover, in _Astacopsis_, the epipodite of the first thoracic appendage (the first maxillipede) bears branchial filaments, and is a sort of reduced gill. These facts may be stated in a tabular form as follows:— _The branchial formula of Astacopsis._ Somites and Arthrobranchiæ. their Podobranchiæ. /───────^───────\ Pleuro- Appendages. Anterior. Posterior. branchiæ. VII. 0 (ep. r.) 0 0 0 = 0 (ep. r.) VIII. 1 1 0 0 = 2 IX. 1 1 1 0 = 3 X. 1 1 1 0 = 3 XI. 1 1 1 1 = 4 XII. 1 1 1 1 = 4 XIII. 1 1 1 1 = 4