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:—
| Somites and their Appendages. | Podobranchiæ. | Arthrobranchiæ. | Pleurobranchiæ. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Anterior. | Posterior. | ||||||||
| 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 | |||
| XIV. | 0 | 0 | 0 | 1 | = | 1 | |||
| 6 + ep. r. | + | 6 | + | 5 | + | 4 | = | 21 + ep. r. | |
{265}
This tabular “branchial formula” exhibits at a glance not only the total number of branchiæ, but that of each kind of branchia; and that of all kinds connected with each somite; and it further indicates that the podobranchia of the first thoracic somite has become so far modified, that it is represented only by an epipodite, with branchial filaments scattered upon its surface.
In Palinurus, these branchial filaments are absent and the branchial formula therefore becomes—
| Somites and their Appendages. | Podobranchiæ. | Arthrobranchiæ. | Pleurobranchiæ. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Anterior. | Posterior. | ||||||||
| VII. | 0 (ep. r.) | 0 | 0 | 0 | = | 0 (ep.) | |||
| 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 | |||
| XIV. | 0 | 0 | 0 | 1 | = | 1 | |||
| 6 + ep. r. | + | 6 | + | 5 | + | 4 | = | 21 + ep. | |