We have only further to add that in a great number of species one or seveml pairs of the thoracic extremities are modified so as to become instruments of prehension; some times it is the last segment of the limb which, acquiring more than usual mobility, bends in such a manner as to form a hook with the preceding segment ; sometimes it is this penul timate segment which extends below or hy the side of the last, so as to form a kind of im moveable finger with which it is placed in opposition. In the first instance these instru ments are denominated subcheliform claws, in the second ehela simply, or cheliform claws. We shall revert to these organs when we come to treat of the apparatus of' digestion.
§ 2. Apparatus of Sensation.
A. Nervous System.—When endeavouring to form as accurate and complete an idea as possible of the tegumentary skeleton of the Crustacea, we began by studying it in its suc cessive states of development in the embryo, and then compared the various stages of tmnsition in which it met our observation, with the permanent conditions in which it finally remains in the organic series, classed in conformity with the structural affinities of the different genera. In the study of the nervous system, upon which we are now about to enter, the same mode of proceeding will lead us to analogous results.
The deep situation of the nervous system, and the transparency of the filaments and various masses which compose it, are each obstacles to its observation until it has arrived at a somewhat advanced stage of development. It was, in fact, only after the sternal canal had begun to appear under the form of an enlarge ment, edged by a double series of tubercles, which prove to be the rudiments of the motor muscles of the extremities, that Rathke was able to catch a sight of the earliest traces of the nervous system in the Astacus fluviatilis, and even this was no more than the portions be longing to the head and thorax. All that can be seen then amounts to very little ; in the part behind the mouth, eleven pairs of whitish spots are arranged in two longitudinal series perfectly distinct from one another, and situated on either side of the mesial plane. It is otherwise easy to perceive that a pair of these spots corres ponds to each ring, setting out from, but in cluding those of the mandibles. Neither the cesophageal cords nor the cephalic ganglions are then distinct.
At a later 'period these rudiments of the nervous system undergo remarkable modifica tions. The six first ganglions of eacli series approach those that are symmetrical with them severally, so as to become united along the median line, and, at length, to form a simple chain of ganglions corresponding to the six rings, whose appendages are the mandibles and the five pairs of maxillary extremities. The ganglions, on the contrary, which corres pond to the five posterior thoracic rings, continue to form a double series. During this time the sternal canal is evolved so as to surround the nervous system with a firm and solid sheath. At a period of the incubation still farther ad vanced, that is to say, during the time which elapses frotn the birth of the young Crustacean to that at which it attains its full growth, new and important changes take place. First, the four most anterior cesophageal tubercles, in other words, those which correspond to the mandibles, to the jaws, and to the first pair of maxillary limbs, become united, by approach ing one another along the mesial line, so as finally to constitute a single continuous mass only. The same thing happens in re gard to the fifth and sixth which soon form no more than a single ganglion. As to the other pairs they always remain completely distinct, and some way parted from one another.
Thus the study of the gradual evolution of the nervous system in the Astacus fluviatilis, al though by no means belonging to the type in which this system is most completely developed, presents us with three distinct and successive facts, which we shall find reproduced in the most perfect manner in the natural series of genera, and which will put us into a position to give a satisfactory explanation of those very striking variations in the organization which we shall encounter.
These are in the first place, the isolated for mation of die nervous centres, independently one of another. We now acknowledge this independence of the several organs at the moment of their appearance, and their ulterior conjunction is one of the most interesting and important facts with which modern science has been enriched ; it constitutes the law of centri petal development, as it has been established by M. Serres.