The Lobster( Astacus marinus) (fig. 39 3) pre sents us with another step in the system of cen tralisation. Here, in fact, the longitudinal cords of communication are entirely consolidated along the median line through the whole of the abdomen, although they are still to be found double in the thorax. Moreover, the first thoracic ganglion (t4), and the last of the abdominal series of ganglions (a6), are conspicuously formed by the reunion of several distinct ner vous centres, in the way we have already indi cated as happening, although in a minor degree and less perfectly, in the Amphipoda and the Isopoda. Before we pass, however, to the con sideration-of more complicated systems, we shall pause a moment to describe somewhat at length the one which we have but just mentioned, the more as it is among the number of those which have been most attentively studied.
The cephalic ganglion (g', fig. 393), situated above the base of the internal antennm, is of con siderable size, and appears to be simple ; it gives origin to five pairs of nerves and to two cords, which connect it with the rest of the ganglionic nervous system. The first of these pairs (o)arises from its anterior edge : this is the optic pair, which, after having penetrated the peduncles of the eyes, increase in size, and traverse a mem branous diaphragm, which may be likened to the sclerotic coat.
The second pair of nerves correspond to the ocular motors ; they run parallel to the pre ceding pair, and are distributed to the muscles of the eyeball.
The third pair proceed to the internal anten nee (b); but before they enter these appendages they send off a branch to the muscles which move them. A like ramification is sent off from the principal trunk to each of the rings of which these antennm are composed, and the nerve ends by becoming bifurcated, in order to penetrate the two filaments in which the an tennw terminate.
The fourth pair of nerves (e) are distributed to the tegumentary membranes of the anterior extremity of the animal. Behind the fourth a fifth pair is seen (d), which proceeds anteriorly to the fourth pair almost immediately after its origin, sends one branch to the cake-like organ of doubtful function which covers the ear, a second branch to the organ of hearing itself, and finally terminates in a trunk of considerable size, which traverses the external or second antenna through its entire length.
A sixth pair is destined to establish con nexions between the cephalic ganglion and the first of the thoracic ganglions, after having sur rounded the cesophagus ; but instead of ap pearing as simple nervous cords through their whole length, as in types which we have hitherto studied, each of them presents an enlargement in its middle, which is neither more nor less than a ganglion, from which there is sent off, first, a nerve that proceeds to the mandibles (f); next, a gastric nerve (g), of large size, which gives many filaments to the coats of the stomach, and finally anastomoses with the corresponding cord of the opposite side ; after this the two form a single nerve, which by-and-by presents an enlargement having the appearance of a small median ganglion, and then remounts upon the dorsal aspect of the stomach to ramify there, and ultimately to lose itself upon the intestine (h). Behind
the stomach a transverse cord (i) is seen, which connects the two nervous filaments, and appears to be the cord of communication be tween the ganglion of which mention has just been made, pushed backwards, in the same way as the ganglions themselves have been kept apart, to wit, by the resistance of the cesophagus, interposed at the time when that process is going on by which the pairs gene rally are approximated in the course of the median line.
The first of the thoracic nervous masses (0) is oval-shaped, and gives origin to ten pairs of nerves, five of which issue from the anterior aspect. The first run to the mandibles and to their muscles; the second to the auditory apparatus ; the third to the first jaw, the fourth to the second jaw ; the fifth to the cells of the flancs, to the muscles and neighbouring inte guments ; the sixth and seventh arise from the inferior aspect of the nervous mass to proceed to the maxillary feet; the nerves of the eighth pair are extremely slender, and are distributed to the muscles of the thorax ; the two succeed ing pairs belong to the third pair of maxillary extremities; lastly, two cylindrical cords arise from the posterior extremity of this nervous centre, and connect it with the second thoracic ganglion, giving origin themselves in their passage to a pair of extremely minute filaments, which run to be distributed to the muscles of the thorax.
'This first thoracic nervous mass represents, therefore, the five pairs of ganglions which follow the mandibular ring, and must be yiewed as resulting from the concentration of the five pairs of medullary nuclei belonging to the five rings which bear the accessory masticatory or gans. In the adult Lobster the different ele mentary constituents are not traceable, and the whole mass appears to be composed of no more than two ganglions closely connected in the median plane; but in a species very nearly allied, namely, the River-crab (Astacus fluviatilis), very obvious traces of the existence of several medullary nuclei can always be demonstmted in its interior. The five pairs of ganglions that follow (t2—t6), and that belong to the five last thoracic rings, have, on the contrary, continued distinct ; although simple, these nervous centres still exhibit manifest indi cations of their composition severally by two nuclei; from either half we have a cord of communication sent off, similar to those which we have already pointed out as exist ing between the first and second thoracic ganglions ; the whole of these inter-ganglionic cords are in contact along the median line, except the penultimate or antepenultimate pairs, which are separated from one another by the sternal artery, in the same manner as those of the head are kept a:sunder for the Os sage of the cesophagus.