Each of these five thoracic ganglions sends two pairs of nerves to the ambulatory extre mities which correspond to them severally. Of these two nerves, the posterior and larger sends branches to the basilar articulations of the extremities; the anterior, again, distributes twigs to the muscles of the flancs; the two soon anastomose, and form a single trunk before penetrating into the extremity itself, which then traverses the whole limb, send ing a branch to the muscles of each arti culation.
The abdominal ganglions (ai—a6) are smaller than the preceding ones, and are connected by simple longitudinal cords. They also supply two pairs of nerves, the one destined to the muscles of the abdomen, the other to the ap pendages of the ring with which it corresponds. As in the thorax, nervous fibres, distributed to the median and superior part of the abdomen, are observed proceeding from the cords which establish a communication between one gan .glion and another.
The last ganglion (a 6), which appears to be made up of the medullary nuclei be longing to the sixth and seventh segments of the abdomen, gives origin to four pairs of nerves, which run to the penultimate articu lation of the abdomen, and to the last, which is of a flattened form, and along with the ap pendages of the former constitutes the kind of horizontal oar which terminates this part of the body.
Such is the nervous system in the Lobster. If we study it in the Palemon, we shall find precisely the same elements, but with a still higher degree of centralization, for the ganglia of the three lowest thoracic rings are conso lidated into one, and situated much forWards, so that the nerves to which they give origin have to pursue a very oblique course, in order to reach the parts to which they are distributed respectively. The ganglion of the second pair is isolated ; that of the first pair of ambulatory extremities blends and is confounded with that of the third pair of maxillary limbs. The five anterior pairs of cesophageal ganglions, in fine, are united into a single nervous centre. There are consequently, properly speaking, no more than four medullary masses in the whole length of the cephalo-thoracic portion of the Palemon ; and even these are very close to one another, and all but united, their longi tudinal cornmissures being thick and simple, and bearing as close a resemblance to constric tions in a single nucleus as to bands of communication between distinct nuclei. The fourth of these four ganglions presents a longi tudinal cleft through its centre, a structure which is easily explained by the presence at this point of the sternal artery, which existed there before the ganglia became conjoined in the course of the median line, and necessarily opposed a merely inechanical obstacle to their entire union.
In the Palinurus the whole of the thoracic ganglia, strictly speaking, are united into a single mass of a greatly elon gated form, and presenting a little way back, like the fourth ganglion of the Palemon, a cleft for the trans mission of the sternal artery.
The transition from the Deca poda Macroura to the Brachyura takes place by the Homola, and certain Anomou ra,* in which the constantly in creasing concen tration of the thoracic nervous centres coincides with the almost rudimentary state of the abdominal ganglionic sys tem, which is here reduced, to a kind of median trunk without en largements.
This, too, is . the disposition ;presented by the .nervous system in the Carcinus mcenas among the Brachyura, with this difference only, that the medullary nuclei are rather closer to one another, and more intimately connected.f The tho racic ganglion has the form of a ring, the cir cumference of which gives origin to the nerves of the thoracic appendages. The .single abdomi nal cord is in its rudimentary state, in obvious relation with the abdomen itself, and therefore reduced to very insignificant dimensions.
It is in the Maja,T in fine (fig. 95), that the nervous system is found in its highest degree of centralization ; for the elements of which the tl,vo masses tbere encountered are composed, are so intimately conjoined, that no trace can be found of their ever having existed indepen dently, although among neighbouring genera several of them may still be discovered isolated ly. The cephalic ganglion (a) is a sufficiently faithful counterpart of that of the Lobster. The nervous cords (g) vvhich connect this first portion of the system with the thoracic portion also present the same arrangement as in the Lob ster ; there are similar mandibular nerves, a like gastric pair, the same transverse band (g') behind the cesophagus, &c. But the thoracic ganglion (/), instead of the ring which it presents in the Carcinus mcenas, here appears as a solid circu lar and flattened nucleus giving origin to the whole of the nerves of the thorax and abdo men, which radiate from it to the number of nine pairs, and one azygous nerve situated in the median plane. There is nothing very remarkable in the distribution of these nerves, unless it be that several pairs,and amongthe number the first and second, are distributed simultaneously to several rings, which proclaims that in the species which engages us the work of con centration has extended from the ganglions to the nervous cords.