CRUSTACEA. 



765 



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 (A). 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 

 oesophagus, 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 (f 1 ) 

 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 mandibularring, and must be viewed 

 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 jluviatilis), 

 very obvious traces of the existence of several 

 medullary nuclei can always be demonstrated 

 in its interior. The five pairs of ganglions 

 that follow (Z 2 6 ), 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 asunder for the pzts- 

 sage of the oesophagus. 



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 (a 1 a 6 ) 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 commissures 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 mechanical obstacle to their 

 entire union. 



In the Palinurus the whole of the thoracic 



