NERVOUS SYSTEM. (COMPARATIVE ANATOMY ) 



609 



merous forms and degrees of organization. 

 In the common Talitrus, an inferior genus, 

 it consists of a regular series of ganglia, 

 developed at an equal distance from each 

 other, united by two distinctly separated 

 longitudinal cords, from which are given 

 oft" transverse nerves. 1 have found the same 

 arrangement in the genus Oniscus, in which 

 also a close analogy to the nervous system 

 of the Annelides was apparent. In the 

 Cymothoa, an animal a little higher in the 

 scale, these longitudinal columns have become 

 closely approximated, and the ganglia have 

 coalesced transversely. Rising higher in the 

 scale, we find a still greater degree of concen- 

 tration and coalescence in the Decapoda, this 

 being directed to two principal points the 

 thorax in the long-tailed Decapods, and the 

 thorax and abdomen in the short-tailed ones. 

 With regard to the former, I have examined 

 the nervous system in the genera Crangon, 

 Processa, and Pagurus, in all of which it pre- 

 sented a similarity in developement. In the 

 lengthened abdomen the longitudinal cords 

 were very closely approximated, and the gan- 

 glia developed were nearly of an equal size, 

 and equidistant from each other; from them 

 were given ofl' transverse nerves. In the thorax, 

 the ganglia were very closely approximated 

 indeed, longitudinally, so as nearly to have 

 the appearance of one nervous mass, from 

 which were given off large transverse nerves to 

 the neighbouring parts, and from the anterior 

 part of which there passed off two long nervous 

 cords, which encircled the oesophagus, and de- 

 veloped a ganglion on its superior part. In 

 the short-tailed Decapods, as in the common 

 edible crab, the abdominal ganglia have co- 

 alesced into one large nervous mass, from 

 which radiate nerves to the legs, &c ; from its 

 anterior part there pass two long filaments, 

 connecting it with the coalesced ganglia in the 

 thorax. There is a supra-cesophageal gan- 

 glion, as in the preceding, but it is compara- 

 tively small. 



2. Myriapoda. 'Amongst the Myriapoda, 

 the next class, we find the nervous system 

 again beginning by a low state of organization, 

 similar to the lower Crustacea, this being prin- 

 cipally characterized by a considerable number 

 of ganglions. In the Scolopendra morsitans, 

 the nervous system consists of a series of 

 twenty-one double ganglia, situated on the 

 ventral surface of the body, connected by in- 

 tervening distinctly double longitudinal cords. 

 From each ganglion are given off lateral nerves 

 to supply the neighbouring muscles, viscera, 

 and feet. Those ganglia are nearly all equal 

 in size excepting the first, which is the largest, 

 and from which are given off additional nerves 

 to supply the maxillae, &c. Beyond this first 

 sub-cesophageal ganglion, and from its anterior 

 part, proceed the longitudinal connecting cords, 

 which diverge to encircle the oesophagus, above 

 which they meet and develope a bilobate supra- 

 cesophageal ganglion. (See MVRIAPOHA, Jig. 

 313.) [Mr. Newport's recent researches on 

 the nervous system of Myriapoda favour the 

 opinion that a distinct series of excitomotory 



VOL. III. 



fibres connected with the ganglia of the seg- 

 ments (and not with the cerebral ganglion) exist 

 in these animals. See the forthcoming volume 

 of the Philosophical Transactions, 1843.] 



3. Araclmidu. In the Arachnida the ner- 

 vous system is more concentrated, and the gan- 

 glia are fewer; they may be considered, indeed, 

 as intermediate in the developement of this sys- 

 tem between the Insecta and Crustacea. In the 

 Scorpions, according to Dr. Grant,* '' the gan- 

 glia of the trunk have formed one large nervous 

 mass, from which all the nerves of the legs and 

 the surrounding parts take their rise as from 

 a single ganglion." The cerebral ganglion is 

 comparatively small, and, according to Cuvier 

 and Carus,f the two nervous cords, proceeding 

 thence, unite at intervals to form seven gan- 

 glions, the last of which belongs to the tail. 

 Grant observes,]; that the motor column is very 

 loosely connected with the two inferior or sen- 

 sitive columns, particularly in the region of the 

 abdomen, and that this conformation is more 

 obvious here than in any other of the Articu- 

 lata. In Spiders, the nervous system consists, 

 according to Professor Owen's description, of a 

 brain, a bilobed ganglion which supplies the 

 optic nerves, and also two large nerves to the 

 mandibles. From it a short and thick collar, 

 embracing the gullet, extends to a second very 

 considerable stellate or radiated ganglion, si- 

 tuate below the stomach upon the plastron. 

 From this ganglion five principal nerves are 

 sent off on each side, " the first to the pedi- 

 form maxillary palpi, the second to the more 

 pediform labial palpi, which are usually longer 

 than the rest of the legs, and used by many 

 spiders rather as instruments of exploration 

 than of locomotion ; the three posterior nerves 

 supply the remaining legs, which answer to the 

 thoracic legs of Hexapod Insects. The ner- 

 vous axis is prolonged beyond this great gan- 

 glion, as two distinct chords, into the beginning 

 of the abdomen, where, in the Epira diadema, 

 it divides into a kind of cauda equina; but in 

 the Mygale a third oanglion of very small size 

 is formed from which the nerves diverge to 

 supply the teguments of the abdomen and its 

 contents." * * * " The stomatogastric nerves 

 are sent off from the posterior and lateral parts 

 of the brain and form on each side a reticulate 

 ganglion, which distributes filaments to the 

 stomach." 



4. Insecta. We have now to examine the 

 last and highest class of articulated animals 

 the Insecta, in which we shall find the nervous 

 system very highly organized, leading us by 

 strict analogies to the Vertebrata. It consists, 

 in almost every order, of a ganglionic nervous 

 cord, running along the abdominal surface, as 

 in the preceding classes, and of a similar supra- 

 cesophageal nervous mass, called by Cuvier the 

 brain, from which are given off eight pairs of 

 nerves and two single ones. This nervous cord 

 consists of a varied number of ganglia, giving 

 off lateral nervous filaments, and connected to 



* Lectures on Comparative Anatomy. 

 t Op. cit. 

 t Op. cit. 



Owen's Lectures, by White, p. 255. 



2 R 



