608 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



of an irregular white line, the structure of 

 which, when viewed under a lens, appeared 

 to consist of a number of closely approxi- 

 mated nervous threads. In a larger species the 

 nervous cord had a more jointed, knotted ap- 

 pearance, and its structure was more homoge- 

 neous; the ganglia were, however, but very im- 

 perfectly developed, for at each segment of the 

 body of the worm, the nervous cord only offered 

 an enlargement or swelling of rather an elongated 

 form (Jig. 344, d) : the first or most anterior 



Fig. 344. 



Upper fourth of the nervous cord nf Lumbricus 

 terrestris (earthworm), 



a, supra-oesophageal or cerebral ganglion. 

 c, infra-oesophageal ganglion. 6, b, lateral ner- 

 vous cords, forming the oral primary nervous 

 ring, d, d, d, d, enlargements of the ventral cord, 

 or imperfect ganglia, developed on the ventral 

 surfaces of the series of primary nervous rings. 



of these (the infra-cesophageal ganglion) was 

 the largest (c). These enlargements were more 

 closely approximated towards the anterior part 

 of the body ; from each of them were given off 

 two pair of nerves, one passing to trie right, 

 the other to the left, to supply the integuments. 

 From the more attenuated intervening portions 

 of the cord, a single pair of filaments was given 

 off; these filaments we at once recognize as the 

 commissures of the nervous ring of the Radiata 

 and Mollusca ; they pass round the body and 

 approach each other on the dorsal surface, but 

 do not unite : in this way is an open nervous 

 ring formed. The infra-oesophageal ganglion 

 before mentioned diverged at its anterior part, 

 sent upwards two lateral nervous cords (6), 

 which developed a large bilobate cerebral or 

 supra-cesophageal ganglion (a) of a transversely 

 elongated oval form, thus forming a distinct 

 nervous ring, embracing the oesophagus. From 

 the angles formed by the divergence of the 

 infra-oesophageal ganglia, a nerve of rather firm 

 texture was given off; another filament had its 

 origin from the lateral and ascending portions 

 of the nervous collar ; and from the cerebral 

 ganglion two pairs of nerves passed forwards to 

 supply the parts about the head. In the 

 Aphrodita, a marine Annelide, the nervous 



cord had a more flattened appearance : it was 

 about equal in width throughout its whole 

 length, and presented similar enlargements in 

 its course, from which nerves were given off, 

 as in the Lumbricus ; no nerve, however, arose 

 from the intervening spaces. In the Leech, 

 the abdominal nervous cord was surrounded 

 by several delicate vessels, and at the anterior 

 part of the commissures was distinctly seen to 

 be composed of two separated columns : the 

 ganglia were very distinct, of a round form, 

 and twenty-four in number, the last four or 

 five were more closely approximated ; from 

 each were given off two diverging pair of lateral 

 filaments, which passed to supply the muscles 

 and viscera. The cerebral ganglion is bilobed, 

 and sends off ten distinct optic nerves besides 

 many smaller filaments to the integument and 

 other parts of the head ; each optic nerve ter- 

 minates by expanding; upon the base of a black 

 eye-speck or ocellus.* 



According to Brandt, a- simple nervous fila- 

 ment is continued from the oesophageal ganglion 

 along the dorsal aspect of the alimentary canal. 

 " This," says Professor Owen, " is an inte- 

 resting structure, since it offers the first trace 

 of a distinct system of nerves, usually called 

 the stomato-gastric in Entomology, arid to 

 which our great sympathetic and nervus vagus 

 seem answerable." 



These four classes comprise the whole of 

 the helminthoid Articulata; and on reviewing 

 the statements just made with regard to their 

 nervous system, we observe that the lowest of 

 them bear a great analogy to the Acrita and 

 some of the Tunicata, in having no distinct 

 nervous system discoverable; that where such 

 does exist in the higher orders of them, it bears 

 more or less the character before noticed as the 

 type of the Articulata, which was, it will be 

 recollected, a series of primary nervous rings 

 connected by commissures ; that in the Ascaris 

 the ganglia are so imperfectly developed, and 

 consequently their commissures, that the whole 

 has the appearance of only a single nervous 

 filament. As we proceed through the Rotifera 

 and Cirrhopoda, we observe the ganglia rather 

 more perfectly developed, but undetermined in 

 number. In the highest of the Annelides, as 

 the Leech, we find distinct ganglia developed 

 and arranged in a determinate numerical series. 

 Again, the cerebral ganglion, from being at 

 first little more than a simple enlargement on 

 the superior surface of the anterior primary 

 nervous rins, has acquired, in the Leech, the 

 form of a distinct ganglion. 



We next pass to the examination of the En- 

 tomoid Articulata, in which we shall find the 

 cerebral ganglion becoming more and more 

 developed, and the ventral ganglia more de- 

 termined as to number, and more concentrated 

 as to situation ; their longitudinal commissures 

 have been supposed to possess two distinct 

 nervous tracts, composed of motor and sen- 

 sitive columns, giving origin to nerves having 

 sentient and motiferous properties. 



1. Crustacea. In the lowest of them, the 

 Crustacea, the nervous system presents nu- 



* Owen's Lectures, p. 141. 



