NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



611 



In the larva of Dyticus marginalis (fig. 347), 

 the nervous system consists of a distinctly 

 bilobate supra-oesophageal ganglion (a), from 



Fig. 347. 



Ventral nervous cord of Dyticus marginalis (larva 

 iitate), magnified to about twice the natural size. 



a, bilobate supra-resophageal or cerebral gan- 

 glion, b, nerves passing to the antennae, c, c, 

 nerves passing to the eyes, d, int'ra-oesophageal 

 ganglion, with the nerves passing to the maxilla, 

 mandibles, and labium. e,e,e,e, thoracic and ab- 

 dominal ganglia, f, f, nervous filaments passing 

 to the caudal extremity of the larva. 



which are given off nerves to the antennas, and 

 and the eyes (b, c), and of twelve abdominal 

 ganglia, connected by longitudinal cords (c). 

 The difference in distance of these ganglia is 

 very remarkable, and worthy of mention. The 

 first, or true infra-oesophageal ganglion (d) is 

 situated as usual ; between this and the second 

 a long space intervenes, and the connecting 

 cords are firm and distinct; the spaces between 

 the second, third, fourth, and fifth are about 

 equal, and not more than one-third the distance 

 between the first described ; the remaining seven 

 are so closely approximated as to touch each 

 other: from the terminal ganglion are given 

 off long and minute nervous filaments, which 

 may be traced down to the caudal extremity 

 of the larva. 



In the Ilymenoptera a very great concen- 



tration and increased developement of the ner- 

 vous system is met with. In the Bee the 

 cerebral ganglion is of a large proportional size; 

 from its anterior part are given off two nerves, 

 which pass forward to the base of the antenna, 

 and have their origin marked by a very distinct 

 conical-shaped ganglionic enlargement. In the 

 thorax all the ganglia coalesce into one central 

 large ganglion, and a smaller one closely at- 

 tached to it, giving off lateral nervous filaments; 

 in the abdomen there are five smaller ganglia ; 

 they are connected by commissures, as in the 

 preceding classes, the double nature of which 

 is distinctly seen by a lens : the first abdominal 

 ganglion is situated at some distance from the 

 thoracic ganglion ; the second and third are 

 much nearer together, but the fourth and fifth 

 are quite closely approximated : from them are 

 given off radiating nerves. 



The most highly developed nervous system 

 in the Articulata occurs in the Lepidoptera, 

 the characters of which we shall next describe. 

 In the larva of the Saturnia pavonia minor 

 (Jig. 348), the nervous system consists of a 

 bilobate supra-cesophageal or cerebral ganglion, 

 and of twelve sub-oesophageal or abdominal 

 ganglia (a), united by longitudinal fissures. 



The cerebral ganglion consists of two closely 

 approximated oblong ovate medullary masses, 

 giving off nerves supplying the eyes and the 

 antennae, and a pair of nerves from its anterior 

 and lower parts, which takes a direction for- 

 wards, and which meeting inwards and joining, 

 forms a ganglion : from the posterior surface 

 of this ganglion arises a nerve (the recurrent of 

 Lyonnet) which passes backward beneath the 

 cerebral ganglion along the oesophagus, and 

 gives off filaments to it and the stomach.* The 

 existence of this nerve, and particularly its si- 

 tuation, is of very high importance, according 

 to the laws of philosophical anatomy : the 

 branching filaments it sends off form nervous 

 rings, which are important in being open be- 

 low and not above, and in developing a gan- 

 glion on the dorsal aspect of the animal ; this, 

 we shall find presently, leads by strong ana- 

 logies to the Vertebrata. The cerebral gan- 

 glion is supported or produced by two lateral 

 nervous filaments, which, having their origin 

 at its posterior part, pass downwards by the 

 sides of the oesophagus, at the inferior part of 

 which they converge, and are connected with 

 the first sub-oesophageal ganglion ; in this way 

 a nervous collar or ring is formed, which en- 

 circles the oesophagus. This first inferior gan- 

 glion (/>) is of rather a quadrilateral form; it 

 gives off a pair of nerves to the maxillae and 

 to the labium (c, d), which have their origin 

 within the termination of the lateral commis- 

 sures just described. The second ganglion is 

 longitudinally continuous with the first; from 

 them are given off lateral nerves. The (supposed) 

 motor and sensitive connecting nervous columns 

 are widely separate between the second and third 

 ganglia, and between the third and fourth ; 

 between all the others they are very closely 



* This nerve is considered by Mr. Newport as 

 the analogue of the par vagum. 



2 n 2 



