NATURE OF THE INVERTEBRATE BRAIN. 7 c 7 



on either side, whose branches are distributed to the muscles and 

 parietes of adjacent segments. 



In this animal also a simple filament is given off from the poste- 

 rior part of the supra-cesophageal ganglion, and is distributed along 

 the dorsal aspect of the alimentary canal. It foreshadows an impor- 

 tant system of nerves corresponding partly with that of the " sympa- 

 thetic," and partly with the pneumogastric (or lung and stomach) 

 nerves in higher animals. This system is known among invertebrates 

 as the "stomato-gastric system." In other members of the inverte- 

 brate series it frequently takes its origin from the commissures con- 

 necting the upper and lower ganglia, rather than from the upper gan- 

 glion itself. The more complicated stomato-gastric system of the 

 earthworm has an origin of this kind. 



The kind of nervous system which pertains to the earthworm and 

 to the leech exists, with only comparatively trivial variations, through- 

 out the whole sub-kingdom Vermes. 



v 



The next sub-kingdom the Arthropoda comprises centipedes, 

 crabs, spiders, and insects. They are all characterized by the pos- 

 session of hollow and jointed organs of locomotion, containing dis- 

 tinct muscles, these appendages being represented among Vermes 

 only by lateral seta? or bristles of different kinds. The lowest types 

 of these various classes possess a nervous system closely analogous to 

 that existing among the various kinds of worms. In the more com- 

 plex types of crabs, spiders, and insects, however, we meet with a 

 great increase in the complexity of animal organization, and this in- 

 crease of complexity is shared in by the nervous system. Among 

 insects, for instance, the respiratory organs assume a marvelous de- 

 gree of elaboration, and the development of this system, together 

 with a correlated development of their nervous and muscular sys- 

 tems, contributes greatly to the enormous powers of locomotion for 

 which these denizens of the air are remarkable. The acuteness and 

 structural elaboration of their sense-organs is almost sure to be greatly 

 increased in such active creatures ; and, looking to the nature of the 

 intelligence in these lower animals, there is thus afforded an increas- 

 ing stimulus to brain-development and slightly higher brain-functions. 



Among the lower centipedes, such as lulus and Geophihts, in which 

 the limbs, though very numerous, are feeble and ill-developed, the 

 nervous system exhibits only a slight advance over the forms which it 

 presents among the higher Annelida (Fig. 3). But in the more pow- 

 erful predatory forms, of which the common centipede may be taken 

 as a type, a distinct advance is met with. This carnivorous animal 

 has a smaller number of well-developed limbs, aud its nervous system 

 closely resembles that found among caterpillars or the larvas of higher 

 insects. 



The supra-oesophageal ganglia receive nerves from the two pairs 



