Page:Popular Science Monthly Volume 9.djvu/735

Rh 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 posterior part of the supra-œsophageal ganglion, and is distributed along the dorsal aspect of the alimentary canal. It foreshadows an important system of nerves corresponding partly with that of the "sympathetic," 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 invertebrate series it frequently takes its origin from the commissures connecting the upper and lower ganglia, rather than from the upper ganglion 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, throughout the whole sub-kingdom Vermes.

The next sub-kingdom—the Arthropoda—comprises centipedes, crabs, spiders, and insects. They are all characterized by the possession of hollow and jointed organs of locomotion, containing distinct muscles, these appendages being represented among Vermes only by lateral setæ 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 complex types of crabs, spiders, and insects, however, we meet with a great increase in the complexity of animal organization, and this increase of complexity is shared in by the nervous system. Among insects, for instance, the respiratory organs assume a marvelous degree of elaboration, and the development of this system, together with a correlated development of their nervous and muscular systems, 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 increasing stimulus to brain-development and slightly higher brain-functions.

Among the lower centipedes, such as Iulus and Geophilus, 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 powerful 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, and its nervous system closely resembles that found among caterpillars or the larvæ of higher insects.

The supra-œsophageal ganglia receive nerves from the two pairs