Page:Popular Science Monthly Volume 9.djvu/736

708 of antennæ, and from the groups of ocelli on each side of the head, and they are connected by œsophageal cords with a bilobed infra-œsophageal ganglion, which distributes nerves to the jaws and other parts about the mouth. This bilobed infra-œsophageal ganglion is the first and largest of a series of ventral ganglia, numbering twenty-two in all, which are connected together by a double ventral cord. Every ganglion sends off nerves on each side to a pair of limbs.



From the posterior part of the brain, or from the œsophageal cords, the stomato-gastric nerves are given off, and distribute themselves over the alimentary canal in the usual manner.

Organs of vision become much more elaborate in crabs, spiders, and insects, than among worms or centipedes. And, while organs of touch and taste are further perfected in these higher arthropods, two new sensory endowments also become manifest. . These organisms, or at least all the higher forms of them, are capable of being impressed by and of discriminating the different odors of some substances anterior to the contact of such substances with their gustatory surfaces. This new power aids them in their search for or recognition of food. Such organisms are, in addition, capable of appreciating those vibrations of the medium they inhabit, which induce in us impressions recognized as sounds or noises. In other words, they acquire a rudimentary power of hearing.

These additional sensory endowments are of high importance to all organisms, but more especially to those possessing active powers of locomotion—serving, as they do on the one hand, to help to bring their possessors into relation with food, and, on the other, to warn them of the approach of enemies, of friends, or of sexual mates.

Among Crustacea great differences are met with in the degree of concentration of the nervous system, the variations being in the main dependent upon differences of external form in the respective members of the class. In some of the lower terms of the series allied to