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THE POPULAR SCIENCE MONTHLY. 



of antennae, and from the groups of ocelli on each side of the head, 

 and they are connected by oesophageal cords with a bilobed infra- 

 oesophageal ganglion, which distributes nerves to the 'jaws and other 

 parts about the mouth. This bilobed infra-oesophageal 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. 



Fig. 3. Brain and Adjacent Parts op Nervous System of Iulds. 



From the posterior part of the brain, or from the oesophageal 

 cords, the stomato-gastric nerves are given off, and distribute them- 

 selves 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 ante- 

 rior 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 vibra- 

 tions of the medium they inhabit, which induce in us impressions 

 recognized as sounds or noises. In other words, they acquire a rudi- 

 mentary 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 witn in the degree of 

 concentration of the nervous system, the variations being in the main 

 dependent upon differences of external form in the respective mem- 

 bers of the class. In some of the lower terms of the series allied to 



