CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS. 101 



boundary of the ' medulla/ so that the latter part may 

 be regarded as the more highly developed portion of the 

 spinal cord by which fusion with the brain is effected. 



The visceral nerves are well developed in the higher 

 Arachnida. They consist of one or two filaments, on 

 which a ganglion may exist, in connection with the posterior 

 part of the brain, and thence proceeding to the stomach 

 and other internal organs. There are, moreover, two or 

 three branches given off from the great ventral ganglion 

 which, after passing through smaller ganglia, distribute 

 numerous filaments to the intestines, the respiratory 

 and genital organs, as well as other viscera. The former 

 set may be in the main afferent, and the latter perhaps 

 principally efferent visceral nerves. 



Organs of vision are much more elaborate in Crustacea, 

 Spiders and Insects, than among Worms or Centipedes. 

 And, whilst organs of touch and taste are further perfected, 

 two sensory endowments, found among higher Mol- 

 lusks, seem also to manifest themselves. These higher Ar- 

 thropods are capable of being impressed by, and of discri- 

 minating, the different odours of some substances anterior 

 to their contact with the mouth. This power must mate- 

 rially aid them in their ' search ' for or recognition of food. 

 Some Arthropods seem to be also capable of appreciating 

 those vibrations of the medium they inhabit which induce 

 impressions recognizable by us as sounds or noises. Still, 

 in some of the most highly organized forms of Insects 

 a sense of hearing appears to have no existence. Much 

 uncertainty, in fact, exists in regard to this sense-endow- 

 ment.* Extreme sensibility of the tactile order may 

 cause the organism to display an apparent sensitiveness 

 to sounds. A delicate general ability to appreciate aerial 

 vibrations, therefore, must not be confounded with the 

 * See pp. 65 and 205. 



