284 ZOOLOGY 



ments there is a massing and fusion of the corresponding ganglia; 

 and in general, everything considered, those ganglia are best de- 

 veloped which lie in the best-developed somites. The concen- 

 tration of the ganglia of the ventral cord may continue until 

 they form practically one mass. Nerves arise from the brain, 

 from the connective about the gullet, and from the ventral 

 ganglia, and pass to sensory organs and to the muscles of the 

 body and the appendages. 



325. Organs of Special Sense. As the thickened cuticular 

 covering of the arthropods develops, it is apparent that much 

 of the sensitiveness of the surface to external conditions must 

 be lost unless special structures are produced to compensate 

 for this by the reestablishment of connection between the 

 internal organs and the outside world. Such structures we 

 find in the chitinous hairs of various shapes which project be- 

 yond the surface and in pits or canals which pierce the skeleton. 

 These all have nervous connections and have been variously 

 interpreted as tactile, taste, auditory, and olfactory organs. 

 They are especially abundant in the more movable portions, 

 particularly those about the mouth, though they may be found 

 over the whole surface of the body. Figures illustrating the 

 great variety of forms of such hairs should be sought in the 

 reference texts. 



It has been shown that the crayfish and many insects are 

 sensitive to chemical stimuli. These sensations have to do with 

 selection of food and with testing the character of the water 

 or air. They are analogous to our sensations of taste and smell. 

 The organs may be scattered over the body or more commonly 

 localized on the appendages about or in front of the mouth. 



At least three classes of organs have been described as audi- 

 tory among the arthropods : (a)* vibratile hairs, as in the case 

 of the male mosquito (Fig. 42); (b) otocysts, as in many aquatic 

 forms; (c) a tympanum or membrane in connection with which 

 are special nervous cells for the reception of the vibrations (as 

 in the grasshopper and other insects) . The otocysts of the Crus- 

 tacea may be open or entirely closed. In the former case the 

 animal itself may place the otoliths in the otocyst in the form 



