ARTHROPODA. 28 1 



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

 from the connective about the gullet, and from the ventral 

 ganglia. 



318. Organs of Special Sense. As the thickened cuti- 

 cular 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 com- 

 pensate 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 

 beyond 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. Figures illus- 

 trating the great variety of forms of such hairs should be 

 sought in the reference texts. 



At least three classes of organs have been described as 

 auditory among the arthropods: (a) vibratile hairs, as in the 

 case of the male mosquito (Fig. 40) ; (&) 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 Crustacea may be open or entirely closed. In 

 the former case the animal itself may place the otoliths in the 

 otocyst in the form of grains of sand. Recent investigations, 

 however, tend to show that the function of this organ is not 

 hearing, so much as that of informing the animal of its rela- 

 tion to the pull exerted by gravity, thus enabling it to keep 

 its equilibrium. 



There are two classes of eyes in the group: (a) compound 

 eyes, made up of numerous similar elements, as in the insects 

 and Crustacea, and (&) simple eyes ocelli found alone in 

 spiders and in many larvae, or in connection with the com- 

 pound eyes, as in many insects. 



