i86 



ANIMALS 



sense organ is sufficiently stimulated a process is set up which 

 travels along the receptor element to the central nervous 

 system. Here connection may be made directly with an effec- 

 tor element, and the stimulus pass out on another fibre to a mus- 

 cle or other response organ and bring the latter into action. 

 Or, the stimulus may be taken up by an association element, 





Fig. ioo. — A photomicrograph of a section through the ventral edge of the 

 brain of the crayfish. The black line represents the plane of symmetry. On 

 either side of it are groups of ganglionic cells and, farther out, part of the circum- 

 oesophageal connectives in cross section. Among the larger cells may be dis- 

 tinguished at least six pairs of cells. The two cells of each pair are alike in 

 size and exactly symmetrical in position and presumably their functions are 

 identical with regard to the corresponding side of the body. This example 

 seems to indicate that within the nervous system diflferentiation may extend 

 to individual cells. 



and through it transferred to an effector of the same or a distant 

 ganglion. Thus a response may occur in a distant portion of 

 the body. A stimulus originating in the periphery may first 

 pass to the brain, and from there return to the organs of response. 

 442. The superiority of the nervous system of the worm over 

 that of the Coelenterate is evident in the more complete dif- 



