RESPONSIVE LIFE OF ORGANISMS 



449 



Ganglia are local centers, which exercise a regulatory 

 function within a more or less localized portion of the body. 

 Their number and arrangement differs according to the 

 structural plan of the body to which they belong. In 

 segmental animals, like the earthworm, the ganglia are 

 arranged segmentally, each ganglion pair of the ventral 

 chain (nerve cord), controlling primarily its own segment. 

 In all arthropods, as we have seen, they are arranged upon 

 the ventral side; in vertebrates, upon the dorsal side. 

 They are arranged so differently in the different animal 

 phyla as to indicate that they have been brought together 

 on different developmental lines, in accordance with the 

 conditions offered by each structural type. 



The neurone. — Nerve 

 cells by migration to the 

 centers, have become 

 widely removed from the 

 receptors at the surface 

 of the body. That there 

 are nerve fibers connect- 

 ing the two together has 

 long been known (the 

 comparison of the fibers 

 to telegraphic lines being 

 a very old and familiar 

 one); but that these 

 fibers ar$ an integral part 

 of the central cells is a 

 matter of recent knowl- 

 edge, derived from the 

 T. ^ro ^ . , , , . study of their develop- 



FiG. 258. Ontogeny and phyloReny in neu- •' ^ 



rones of the middle layer of the cerebral ^lent. Thc flbcr aHsCS aS 

 cortex. A, B, C, Corresponding cells trom 



the adult brain in frog, lizard and rat g^^ OUterOWine prOCCSS 

 respectively, a, to e. successive stages ot ^ , 



development of cells of the same sort in fj-Qj-Q the CcU bodv (fig- 

 a mammal: the axone appears in a, den- - ^ o* 



drites in b and collaterals, in d, 2 c; 8) There maV bc One 



(after ajal) / ' J 



