170 GENERAL ZOOLOGY 



responses, such as one might expect in the early stages of the 

 evolution of the nervous system. The system has remained at 

 its site of origin in the surface layer of the body (Fig. 111). 



Primitive Ganglionic Nervous System. — The nervous system 

 of a flatworm, such as Euplanaria, illustrates an early step in the 

 process of centralization of a portion of the nervous mechanism. 

 This has been brought about by the development of a pair of 

 cephalic ganglia, in the anterior end of the animal, from which a 

 pair of nerve cords extends posteriorly to the tip of the body. 

 Both the ganglia and the nerve cords are completely covered 

 over by the integument. From both ganglia and nerve cords, 

 nerve fibers extend to various parts of the body. The anterior 

 end of the body is provided with special sense organs, such as 

 eyes, tactile organs, and organs of chemical sense. The concen- 

 tration of sense organs at the anterior end is correlated with the 

 presence there of the cephalic ganglion, the general result of 

 which is to convert this region of the body into a "head end." 

 When Euplanaria explores new territory, the part of the body 

 that leads the way is the head end, which is the part best fitted 

 with means of sensing the environment. Thus the cephalic 

 ganglion may be regarded as a kind of brain that serves as a 

 center for correlating the sensory and motor mechanism. As a 

 result the head end assumes a position of dominance or control 

 over the activities of the rest of the body. That the head end 

 of Euplanaria is dominant in a physiological sense has been 

 demonstrated by experiments that show that the rate of meta- 

 bolism is higher in the head end than elsewhere. The evolution 

 of a head end, i.e., cephalization, is bound up with the evolution 

 of the nervous system and sense organs (Fig. 52). 



Ganglionic Nervous System. — A further elaboration of the 

 type of nervous system found in flatworms occurs in a large num- 

 ber of invertebrates such as segmented worms (Annelida), and 

 crabs, spiders and insects (Arthropoda). In these forms the 

 general functional significance of the ganglia is the same as in the 

 flatworms, but the arrangement and specific forms of the ganglia 

 are influenced by a morphological factor, segmentation, that is 

 absent in flatworms. A segmented worm, such as an earthworm, 

 differs from a flatworm, among other things, in that the body of 

 the earthworm is composed of a linear series of parts, called 

 segments or metameres, of which mention was made in connection 



