140 



PRINCIPLES OF ANIMAL BIOLOGY 



oes- 



oes- 



highly developed but it is not highly centrahzed. It consists of a chain 



of ganglionic masses extending ventral to 

 the alimentary tract from near the anterior 

 end to the posterior end of the worm. In 

 the anterior end the nerve cord divides, 

 the two halves together encirchng the 

 pharynx and uniting above it, at the junc- 

 ture of the pharynx and buccal pouch, to 

 form the so-called brain, which in reality 

 is no more than a pair of ganglia. From 

 the brain, branches extend forward to the 

 prostomium and to the other organs of the 

 first few somites. There is a ganglionic 

 mass in each somite, from which paired 

 nerves extend out to the body wall and 

 there branch out to all parts of the somite. 

 The nervous system of the Arthropoda 

 has certain points in common with that of 

 the Annelida. It lies ventral to the digest- 

 ive system, which it encircles at the an- 

 terior end, and is composed of ganglia from 

 which nerves arise. Whereas in the an- 

 nelids the paired ganglia of each somite 

 are fused into one ganglionic mass, in some 

 Arthropoda they have a ladder-like arrange- 

 ment as in Fig. 110, A, and in others the 

 ganglia are fused into a chain (Fig. 110, B) 

 like that seen in the earthworm. A more 

 advanced type of nervous system in which 

 the ganglia of the chain are fused into larger 

 ganglionic masses from which nerves arise 

 .u'""- 110-— Nervous systems of j^ay be seen in Fig. 110, C. In this type 



annropods, showing advance in ... . 



centralization has gone further than m the 

 preceding forms. 



Concentrated System of Vertebrates. — 

 Concentration of the main masses of the 

 nervous system into organs of limited ex- 

 tent is carried farther in the Vertebrata 

 where the nervous system is highly organ- 

 ized. Even within the group of the Verte- 

 brata, the nervous system shows a con- 

 siderable increase in complexity and in the 

 degree of centralization from the condition in the lower vertebrates to 

 that in man. The central nervous system of the frog (Fig. Ill) consists 



B 



centralization. A, ladder-like 

 nervous system of Limnadia, an- 

 terior ' portion only {after 

 Klunzinger) ; B, Astacus fluviatilis 

 {after Vogt and Yung) ; C, Maia 

 squinado {after Milne-Edwards) ; 

 hg, subesophageal ganglion; g, 

 brain or supraesophageal ganglion; 

 oes, esophagus; so, esophageal com- 

 missure; y, postesophageal com- 

 missure. {From Sedgwick's 

 Student's Textbook of Zoology, 

 after Lang.) 



