124 



GENERAL PRINCIPLES OF ZOOLOGY. 



Ladder Nervous System. — A widely recurring arrangement is 

 that termed the ladder nervous system (of annehds and arthropods) 

 (fig. 75). Numerous pairs of ganglia (in 

 the examjilo Ijefore us, nine) lie in serial 

 order on the ventral side of the animal, and 

 are connected by longitudinal commissures 

 (connectives), and also by transverse com- 

 missures connecting the left and right 

 ganglia. The first pair of the series is 

 formed by the infra-oesophageal ganglion, 

 which sends out commissures riglit and left, 

 surrounding the pharynx, to the supra- 

 ffisoiihageal ganglion. The supra- and infra- 

 o3sopliageal ganglion together with the 

 ffisophageal commmissures form the cesoplta- 

 (jeid rintj, a nerve-ring surrounding the 

 ojsophagus. 



Tubular System. — The tubular type of 

 nervous system is found only in the chordates 

 (fig. 70). The vertebrate brain and spinal 

 cord may be regarded as parts of a tube with 

 greatly thickened walls, developed in difi^er- 

 ent ways. In the centre lies the extremely 

 narrow central canal, which widens anteriorly 

 into the several ventricles of the brain. In 

 a transverse section the nervous elements 



Fia. 7o. — Ladder nervous n n , i ,1 1 • 



system of Porftnin wnfier are Seen grouped around the central canal m 

 *4Tr\ta;A^"utr^a?<foni! a manner almost the reverse of that of the 



On the periphery lies 



r?hf »so"p'£Jil''cr ganglionic type. 



Seri"r'ega?dedaT.tm^;a:l^yer of uerve-fibres (the 'white matter -of 



^'"''"^- human anatomy) ; next is a central portion 



formed of ganglion-cells and nerve-fibres (the 'gray matter'), 



which is marked off from the central canal by a sj^ecial epithelium 



(ejiendyma). 



Relations between the Nervous System and the Skin. — For 

 almost all animals it has been ascertained that the nerrous system 

 arises from the ectoderm. Therefore, in many animals, the nerve- 

 cords and the ganglionic masses lie pemanently in the skin: in 

 others only during the development, later becoming separated by 

 splitting off or by infolding, and thus coming to lie in the deeper 

 layers of the body (fig. '••). 



