124 



GENERAL PRINCIPLES OF ZOO LOOT. 



Ladder Nervous System. A widely recurring arrangement is 

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

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

 the example before 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 right and left, 

 surrounding the pharynx, to the supra- 

 O3sophageal ganglion. The supra- and infra- 

 oesophageal ganglia together with the 

 oesophageal commmissures form the cesopha- 

 geal ring, a nerve-ring surrounding the 

 oesophagus. 



Tubular System. The tubular type of 

 nervous system is found only in the chordates 

 (fig. 76). The vertebrate brain and spinal 

 cord may be regarded as parts of a tube with 

 greatly thickened walls, developed in differ- 

 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 



FIG. 75. Ladder nervous -i T , -, -> ^ . 



system of Porceiuo scatter are seen grouped around the central canal m 





a manner almost the reverse of that of the 

 ionic type. On the periphery lies a 

 f nerve-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 oil from the central canal by a special epithelium 

 (ependyma). 



Relations between the Nervous System and the Skin. For 

 almost all animals it has been ascertained that the nervous 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 tlje body (fig. 9). 



