148 



GENERAL PRINCIPLES OF ZOOLOGY. 



cesophageal commissure form the oesophageal ring, a nerve- 

 ring surrounding the oesophagus. 



Tubular System. The tubular 

 type of nervous system is found only 

 in the vertebrates (Fig. 73), and in 

 the larva of tunicates, which are 

 closely related to the vertebrates. 

 The vertebrate brain and spinal cord 

 may be regarded as parts of a tube 

 with greatly thickened Avails, devel- 

 oped in different ways. In the centre 

 lies the extremely narrow spinal canal, 

 which anteriorly widens into the sev- 

 eral ventricles of the brain. In a 

 transverse section the nervous ele- 

 ments are seen grouped around the 

 spinal canal in a manner almost oppo- 

 site to that described above for the 

 ganglionic type. On the periphery 

 lies a layer of 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 off from the central 



FIG. 72. Ladder nervous sys- . . . , .... 



tem of Porcciiio scaber (AS- canal by a special epithelium. 



sel). A, brain; B, ventral 



cord, connected with the brain RelatlOHS bctWCCn the NCfVOUS 



by the oesophageal commis- 



sures; 6, a cord formerly re- System and the Skin. r or almost 



garded as sympathicus. (Af- 

 ter Leydig.) 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 permanently 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. 9, p. 46). 



III. Sensory Organs. 



Sensations of the Lower Animals. What we know 

 of the character of the external world is founded upon ex- 



