12 



figure of 



(Nereis). 



MEDUSA. 



a part of the skin of a small worm 



Fig. 12.— Half of a cross section tlirongb the brain and hinder pair of eyes of Xereis 

 cullrifera (after Carriere). 1, Hypoderra ; 2, cuticle ; 3, retina ; i, outer coniPal 

 cells; 5, inner corneal cells; 6, brain ; .s, 8a, two places to which the brain sends 

 large nerves (9), but where the cuticle is unaltered ; g, gelatinous body. 



Amjsng the Medusae (jelly-fishes), also, the supposed 

 tactile organs are ciliated cells (Fig. 13), which scarcely 

 differ from the other epithelial cells, but which ter- 

 minate externally in a cilia, and internally in a nerve- 

 fibril. 



Fig. 13.— Part of upper nerve-ring and tactile epithelium of Lizzia (after Hertwig). 

 a, Tactile epithelium ; g, ganglionic cell ; nr'y upper nerve-ring. 



In other cases, the tactile hairs scarcely differ from 

 those covering the general surface. Fig. 14 represents 

 part of the skin of a sea-anemone, the lono- cylinders 

 are nematocysts, or thread-cells— elastic sacs in the 



