12 



MEDUSiE. 



figure of a part of the skin of a small worm 

 (Nereis). 







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

 cultrifera (after Carriere). 1, Hypoderni ; 2, cuticle ; 3, retina ; 4, outer corneal 

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

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



Among 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. 



r\^^^. 



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

 a, Tactile epithelium; g, ganglionic cell; nr\ upper nerve-riug. 



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 long cylinders 

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



