396 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



both the upper and lower surfaces of the velum have 

 some of the constituent cells converted into sense 

 cells ; the basal ends of these are of some length, and 

 pass into a nervous ring which runs round the edge 

 of the bell. The several sensory cells are thereby 



brought into connection 

 with one another, and the 

 consentaneous action of 

 all parts of the jelly-fish 

 is thus ensured. 



Underlying the epi- 

 thelium of the lower sur- 

 face of the bell, and placed 

 between it and the mus- 

 cles, is a network of nerve 

 fibres, among which there 

 are scattered ganglionic 

 cells (Fig. 167); this net- 

 work is connected with 

 the marginal nerve-ring. 

 Here, then, we have a 

 simple example of an 

 aggregated central ner- 

 vous system, together 

 with a peripheral 

 system of fibres and cells, 

 which is diffused over the whole of the under surface 

 of the bell of the medusa. Some of the Craspedota 

 (e.g. Carmarina) present us with an important advance 

 in structural differentiation, for some of what, in all 

 other particulars, resemble the sense cells, are found to 

 have lost their free projecting process, and to be now 

 moved a little away from the surface of the body. 

 Here, then, we have nervous epithelial cells which 

 are beginning to lose their superficial position, and 

 sinking deeper into the substance of the or- 

 ganism.. 



Fig. 168. Transverse Section 

 through aTentacle of Tealia cras- 

 sicornis ; to show (a) Sensory 

 Cells with their free Projecting 

 Processes, and their Bases con- 

 tinued into the Nervous Layer; 

 (b), supporting cells. 



