396 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



both the upper and lower surfaces of the velum have 

 some of the constituent cells converted into -aease 

 _fft1lR-; 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 



* 



another, an 



" 



pP tiisLr>f 



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 gangl ionic 

 (Fig. 167); tlii3_Tiftt r 

 with 



Fig. 168. Transverse Section 

 through aTentacle of Tealiacras- 



marginal nerve-ring. 

 Here, then, we have a 

 simple example of an 



sicornis ; to show (a) Sensory a o Te o> ated f,p*'s*l 





Cells with their free Projecting 

 Processes, and their Bases con- 

 tinued into the Nervous Layer; 

 (b), supporting 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. 



