18 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



The second kind lie for the most part under the 

 upper surface. They are of a large size, and pre- 

 sent, coursing towards the upper surface, a long 

 continuation, which at its free extremity supports a 

 hair. In some cases this continuation is smaller, 

 and stops short before reaching the outer surface. 

 Drs. Hertwig observe that in these peculiar cells we 

 have tissue elements which become more and more 

 like the ordinary ganglion-cells of the nerve-ring 

 the more that their lona: continuation towards the 

 surface epithelium is shortened or lost, and these 

 authors are thus led to conclude that the upper 

 nerve-ring was originally constituted only by such 

 prolongations of the epithelium-cells, and that after- 

 wards these prolongations gradually disappeared, 

 leaving only their remnants to develop into the 

 ordinary ganglion-cells already described. 



Beneath the upper nerve-ring lies the lower 

 nerve-ring. It is inserted between the muscle- 

 tissue of the veil and umbrella, in the midst of 

 a broad strand wherein muscle-fibres are entirely 

 absent. It here constitutes a thin though broad 

 layer which, like the upper nerve-ring, belongs to 

 the ectoderm. It also consists of the same elements 

 as the upper nerve-ring, viz. of nerve-fibres and 

 ganglion-cells. Yet there is so distinct a difference 

 of character between the elements composing the 

 two nerve-rings, that even in an isolated portion 

 it is easy to tell from which ring the portion has 

 been taken. That is to say, in the lower nerve- 

 ring there are numerous nerve-fibres of considerable 

 thickness, which contrast in a striking manner with 



