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 eases 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 long 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 
