STKUCTUHE OF THE MEDUSAE. 17 



is somewhat thickened where it is in contact with 

 the veil. In these species the nerve-ring is only 

 indistinctly marked off' from the surrounding tissues. 

 But in other species the crowding together of the 

 nerve-fibres at the insertion of the veil gives rise to 

 a considerable concentration of nervous structures ; 

 while in others, again, this concentration proceeds 

 to the extent of causing a well-defined swelling of 

 nervous tissue against the epithelium of the veil 

 and umbrella. In the Geryonidse this swelling is 

 still further strengthened by a peculiar modification 

 of the other tissues in the neighbourhood, which had 

 been previously described by Professor Haeckel. In 

 all species the upper nerve-ring lies entirely in the 

 ectoderm. Its principal mass is composed of nerve- 

 fibres of wonderful tenuity, among which are to be 

 found sparsely scattered ganglion-cells. The latter 

 are for the most part bi-polar, more seldom multi- 

 polar. The fibres which emanate from them are 

 very delicate, and, becoming mixed with others, do 

 not admit of being further traced. Where the 

 nervous tissue meets the enveloping epithelium it 

 is connected with the latter from within, but diflTers 

 widely from it ; for the nerve-cells contain a longi- 

 tudinally striated cjdindrical or thread-like nucleus 

 which carries on its peripheral end a delicate hair, 

 while its central end is prolonged into a fine nerve- 

 fibre. There are, besides these, two other kinds of 

 cells which form a transition between the ganglion 

 and the epithelium cells. The first kind are of a 

 long and cylindrical form, the free ends of which 

 reach as far as the upper surface of the epithelium. 



