﻿246 CONTRIBUTIONS TO THE NATURAL HISTORY 



I have little to add upon the nervous system, after what has been remarked above. 

 Its structure is the same wherever it occurs in naked-eyed Medusae, consisting of ovate 

 cells, arranged in strings, which are, however, more elongated in Sarsia than in Hippo- 

 crene. The main cord is that which extends along the lower margin of the body, form- 

 in^ a circle close to the inner side of the circular chymiferous tube. This cord, how- 

 ever, is not continuous all round the animal, as the four sensitive bulbs are interposed in 

 its circumference. We should therefore view it, in this type, as four strings connected 

 by four ganglia, in which the thread-like arrangement is lost in the mass of the bulb. 

 As for its position, it follows the inner margin of the chymiferous tube, and may be easily 

 recognized upon any point of the circumference, close to that tube, on its inner margin ; 

 near the bulb it rises distinctly towards its base on its posterior side, into which it merges. 

 Contractile fibres placed on the sides of this ganglion, and a thickening of the substance 

 of this part, prevent a clear perception of the sensitive elements in their connection with 

 the bulb ; but parts of this system rise along the vertical tubes on their inner side, and 

 follow their whole tract up to the central cavity, around which they form another ring, 

 connecting the four vertical threads and encircling here, again, the central parts of the 

 alimentary cavity. From this upper ring, thread-like cells may be traced downwards 

 along the proboscis ; and, as already mentioned, I am almost convinced that they follow 

 the whole tract of the proboscis between the epithelial and muscular cells. Again, along 

 the main bundles of vertical muscles, there are similar threads, which seem also to 

 belong to the sensitive system, and to unite with the circular cord below, between the 

 main bulk of these bundles, at their origin from the lower margin. 



The resemblance, however, between the many variations of form in the contractile 

 cells, (from the appearance of mere caudate cells to that of bicaudate cells, or mere 

 thread-like cells,) and the numerous modifications of a similar kind observed among the 

 sensitive cells, is so close, that I do not yet venture, in every case, to distinguish be- 

 tween them ; so that, among these various sets of apparatus, which I have referred to 

 two distinct systems, there may be some of the elements which belong truly to but one 

 system, and it will require still more extensive investigations to decide with any cer- 

 tainty whether the proboscis is provided with sensitive cells at all, and whether the 

 threads between the muscular cells are also muscular, or whether the shorter cells be- 

 tween the vertical, muscular bundles are nervous cells. Even some cells joining the 

 upper ring, and accompanying it, may be only motory cells, and not sensitive elements. 



But whatever be the final result of the investigation upon these few points, so much 

 is already ascertained, — that there are rows of sensitive cells distinct from the contrac- 

 tile cells, though their constituent elements are very closely allied in their structure ; and 



