232 CONTRIBUTIONS TO THE NATURAL HISTORY 



and of their developments, that, however diversified their parts are, their tissues remain 

 in the lowest degree of structure, — in a condition simihir to that in which all animals 

 are found at a certain period of their life when developing from the egg ; and if we 

 apply such a standard to tlie appreciation of the structure of the Medusae, we shall ac- 

 knowledge that they are animals of a low character of structure, although this structure 

 be highly diversified in itself. 



There is unquestionably a nervous system in INIedus^, but this nervous system does 

 not form large central masses to which all the activity of the body is referred, or from 

 wliicli it emanates. There is no regular communication by nervous threads between the 

 centre and periphery, and all intervening parts ; and the nervous substance does not con- 

 sist of heterogeneous elements, of nervous globules and of nervous threads, presenting 

 the various states of complication and combination, and the internal structural differences, 

 which we notice in the vertebrated animals, or even in the Mollusca and Articulata. 



In Medusae the nervous system consists of a simple cord, of a string of ovate cells, 

 forming a ring around the lower margin of the animal (Plate V. Fig. 11, 2, 4, 5), ex- 

 tending from one eye-speck to the other, following the circular chymiferous tube, and 

 also its vertical branches, round the upper portion of which they form another circle. 

 The substance of this nervous system, however, is throughout cellular, and strictly so, 

 and the cells are ovate. There is no appearance in any of its parts of true fibres. 



I do not wonder, therefore, that the very existence of a nervous system in the Me- 

 dusae should have been denied, and should not be at all surprised if it were even now 

 further questioned after this illustration. I would only urge those interested in this 

 question to look carefully along the inner margin of the cliymiferous tubes, and to search 

 there for a cord of cells of a peculiar ovate form, arranged in six or seven rows, forming 

 a sort of string, or rather similar to a chain of ovate beads placed side by side and point 

 to point, but in such a manner that the individual cells would overlap each other for one 

 half, one third, or a quarter of their length, being from five to seven side by side at any 

 given point upon a transverse section of the row; and would ask those who do not rec- 

 ognize at ence such a string as the nervous system to trace it for its whole extent, es- 

 pecially to the base of the eye-speck, where these cells accumulate in a larger heap, with 

 intervening colored pigment forming a sort of ganglion ; then, further, to follow it up 

 along the inner side of the radiating chymiferous tubes which extend from the summit of 

 the vault of the body, and to ascertain that here, again, it forms another circle around 

 the central digestive cavity, from which other threads, or rather isolated series of elongated 

 cells, run to the proboscis ; they will then be satisfied that this apparatus, in all its com- 

 plication, is really a nervous system of a peculiar structure and adaptation, with peculiar 

 relations to the other systems of organs. 



