BEGINNING OF NERVES. 



305 



Fig. 1. 



contractile jelly ; but the whole surface of the polypite, and the whole 

 co?icave surface of the bell, are overlaid by a thin layer, or sheet, of 



contractile tissue. This tissue con- 



stitutes the earliest appearance in the 

 animal king-dom of true muscular 

 fibres. The thickness of this continu- 

 ous layer of incipient muscle is pretty 

 uniform, and is nowhere greater than 

 that of very thin paper. The mar- 

 gin of the bell supports a series of 

 highly contractile tentacles, and also 

 another series of bodies which are of 

 great importance in the following re- 

 searches. These are the so-called 

 marginal bodies, which are here rep- 

 resented, but the structure of which I 

 need not describe. Lastly, it may 

 not be superfluous to add that all 



the Medusm are locomotive. The mechanism of their locomotion is 

 very simple, consisting merely of an alternate contraction and relaxa- 

 tion of the entire muscular sheet which lines the cavity of the bell. At 

 each contraction of this muscular sheet the gelatinous walls of the bell 

 are drawn together ; the capacity of the bell being thus diminished, 

 water is ejected from the open mouth of the bell backward, and the 

 consequent reaction propels the animal forward. In these swimming 

 movements systole and diastole follow one another with as perfect a 

 rhythm as they do in the beating of a heart. 



Previous to my researches, the question as to whether or not the 

 Medusae, possess a nervous system was one of the most vexed questions 

 in biology — some eminent naturalists maintaining that they could de- 

 tect microscopical indications of nervous tissues, and others maintain- 

 ing that these indications were delusive — the deliquescent nature of 

 the gelatinous tissues rendering microscopical observation in their case 

 a matter of great difficulty. But amid all this controversy no one ap- 

 pears to have thought of testing the question by means of physiologi- 

 cal experiments as distinguished from microscopical observations. Ac- 

 cordingly, I made the experiment of cutting off now one part and now 

 another part of a jelly-fish, in order to see whether by so doing I could 

 alter the character of its movements in such a way as to show that I had 

 removed nerve-centres or ganglia. The results which I obtained were 

 in the highest degree astonishing. For, on removing the extreme 

 margin of the swimming-bell, I invariably found that the operation 

 caused immediate, total, and permanent paralysis of the entire organ. 

 That is to say, if, with a pair of scissors, I cut off the whole marginal 

 rim of the bell, carrying the cut round just above the insertion of the 

 tentacles, the moment the last atom of the margin was removed, the 



VOL. XIT. — 20 



