contractile jelly; but the whole surface of the polypite, and the whole concave surface of the bell, are overlaid by a thin layer, or sheet, of 
Fig. 1 contractile tissue. This tissue constitutes the earliest appearance in the animal kingdom of true muscular fibres. The thickness of this continuous layer of incipient muscle is pretty uniform, and is nowhere greater than that of very thin paper. The margin of the bell supports a series of highly contractile tentacles, and also another series of bodies which are of great importance in the following researches. These are the so-called marginal bodies, which are here represented, but the structure of which I need not describe. Lastly, it may not be superfluous to add that all the Medusæ are locomotive. The mechanism of their locomotion is very simple, consisting merely of an alternate contraction and relaxation 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 Medusæ, possess a nervous system was one of the most vexed questions in biology—some eminent naturalists maintaining that they could detect microscopical indications of nervous tissues, and others maintaining 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 appears to have thought of testing the question by means of physiological experiments as distinguished from microscopical observations. Accordingly, 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
