Romanes first carefully observed the movements of the Medusæ, which, it will be remembered, are effected by the dilatation and contraction of the entire swimming bell, and he found that if, in the 'naked-eyed' group, the extreme margin of this swimming bell be excised, immediate, total, and permanent paralysis of the whole organ took place. This result was obtained with every species of this group which he examined; he therefore concluded that in the margin of all these forms there is situated a localised system of centres of spontaneity, having for one of its functions the origination of impulses to which the contraction of the swimming bell is, under ordinary circumstances, exclusively due. This deduction was confirmed by the behaviour of the severed thread-like portion of the margin, which continued its rhythmical contractions quite unimpaired by its severance from the main organism, the latter remaining perfectly motionless. In the 'covered-eyed' forms Romanes found that excision of the margin of the umbrella, or rather excision of the sense organs or marginal bodies, produced paralysis; in this case, the paralysis was of a temporary character, as in the great majority of cases contractions were resumed after a variable period. From this series of experiments he was led to believe that in the 'covered-eyed' Medusæ the margin is the principal, but not the exclusive, seat of spontaneity, there being other locomotor centres scattered throughout the general contractile tissue of the swimming bell.
Having demonstrated the existence of a central nervous system capable of originating impulses, Romanes had yet to prove the identity of this nervous tissue of the Medusæ with that of nervous tissues in general: therefore, he next proceeded to test whether it was also capable of responding to external stimulation by light, heat, electricity, &c.
As regards appreciation of light, he was able to prove conclusively for at least two species of the 'naked-eyed' forms that as long as their marginal bodies remained intact they would always respond to luminous stimulation, and would crowd along a beam of light cast through a darkened bell jar in which they were swimming; if their marginal bodies were removed, they remained indifferent to light. With regard to the 'covered-eyed' forms, he obtained sufficient evidence to induce him to believe they possessed a visual sense localised in their marginal sense organs.
The effects of electrical stimulation agreed in all respects with those produced on the excitable tissues of other animals. He next experimentally investigated in the jelly-fish the paths along which the nervous impulses must pass in their passage from the locomotor centres, where they originate, to the general contractile tissues of the animal.
The results of these experiments led him to infer the existence of a very fine plexus of nerve fibres, in which the constituent threads cross and re-cross one another without actually coalescing. This conclusion, which he arrived at from purely experimental grounds, was some years afterwards confirmed by minute histological research.
Finally, the effect of various poisons, chloroform, alcohol, &c., was tried, and the striking resemblance of their action on the nervous system of the Medusæ with that which they exert on that of higher animals supports the belief that nerve tissue when it first appears in the scene of life has the same fundamental properties as it has in higher animals.
This piece of work was important, as the facts threw light, as Professor Sanderson has said, on elementary questions of physiology relating to excitability and conduction, and it was a characteristic of Mr. Romanes that in all his work, of whatever kind, he was always searching for principles. The minutest detail never escaped his attention if it appeared at all likely in any way to throw light on some biological or psychological problem. Only a trained scientific worker can appreciate the amount of labour these Royal Society papers represented. In 1875 he gave a Friday evening lecture at the Royal Institution on his work on Medusæ.
He was also at this time working on the subject of 'Pangenesis,'[7] and a series of letters to Mr. Darwin and to Professor Schäfer may interest some readers.
18 Cornwall Terrace, Regent's Park, N.W.:
January 14, 1875.