With kind regards to Mrs. Darwin, I remain, very sincerely and most respectfully yours,
Geo. J. Romanes.
To Professor E. Schäfer.
Dunskaith, Ross-shire.
My dear Schäfer,—I am glad to hear that your rest has been beneficial, and also about all the other news you give.
I should like to have your opinion about the meaning of the following facts.
In Sarsia gentle irritation of a tentacle or an eye-speck causes the polypite to respond, but not the bell (stronger irritation, of course, causes both to respond); this seems to show that there are nervous connections between the eye-specks and the polypite. By introducing cuts between former and latter, these connections may be destroyed—the tolerance of the tissue to such sections being variable in different cases, but never being anything remarkable. So far, then, the matter seems favourable to the nerve-plexus theory.
In another disc-shaped species of naked-eyed Medusæ with a long polypite, which I have called Tiaropsis indicans, from its habit of applying this long polypite to any part of the bell which is being injured, the localising function of the polypite is destroyed as regards any area of bell-tissue between which and the polypite a circumferential section has been introduced. In other words, the connections between the bell and the polypite, on which localising function of the latter depends, are exclusively radial. But not so the connections between the bell and the polypite, which render it possible for the one to be aware that something is wrong somewhere in the other. For if the whole animal be cut into a spiral with the polypite at one end, irritation of the other end of the spiral, or any part of its length, causes the polypite to sway about from side to side trying to find the offending body. And here it is important to observe that wherever a portion of one of the radial tubes occurs in the course of the spiral, irritation of that portion causes a much stronger response on the part of the polypite than does irritation of any of the general bell-tissue, even though this be situated much nearer to the polypite. This seems to show that the nervous plexus, if present, has its constituent fibres aggregated into trunks in the course of the nutriment tubes.
Thus far, then, I should be inclined to adopt the nerve-plexus theory. But lastly, we come to another species with a very large bell and a very small polypite. Irritation of margin or radial tubes causes the animal to go into a violent spasm, but irritation of the general muscular layer only causes an ordinary locomotor contraction. On cutting the whole animal into a spiral, and irritating the extreme end of several marginal strips, the entire muscular part of the spiral goes into spasm. On interposing a great number of interdigitating cuts in the course of the spiral, there is no difference in these results. Now the question is, What is the nature of the tissue that conducts impressions from the ganglionic tissue to the muscular, making the latter go into a spasm? A spasm is as different as possible from an ordinary contraction, and will continue to pass long after the ordinary contractions have been blocked by severity of section. It is scarcely possible to suppose a nerve-plexus here—the tolerance towards section being so great, although it varies in different cases. Besides, suppose this to be a segment of animal cut as represented. On irritating margin at a all the bell goes into a spasm, and it is evident that whatever the nature of the conductile tissue, all the connections must pass through the tract of tissue at b. Yet on irritating that tract no spasm is given. I cannot understand this on any view as to the nature of the conductile tissue.
