Page:The Zoologist, 4th series, vol 2 (1898).djvu/224

192 or grappling-iron, and when in the act of gripping the tufts expanded like a hand. It was evident therefore that these worms were specially adapted for clinging firmly to their host, and I found it rather difficult to detach them; but with the assistance of a friend, who is rather clever in manipulating for the microscope, I managed to get one mounted. The head of the worm, I noticed, was ciliated, and there were from four to six worms on this specimen, if I remember correctly. I omitted to make a note of it, although on others I found fully six worms.

The conclusion one would naturally draw from the above facts is, I think, that the irritation produced by several of such worms, or possibly the desire of the mollusc to accommodate them, is sufficient to account for the depression of the head and the consequent distortion of the shell. On examining two normally formed specimens of Planorbis complanatus, I found that one carried worms and that the other did not. It does not, however, follow that because one animal carried worms and had a normal shell, that therefore my theory will not hold good, since the worms might have only recently attached themselves, and the amount of irritation would naturally be proportionate to the number of worms. I may here mention that the Planorbis was only half-grown, and that the head and tentacles were strongly ciliated. It may be that the worms derived some advantage from this circumstance, since in very stagnant water the currents set up would bring both food and oxygen. But I imagine that the main benefit derived would be from the fact of being transported about, and that from a position of great security, Under irritation the worms evinced an uncontrollable desire to divide. This operation was performed once, and almost a second time. A constriction took place at a certain point in the body, and gradually became more and more pronounced. Then the body from time to time gave some spasmodic twitches and bent upon itself at the constricted point. A few more spasmodic twitches followed, and the trick was done.

From the facts here stated I think a fairly good prima facie case is made out, and if other observers who happen to meet with scalariform Helices, &c, would examine the head closely, they would most likely find some irritating parasite to account for the abnormality.

On mentioning the above circumstances to a gentleman at South Kensington Museum, he cited the case of a scalariform Turritella, which when found had on its head a parasitic crustacean. The name of the worm above alluded to is Chætogaster limnæi, Von Baer.— (The Croft, Hastings).