Midland Naturalist/Volume 01/Note On Œcistes Pilula

by Arthur Winkler Wills

At the meeting of the Birmingham Natural History and Microsoppical Society, held June 11th, 1878. (see "Midland Naturalist” at p. 202,) I exhibited the very rare Rotifer Melicerta pilula, or, more correctly, Œcistes pilula, which I had then just found in Sutton Park. The history of this species appears to be as follows:—In the journal of the Quekett Club, 1868, this animal was described by Mr. J. G. Tatem as a variety of Melicerta, in which “only rudely shaped excrementitious masses adherent to the gelatinous investment are observed." but no distinctive specific name was suggested for it. This description was accompanied by drawings, which are fairly accurate so far as they go.

In "Science Gossip," 1872, Dr. F. Collins described the same organism as a new species, and gave a very incorrect account of it, stating that "the pellet with which the animal builds its tube is formed in a kind of sac, situated at the lower extremity of the abdomen,” &c., In the "Monthly Microscopical Journal,” July 1st, 1872, Mr. C. Cubitt takes this species as illustrative of the structural differences between Flosculariæ and Melicertidæ, and speaks of it as a form with which he had bean acquainted for some years, and which he had called M. pilula, from the fact that "she fortifies the gelatinous basis of the theca with her own excrementitious pitules.”

In this paper the author preposed to divide the whole thecated suction of Rotifers into two families only, distinguished primarily by the position of the marginal wreath of setæ and the cingulum or secondary belt of cilia and of the ganglion, relatively to a line or axis drawn from the month to the anus. Of these two families he proposed to make Melicertidæ include the genera Melicerita, Œcistes, Limnias, and Tubicolaris under the term Melicerta, while Canochilus, Lacinularia, and Megalotrocha were ta be grouped together under the common tame of Lacinularia.

But later observers have added several species to each of the old genera, the characters of which are sufficiently distinct ta justify the retention of the older divisions. The differences of the form of the disc is a sufficient (distinction between Melticerta on the one hand and Limnias and Œcistes on the other, while the two latter are separated by the different form of the lobes, the character of the theca, and their general habit.

If we accept the genus Œcistes at all, the species we are now describing should certainly be included in it. My friend, Dr. C. T. Hudson, says on this point—"They are Œcistes, and good specimens of the genus."

Mr. Cubitt’s description of the singular habit of this animal is quite correct, but he does not appear to have observed the precise manner in which the remarkable operation is performed, from which it derives its name. It is self-evident that only a minority of the excrementary pellets discharged by the creature can be required or used to fortify its theca. The larger part are whirled away from the vicinity of the animal in the manner familiar to all who have observed the thecated Rotifers or the freshwater Polyzoa—but those which are utilised for building purposes are ejected between the rotifer and its tube or theca, and received under the lower margin of the ciliated trochus, where they remain for a few seconds as if the animal were making sure of its proper hold, and then by & sudden retraction of its body it dabs the pellet into a proper position on the margin of the theca, and instantly resumes its usual condition. The amount and regularity of the pellets with which the tube is fortified varies very much. One finds occasionally an individual in which they are so few and irregular as only to suffice for the identification of the species, Regular feeding with water containing abundant food produces a corresponding increase in their number and regularity, and a supply of carmine and indigo on alternate days is followed by the deposition of very regular alternate layers of red and blue courses on the outside of the tube, which, when viewed by strong dark background illumination, then forms a very pretty object.

My specimens produced abundant ova, which were formed in the usual manner in the ovary, and thence extruded into the space between the animal and its theca, and deposited upon the lower part of the foot, as is customary with this division of the Rotifers. I have not yet observed their development nor, although I have examined a large number of specimens, have I yet been fortunate enough to see the male of this species.

A. W. Wills.

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