Midland Naturalist/Volume 01/Conochilus volvox

by Anonymous

Conochilus volvox.

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The pond near Redditch which for nearly two years yielded a constant and abundant supply of this interesting and beautiful Rotifer, has for the last month or so, on each visit in search of it, been "drawn blank." It is a curious fact that the "sticks" discovered by the Rev. Lord S. G. Osborne, congregated near the centre of many of the colonies from this pond, and called by his Lordship, (vide letter to the English Mechanic of 1st March, 1878,) "rooks' nests," were for several months in the spring and early summer, when the containing Rotifers were abundant, rarely to be found either floating in the water, in the sediment, or attached to submerged plants. But now that the Conochili have disappeared the sticks are plentiful, both floating freely and entangled in the brown flocculent matter (which by the way is full of interesting microscopic organisms) with which the water plants are at the present time coated.

There cannot be the least doubt that these "sticks" are Diatoms, though Mr. Hy. Davis, (by whom an interesting article on C. volvox was written and published in the "Monthly Microscopical Journal" for July, 1876,) once entertained, I understand, the opinion that they are Desmids, possibly belonging to the genus Docidium. This can scarcely be so, as the bodies divide longitudinally, as do Diatoms, and not transversely, after the manner of Docidium and other allied Desmids. They do not (at any rate the form commonly found) belong to the genus Navicula, as Lord Osborne originally supposed, as they lack the characteristic "median longitudinal line and nodules;" and his lordship's observation that only one form is found in each colony, and that the size of all the sticks in each is the same, the size varying with the dimensions of the colony, does not accord with my own. In one instance I found a decided Pinnularia, if not P. viridula a form excessively like it—one specimen only amongst a number of the common kind. The Diatoms, either owing to the jelly of the Rotifers not being a favourable medium for displaying the markings, or to the fact that the latter are worn away in some mysterious manner, are most difficult to resolve. Those which are free in the water are by no means easy, and though "high powers" have been brought to bear upon them, it is by no means clear which they are. I believe them to belong to the genus Synedra, closely allied to, if not identical with, S. fasedeulate.

It is just possible that the plane spot at the centre of the valves may have been mistaken for the line of transverse division. At first sight it appears inexplicable that other bodies than acicular-shaped Diatems are rarely, if ever, found in those erratic jelly-masses; but when we take into consideration that the jelly expands as the funnel-shaped creatures emerge, and becomes compressed when they are retracted, it is obvious the quasi tubes in which the animals live are always "too tight a fit" to allow freedom of entry for comers of every class, and it is simply owing to "Natural Selection," (if it is lawful to adapt the expression,) and the needle-pointed shape and hard and unyielding substance of the intruders, that they are able to gain a footing at the threshold. The sudden retreat of the irritated hostess draws, perhaps, one of them in a little way, the closely-pressing jelly retains it while its hostess cautiously passes outwards. Another sudden jerk draws the unwelcome visitor farther in, then another jerk and another, till the unbidden and perhaps unwilling guest reaches the middle of the globe and finds himself merely one "stick" in a "rook's nest."

The "Productive Pond," as Mr. Bolton styled it, now its stock of C. volvox is exhausted, has a rich store of the exquisite Polyzoa, Plumatella repeus, growing unattached, twining in and out among the rootlets of Leana minor. This condition is more favourable for observation than when the polypidom adheres to a large plant such as Potamogeton nataus, upon which I usually find it hereabouts, as it is readily seen with transmitted light and without obstruction.—S. S. R.

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