Midland Naturalist/Volume 01/Freshwater Life.—2. Rotifera

by Edwin Smith

Freshwater Life.—2. Rotifera.

By Edwin Smith, Esq., M.A.

 

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In pursuing our studies of the minute forms of Freshwater Life, we cannot fail to encounter a number of creatures much smaller than the Entomestraca before described, yet quite distinguishable with the aid of a simple lens, sailing through the water with an easy, swinging motion; or grazing, like small cattle, among the threads of algae; or moored by the tail to some green spray, while a curious ciliated head is stretched forth in the act of feeding, The cilia will bo seen to move in such rapid and well-timed succession as to look like revolving wheels; and so perfect is the optical illusion that you feel how appropriately these creatures have been named "Wheel-bearers," or Rotifera.

Their exact place in the animal series has not yet been finally determined. Provisionally they may be referred, as an order, to the somewhat miscellaneous class Scolecida, in the sub-kingdom Annutoida. On the whole, they appear to have strong affinities to the worm-like animals of the class just mentioned, as well as certain points of resemblance to the lower crustaceans, and to the larval forms of Echinodermata.

The Rotifera attain a maximum size of about 1-86th of an inch. Some are as small as 1-400th of an inch, or smaller. They are of a higher type of organization than the Infusoria, with which they were formerly grouped, since they have an obscure segmentation of the body, a completely separate alimentary canal, and a water-vascular system; and they never multiply by budding oy self-division. They have a right and left, a dorsal and ventral, and a head and tail aspect, the end answering to the head moving habitually forwards, with the back upwards. Some are fixed by a foot-stalk to water-plants during the whole of the greater portion of their existence; but the majority are free-swimming. The former use the ciliated wreath to urge food to their mouth; the latter use it as a locomotive organ when in motion, and a feeding organ when at rest. Few things are more strikingly beautiful than this ciliated wreath in full activity. At intervals it is drawn in and tucked out of sight, so as completely to alter the look of the animal. But after a time it issues forth again, expands, and resumes its work. The food, received by a distinct mouth, is caught by a sort of champing gizzard, which has been likened to a pair of toothed hammers and a double anvil, and is there crushed small before admission to the stomach. In this receptacle, which is of variable size, the food is digested. The refuse of digestion then passes along an intestine; and finally, in most kinds, though not in all, is got rid of by a distinct orifice, connected with a cloaca, into which the ovary and a contractile vesicle also open. There is no heart; but a water-vascular system is present, consisting of two convoluted tubes, one on each side, furnished at intervals with short pipes lined with cilia, which lead into the general cavity of the body. The lower ends of these tubes open into the aforesaid contractile vesicle; and thus, the pulsations of the vesicle and the vibrations of the cilia Keeping up a current, the fluids of the body are refreshed by being brought into communication with the outer water.

Of the nervous system, the following are the main features:—Near the back of the neck there is found a ganglionic mass, on which are mostly seated one or more eye-spots, generally of a bright crimson colour. Projecting from about the same place may often be seen a little telescopic feeler, armed at the tip with minute bristles; or the bristles may be sessile in a small hollow. Muscles pass lengthwise from end to end of the body, and ring-wise at intervals round it, by which the external shape can be more or less altered, enabling some species ta crawl about like a leech. Other muscles assist the movements of the ciliated wreath, as well as those of the tail-foot. Under careful illumination I have occasionally observed muscular striæ.

The body-cavity, with its various internal organs, is protected by an integument of greater or less firmness, shaped like a shield, a boat, a spindle, a vase, and so forth, in those cases where the integument is hardest, it may be termed the lorica, corresponding to the carapace of Enntomostraca, and having a similar chitinous composition. Its surface is bare of cilia, but it is not infrequently armed with lateral or terminal spines. The only approach to an articulate appendage possessed by Rotifera is that which I have called the tail-foot. It is not a prolongation of the back, and is, therefore, not a true tail. Coming off from the under surface of the body, below the anal orifice, it may be regarded as a kind of foot. It is capable of being shortened, either on the telescopic principle, or, when soft, by contracting in wrinkles. The basal portion varies considerably in length, being reduced sometimes to a mere stump. At its narrower end are often inserted, movably, one, two, or three styles, or dagger-like bristles, which may be very long. When these are two in number, they strongly remind us of a pair of scissors; when they are three, the middle one is small. With this organ the Rotifers steer themselves in the act of swimming, or hold on to some support while groping about for food, or even, in a few instances, take veritable leaps.

With regard to reproduction, the same phenomena of parthenogenesis have been observed, which we noticed when describing the Entomastraca. The young are born from two very distinct kinds of eggs; the summer eggs, which are generally, if not always, virgin produce; and the Winter eggs, which have been duly fecundated. The latter are preserved against the cold by a peculiar shell, tl spring returns to hatch them. While the females multiply in enormous numbers, the males are very rarely met with. The latter, moreover, are, as a rule, so unlike the former in appearance, that it is difficult to recognise them as belonging to the same species. It is a curious fact, that in all males the alimentary canal is either absent or rudimentary. Whey are, consequently, short-lived.

Before briefly considering particular examples of Rotifera, I will here give my authorities for the preceding anatomical sketch, while naming a few works to which the student can refer for a fuller account of the subject. He will find that the ablest observers are by no means unanimous on many important points in the anatomy and life-history of these somewhat puzzling animals.

Works of reference:—Pritchard's "Infusoria;" Rolleston's Forms of Animal Life;" Huxley's "Anatomy of the Invertebrated Animals;" Gosse's "Papers in the Philosophical Transactions."

To be continued.

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