Midland Naturalist/Volume 01/Freshwater Life.—2. Rotifera (continued)

by Edwin Smith

Freshwater Life.—II. Rotifera.

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By Edwin Smith, Esq., M.A.

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(Continued from page 96.)

I do not propose any elaborate description of examples, and shall merely mention, with a few notes, those which have occurred to me in the neighbourhood of Nottingham, and which are tolerably common everywhere. I have of course met with our old friends Œcistes, Floscularia, Meliecrta, and other sedentary or case-inhabiting hinds, According to my experience, the sheath of Œcistes is generally of an irregular, somewhat broken form, and more or leas dingy with adhering vegetable matter. Last March I found one with three eggs at the bottom of the sheath, close to the supporting stalk, Floscularia, with its long pedicel, might easily be taken at first sight for a large Vorticella. Round the opening at the free extremity, there are five, or occasionally six, knob-like processes, each armed with a radiating bundle of long cilia, finer than any spun glass. These long filaments, however, have no concern in producing currents towards the mouth; such currents being evidently due to vibratile cilia within the mouth or gullet itself. The species which I have most frequently met with is, F. cornuta, distinguished by a little horn or feeler at the hack of one of the knob-like processes. The eggs cluster in a group of two or three about the pedicel; and through their thin covering may often be seen the eye-spots of the young ones. The outer sheath is perfectly transparent, and has a refractive power nearly the same as that of water. Consequently it is mlmost invisible, except by means of particles collecting on its delicate surface. Meliecrta possesses a ciliated disk, arranged in wavy lobes, presenting a front aspect not unlike the stylish cap one sees in portraits of Mary Queen of Scots. Nothing can exceed the beauty of the general effect, when this graceful wreath is in fall action, Still more noteworthy is the animal's building talent. Into a little pit near the head, particles of selected matter are swept from the water, and there moulded into conical pellets, which the animal then deposits in regular courses one upon another, like rounds of bricks, and so builds up its case, These cases, of a reddish brown colour, are easily detected with the naked eye, attached by one end to branches of myriophyllum, or roots of lemma, By chipping cut the bits of vegetation to which the several specimens cling, four or five may be got together in the field of a two-thirds objective; and then the display, under spot-lens illumination, is simply magnificent. A zoophyte trough made specially shallow from front to back, is the most convenient for showing them. With regard to Stephanoceros, or the Crown-horned Rotifer, it should be noticed that the lobes of the wheel-apparatus take the extreme form of so many tentacles, fringed with whorls of moving cilia. The protecting case is highly transparent. Few specimens have rewarded my search in this neighbourhood.

We, now pass from these kinds which envelope themselves in a sheath of various structure, into which they can retire at will, to the free-swimming group. The latter constitute by far the larger division of Rotifera, Occasionally the observer is startled by seeing move across the field of his microscope a thing with long stiff outstretched tail, like a mouse; and he can hardly believe that he is looking at a creature only the 120th of an inch long. It bears the expressive name of Monecerea rattus, or Rat-single-tail; the tail being, as already explained, a sort of foot prolonged backwards. This species may be looked for among duck-weed in the earliest days of spring, and all through the summer months. Closely allied to it, if not of the same genus, is Mastigocerea earinata, or Keeled-whip-tail; whose chief distinction is a dorsal expansion of the integument, like a keel. The body is about the 160th of an inch long by about the 400th of an inch broad, inclusive of the keel. It has a crimson eye-spot, and is found among confervoid plants.

I well remember the pleasurable surprise with which I saw for the first time, in the month of May, a fine example of Natanenata tigris, so called from having its eye-spots situated on the back of the neck. It may easily be recognised by its long pair of forceps, or double setæ of the tail-loot, the longer blade being the 140th of an inch in length, nearly twice as long as the body. I noticed numerous transverse lines on the thinner half of each seta, about fourteen on the longer, and eight on the shorter of the two, These quasi-jointings evidently contributed to the flexibility of the organ. The outer cuticle of the body was soft, and allowed the mast varied contortions on the part of the animal, On one occasion, in the month of June, I observed in sore water taken from a small pond in our Arboretum, an egg covered all over with hairs, I watched the egg for some hours in a live-box. The enclosed animal fidgetted about in its narrow prison, and appeared to be rasping the membrane at one end of the egg, The crimson eye-spot was beautifully distinct. At last the shell was broken trough, and the tiny prisoner struggled into freedom. In a few moments it unfolded its limbs from their doubled-up posture, and sailed merrily away, unmistakably a young Scaridium longicauda. As its name, Long-tailed-leaper, implies, the Searidium can leap as well as swim, it leaps with the aid of its tail-foot, which attains the considerable length of the 90th of an inch, the body measuring only the 137th of an inch.

It is a good plan for the microscopist to make drawings for future reference of everything noticeable which he meets with, Referring to my notebook, I find sketches of the ventral and posterior aspect of what I take to be Euchlanis triquetra, although Pritchard fails to notice the lateral fissure between the upper and lower valves. The body-shield, seen from behind, looks like a three-cornered hat. It evidently consists of three valves, two dorsal rising in a ridge along the middle and divided by a deep furrow, and one ventral, separated from the foregoing at the sides. There is also a large opening behind, extending some way on the under surface, and giving liberty to the tail-foot to double up beneath the ventral valve, My first observed specimen happened to lay an egg while under examination. The egg was the 190th of an inch long, the length of the parent’s body being the 80th of an inch. A near relation of Eucldanis is Salpina, a very common object in freshwater gatherings. The lorica is three sided, with a dorsal ridge furrowed from end to end.

The front and hinder extremities differ in different species. S. mucronata has a lorica furnished with four spines in front and three behind; S. ventralis has two in front only, and its lorica is spotted. On the back of the neck of {he former will be found a feeler, armed with a bristle. The body of a full-grown Salpina is about the 100th of an inch or more long. A young one has just the reeling side-to-side motion of its parent, but has a soft lorica, which becomes hard and firm with age. Our next example is one which occurred to me in a gathering from the small Arboretum pond before mentioned. Dinocharis pocillum has the basal portion of its tail-foot freely jointed, the joints having spinous processes. There are also two spines near the origin of this organ, and a minute bristle marks where the fork begins, The lorica is rather loose, and stippled all over with open dots. The actions of the animal are queerly angular and vigorous. One of the most beautiful of the loricated Rotifera is Stephanops lamellaris, in which the body-shield undergoes a peculiar expansion on its anterior margin, so as to form a very elegant crown over the animal's head. Behind the lorica is armed with three spines, and there are three bristles at the end of the tail-foot. On each side of the head is a little horn, and the neck has a collar-like thickening, The entire length of the lorica, including spines and hood, is about the 200th of an inch. A familiar friend to the microscopist is Squamella oblonga. Its favourite occupation is to climb about the stems or roots of water-plants, feeding as it goes like a minute species of cattle. It has four crimson eye-spots, and a lorica armed in front with four small spines.

One of the most interesting of all the frees swimming Rotifera is, undoubtedly, the little creature which has given its name to the order, and is known as Rotifer vulgaris. When the wheel-like apparatus is drawn in, the shape of the animal reminds one of a spindle, the forward extremity tapering, as it were, into a blunt-pointed snout. Its movements are then much like those of a leech, The body is alternately wrinkled up telescopic-fashion, and stretched out over the ground intended to be covered. The ciliary wreath is double, and serves both for swimming and feeding, On the back, near the head, is a small feeler. There are two eyes, placed much in advance of the masticatory organ, and, apparently near the tip of the snout, in the retracted state of the ciliary wreath. I was very much puzzled, some years ago, when, having caught a full-grown female specimen, I observed a young one, about halt her length, freely disporting itself in the interior of its parent. Omitting the parent's tail-foot, about two-thirds of her body was occupied by the young one, with eyes, and champing gizzard all complete. Even the ciliary wreath of the latter played at intervals. And all this must have taken place in the maternal ovary, which had been stretched to accommodate the offspring for sometime before actual birth. In fact the common Rotifer is occasionally ove-viviparous; that is to say, the young, though produced from eggs, may be retained within the ovary for a certain time after they are hatched,

In Brachionus urceolaris the lorica is closed at the sides and open at the ends like the shell of a tortoise. Into this the animal can entirely withdraw itself. It is very prolific, and sometimes increases in such numbers as to render the water turbid. The terminal forceps of the tail-foot can be drawn back into a sheath. Both the front and hinder edges of the body-shield are usually toothed, I have often seen this majestic creature, with from two to four eggs attached to the hinder part of her body, sailing about as if proud of her maternal charge. She thus tugs along a precious load of care through a large portion of her existence; while the crimson eye of the parent is prettily imitated by the the eye-spots of her yet unhatched offspring, My last example, Pterodina patina, is found lurking under the leaves of duck-weed. Its generic name refers to certain wing-like processes; its specific name aptly describes the dish-like form of the soft, flat, transparent lorica. The tail-foot comes out through an opening in the lorica near the middle of its ventral surface, giving the creature a profile somewhat like a shield with its handle. The free extremity of the tail-foot acts like a sucker, enabling its possessor to hold on to one spot, while swinging round with the rest of its body, an exercise which it seems to enjoy. Owing to the transparency of the body-shield every internal organ can be discerned with ease, The convolutions of the respiratory canals are particularly wall shown. Two longitudinal muscles can also be seen, crossed by faint stripe.

In bringing my imperfect observations upon Rotifera to a close, I take the opportunity of strongly recommending to the student, as books full of interesting information and excellent figures, Slack's "Marvels of Pond Life," and Gosse's "Evenings at the Microscope."

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