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CHARACTERISTICS OF ROTIFERS.
BY HENRY J. SLACK, F.G.S., HON. SEC. R.M.S.
(With a Plate.) The following paper contains the substance of a lecture delivered by the writer before the Old Change Microscopical Society on the 18th October, and is intended to serve as an introduction to the study of a group of small animals, which from their variety and beauty have always been favourite objects with microscopists of all grades, from young beginners to profound students of natural forms.
We may begin by asking, What is a rotifer? And the answer to this inquiry is rendered somewhat difficult by the important differences exhibited by different members of the group of beings usually designated by the term. If a botanist were asked, What is a chrysanthemum, or “golden flower ?” he would enumerate a number of qualities amongst which the colour indicated by the name would not be found, because it belongs to individuals and not to the entire group. In like manner a rotifer, or “wheel-bearer,” may be a creature which does not exhibit the least semblance of wheels or organs presenting an appearance of rotation, and a considerable number of so-called Rotifera only retain the name because it has become a popular one, and because they possess other organs and structure more or less closely resembling those of the common rotifer, from whose aspect the whole group was designated.
Formerly, rotifers were spoken of as Infusoria, but Ehrenberg distinctly pointed out the superiority of their organization, and observed a great number of important facts concerning their structure and modes of life. In dealing with this, as with other groups, Ehrenberg no doubt made many mistakes, some arising from erroneous interpretations of what he did see, and others resulting from the imperfect instruments at his command, but it is unfair to notice the errors of this distinguished microscopist without remembering with gratitude the enormous amount of his labours, and the success that has attended a great portion of his investigations.
Returning to the question, What is a rotifer? let us endeavour to obtain a reply by bringing together certain facts pertaining to the wheel-bearing group. Rotifers, then, are symmetrical animals, in which a dorsal and ventral, or back and front sides can be distinguished.* They have a flexible, strong skin, and
* Mr. Gogse says, “ the dorsal aspect is always determined by the eye or eyes being turned towards that surface, by the stomach and intestine passing down it, and by the cloaca being on that side of the foot. The ventral aspect has the manducatory apparatus and the ovary."
in many cases enjoy the further protection of a sort of armour, which may be roughly compared to that of the turtle or tortoise, or of dwelling-tubes, more or less simple in construction. They have considerable powers of contracting or expanding their bodies, and in some cases can slide one portion inside another, like the tubes of a hand telescope. Their possession of a certain rank in the animal world is evidenced by a wellmarked digestive canal, consisting of an oral or receiving orifice, an apparatus composed of hard materials for crushing and grinding food, popularly termed a “gizzard,” though it is not exactly analogous to the grinding organ of birds, a gullet, a stomach supplied with secretions from glandular organs, and, except in one genus (asplanchna), an intestine and anal orifice. The term “oral orifice” has been used instead of the“ mouth," to avoid suggesting comparisons which might not be correct between the mouth of the rotifer and that of the higher animals of the vertebrate type. More or less surrounding the oral or receiving orifice, are groups of cilia, which in the common rotifer present the aspect of the well-known“ wheels.” Most rotifers, if not all, possess rudimentary eyes, either at an early period, or throughout their lives. They have all a distinct muscular system, and a nervous system, of which a large cerebral ganglion is frequently conspicuous. None of them possess a true blood circulation, but all have what is termed a “watervascular system” of delicate canals, similar to those found in Turbellaria, to which the Planaria, well-known to microscopists belong, and in the Helminths, or intestinal worms—flukes, tapeworms, and threadworms. The functions of the watervascular system are not positively known, but are conjectured to be respiratory and excretory. The contractile organ, popularly called the “heart” of rotifers, belongs to this system, and so do the “vibratile tags” easily seen in some species.
The rotifers were formerly considered to be hermaphrodites, but Mr. Brightwell and Mr. Gosse discovered distinct males of certain species, and it seems most probable that other males will be found in the course of further research. The rotifers usually obtained by microscopists are females, and their eggs and ovaries are frequently very conspicuous, the eggs often being enormously large in proportion to the dimensions of their parents. The common rotifer is an example of ovovivaparous generation, the egg with its living contents being commonly seen in the body of the parent, and the young coming forth as the exact image of its mother. In the common rotifer, and in many others when the eggs are sufficiently advanced, the jaws of the unborn infant may be seen vigorously working, the little red eyes brightly shining, and a strong ciliary current running down the gullet. Three kinds of eggs have been distinguished—common eggs, intended for immediate hatching; winter or resting eggs, designed for preservation probably till the next season, like the statoblasts of Plumatella and other Polyzoa ; and male eggs. The winter or resting eggs are usually rough and large.
The known males are shortlived, and not provided with any digestive machinery. They seem to be simply locomotive organs of fecundation, whose services are occasionally required, the ordinary process of reproduction being carried on by the females alone, as is the case with those pests of the greenhouse and garden—the Aphides, or plant-lice. Such a method of propagation has more or less analogy to the multiplication of plants by buds or cuttings, instead of by germs fertilized by pollen and giving rise to true seeds.
The existence of eyes in rotifers has already been mentioned. They often exhibit the brilliance and tint of the ruby. and the single large eye of the Brachions may be specially noticed for its conspicuous beauty. In many rotifers the eyes vanish as the individual grows old, and it is often difficult to detect them. Dark ground illumination is frequently very useful for this purpose. The eye is probably of limited use, and may not form true pictures in the highest sorts of rotifers, though a refracting body or crystalline lens is said to be always present. There are many gradations in nature from eyes that seem only capable of making known the presence of light to those which form elaborate pictures on the retina, and by means of appropriate nerves supply definite information to the brain. Probably, the rotifer eye occupies one of the lower, though not the lowest position in the scale.
The calcar or spur of the common rotifer, often seen projecting like a pigtail, and similar organs on other species are probably feelers. They are tubular structures, fitted with a sort of moveable piston, terminating in a tuft of cilia. A sense of touch is probably diffused over the soft parts of the body, and the tentacles may be very sensitive to vibrations affecting the fluid in which the animal lives. It is also probable that the sense of taste exists in a rudimentary form, as it is very common to find rotifers rejecting particles which so far as size and consistency goes might be swallowed for food.
No perfectly satisfactory classification of rotifers has yet been proposed. Ehrenberg divided them into groups, founded upon his conception of the form of the rotary organs, but a modification or rather expansion of Dujardin's classification will be found more convenient for general use, and may stand as a provisional arrangement, pending a more thorough examination of the whole series. Dujardin calls rotifers Systolides, from the Greek, ovoton, contraction, but as many other creatures exhibit contractibility in as remarkable a way, the name has not usually been considered appropriate, and has not come into general use. He divides his Systolides, which include water bears, into orders, as follows:-1. Those which are fixed by the posterior extremity of their body. 2. Those which have only one mode of locomotion by means of their vibratile cilia, or the swimmers. 3. Those which have two modes of locomotion, and which sometimes crawl like leeches, and at others swim like the preceding, or swimmers and crawlers. 4. Those which are destitute of vibratile cilia, but provided with claws and are veritable walkers—water-bears. It is desirable to exclude the water-bears from the group, and we shall then have fixed rotifers, swimming rotifers, and swimmers and crawlers. These divisions, though not founded upon any deep considerations of structure, indicate obvious and important facts. The fixed rotifers comprehend the Floscularians, which have long tufts of cilia, but no wheel-like organs, and the Melicertians, which have four lobes something like the petals of a flower, with cilia round their margins, exhibiting the rotatory appearances. The swimmers comprehend several genera, amongst which the Brachions, or pitcher rotifers, are conspicuous. Dujardin proposes two divisions of swimmers, one characterised by a general flexibility of the skin or integument, Furcularians, and the other, like Brachionus, Salpina, etc., having rigid carapaces or cuirasses. The swimmers and crawlers comprehend the Philodines, of which the common rotifer is the type. Many things might be said against accepting this scheme as a final arrangement of the group, but no harm can arise from its provisional use, provided that it is borne in mind that in addition to the modes of locomotion mentioned by Ehrenberg, jumping must be added as characteristic of several species, such as Polyarthra, Triarthra, and a few others. It would also be advisable to add to Dujardin's groups one of associated rotifers to include Conochilus and Lacinularia, which are inconveniently placed in his Melicertian family. There is an obvious and important difference between the permanent fixture, with confinement to one spot, of the Floscules, Melicerta, Ecistes, etc., and the free swimming of the asso. ciated groups of Conochilus. In both cases the creatures may anchor themselves by their tail-feet, but in one case there is permanence of abode and in the other a roving life.
The fixed rotifers differ very considerably from each other, and those which form simple tubes are easily distinguished from the Melicertoe, which are builders, constructing their abode of separate pellets moulded by an organ specially provided for the purpose, and placed in due order as a mason
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arranges his stones or bricks. There are also obvious and important differences between the ciliary apparatus of the Floscules, which exhibit nothing like rotation, and those of the Ecistes, Limnias, or Melicerta, which do show that remarkable phenomenon, and in which it is subservient to the collection of food. In fact, Dujardin's order of fixed rotifers is so far unnatural, that it groups together families that must be separated in any system founded more upon structure and less upon obvious peculiarities of external appearance.
If we followed Dujardin's arrangement, we should begin by speaking of the Floscularians, which would not have been called “rotifers” or wheel-bearers at all, if they had been the first of the group to attract attention. The most conspicuous and decided wheel-bearer is the common rotifer, in which the rotatory organs serve the two purposes of locomotive engines and food collectors, and probably, also, act in assisting respiration by bringing fresh currents of water to the delicate tissues of the creature. When it pleases this rotifer to anchor herself by her tail-foot, her mode of life for the time resembles that of Ecistes or Linnias. Her ciliary currents cause a convergence of whirlpools, which bring all sorts of particles, living or dead within her reach, and she selects some for reception and others for rejection. This being ended, we find her swimming, or crawling, frequently routing about with the snout-like extremity she presents when her wheels are withdrawn, and apparently exerting discrimination in the selection of places to examine, and of food to take in. From this active, many-motioned animal, with powerful, whirlpool machinery at her command, let us pass to the beautiful Floscule, one of the most exquisite of rotifers, when properly exhibited by dark-ground illumination. If our illuminating apparatus is nicely managed we see an extremely delicate and transparent cylinder like a confectioner's glass, surrounding the creature, and forming her house. Slowly she rises, looking so uncouth that her appellation of “beautiful" seems inappropriate. But we must not be in a hurry to condemn her. Most gracefully she throws out her five lobes, and opens from each one a long tuft of cilia, gleaming and glancing in the light of our apparatus. If we keep quiet, the fans remain expanded and still. Each of the long hairs or cilia, of which it is composed, appears capable of transmitting delicate vibrations, and we frequently see a whole group of them twinkle in succession like the small steel vibrators of a musical-box when the instrument is playing. The animal can direct her head to any quarter, she can change at will the position of her richly-ciliated lobes, and thus cause the inward current which she maintains by less conspicuous cilia to bring to her mouth the particles she requires. Here the long cilia may be roughly