페이지 이미지
PDF
ePub

CONTRIBUTIONS TO THE HISTORY OF THE

ROTIFERA, OR WHEEL-ANIMALCULES,

BY PHILIP HENRY GOSSE, F.R.S.

PART IV.

THE FLEXIBLE CREEPERS (NOTOMMATINA). ITNINITIATED persons are sometimes startled, and not a

little stumbled, by reading in books of scientific natural history such statements as this : “Ab i s a much more perfect form than C d ." They pause and ask whether there can be any degrees of perfection in the handiwork of the All-wise and Almighty God. Are not all His works perfect? The answer must be that they are; understanding the term to express the adaptation of the creature, and of all parts of its organization, to the purposes for which they were designed.

The naturalist, however, does not mean this. He is not speaking of the organism as an individual, with reference to its own objects and requirements; but as one of a series, with reference to an ideal standard of aggregate qualities presented by that series. Thus, when we consider Birds, we have before us a vast assemblage of creatures, the most prominent and characteristic feature of which is that they are constituted for flight, by means of a peculiar development and arrangement of tegumentary organs (feathers), associated with peculiar developments and arrangements of the skeleton, muscles, and breathing organs. Thus, then, we acquire a definite idea of a bird, notwithstanding that this idea is presented under considerable diversity in all these details, when we examine the thousands of species that are known; nor does this diversity at all interfere with the unity of the model or pattern which is involved in the aggregate idea of a bird. The comparison of these details, however, soon shows us that the characteristic features of a bird are much more prominent in some species than in others. In the hummingbird, the frigate-bird, or the swift—in the great breast-bone, the enormous lungs and chains of air-cells, the hard, dense, thick pectoral muscles, the firm rigid feathers, drawn out to long points on the wings and tail, we see a high concentration of all the characters of a bird, as I have enumerated them above. Compare now with these the Guinea-fowl, with its short hollow wings, and small, weak, excavated breastbone; the tame duck, with its soft feathers and short, powerless tail; or the penguin, or the ostrich, in which the wings are reduced to mere rudiments, and have absolutely ceased to be flying organs; and you will see what the ornithologist means when he speaks of the comparative perfection of these birds, when he says that the humming-bird is the most perfect representative of birds, the penguin the least. It is not that the penguin is not most admirably fitted for its mode of life and its requirements ;-nay, it exhibits some adaptations of structure to function which surprise us by their wondrous fitness. As a creature it is perfect, but not as a bird ; there is a manifest departure from the characteristic structure and functions of a bird, and an approach to those of some other great model of animate existence, while it still remains essentially a bird. It is, perhaps, the least perfect of birds.

It is the same in the subordinate divisions. The swallows, taken as a family, are very perfect representatives of their class in the points above-mentioned; but there are degrees of perfection among them. The swifts possess the characters which mark the family in a higher degree than the swallows; while of these latter, how much more completely, “more perfectly," a swallow, is the chimney-swallow than the sand-martin.

Naturalists have been accustomed to use the technical term “type," or its adjective “typical,” to express the greatest aggregation of characteristic features in any given group. Among birds, the perchers constitute the typical order; in this order the cone-billed birds take the typical place as a tribe; the finches, again, are the typical family of these ; and what is the type of these pretty little birds, as I am not writing an essay on ornithology, I shall leave to my readers' acumen to determine.

There must always be some latitude in the determination of what constitute the typical characters of any given group, and therefore of the types themselves. One naturalist may picture to himself a different standard of completeness from that which another of equal eminence would assume; and hence no one ought to speak oracularly; he merely gives his judgment. The great group of tiny creatures of which I am about to treat is, according to my judgment, the typical order of the class RoTIFERA. They may be designated, in homely parlance, The Flexible Creepers ; systematically,

the order Notommatina.* One mark of a typical group, almost constant, is that it contains a larger number of subdivisions than any other group of like rank; and populousness is highly characteristic of the Notommatina.

All these animals are permanently free, never becoming fixed to other objects, and never forming mutually adherent groups. Their bodies are not inclosed in a tube; and their skin is for the most part flexible, not forming a shelly mail, though in one or two genera certain portions of the integument are stiffened, indicating an approach to the structure of the mailed families. The form is generally cylindrical, with a length about twice the diameter on the average, occasionally, however, three or four times the diameter. The front does not expand into a flower-like disk, but is usually no wider than the body, very varying in its shape, sometimes being flattish, oftener convex, with many swellings, beset with strong vibrating cilia, which are so arranged as that their combined action produces two vortices, one on each side of the head. The opposite extremity of the body terminates in a thick foot of several joints, of which the last bears two diverging toes. These are used by the animal for its support, to aid it in crawling. They are in general moderately short, but in a few cases are of great length.

The characteristic appearance of the little animals of this order will be seen by reference to Plate XIX., in which figure a represents a dorsal view of Eosphora aurita, in the act of swimming. This is an elegant and sprightly species; especially beautiful when, as is very frequently the case, the enormous stomach and intestinal canal are filled with translucent green food. I have repeatedly met with it around London; as in a green pond in Greenwich Park, and in the tiny sweltering pools on Hampstead Heath, renowned, for more than a century, as being prolific nurseries of the higher forms of ROTIFERA. They will live and increase their race in a phial of water containing a little growing vegetation, as a sprig of water-moss, a few leaves of duckweed, or a small tangle of converva : the phial being left uncorked, and set in the light, but protected from the direct rays of the sun. Here they will play and rout about, delving in the sediment that

* This order is nearly concurrent with the family Hydatinca of Ehrenberg. But the genus Hydatina, which he considered typical, and whose name he used for the family appellation, consists of only a single species, and cannot be separated from the great genus Notommata, except by the very trivial and valueless character of wanting an eye; or rather, of wanting the speck of coloured pigment which ordinarily accompanies the eye. H. senta is a true Notommata.

esting the Familthouts direct observesund fo

accumulates on the phial's side for food; now and then shooting off into the water, and swimming evenly and gracefully along, slowly rotating on their long axis as they go. Their movements can be watched, and their forms distinctly made out, with the aid of a good pocket-lens, as the phial stands on the window-sill; and this sort of observation, which is highly interesting, can be pursued with great ease and satisfaction, especially if the finger of the disengaged hand be passed behind the phial (without shaking it), so as to intercept those rays of light which pass direct from the sky through the animalcule to the eye of the observer. The effect of this manquvre is to make a dark back-ground for the object, while it is yet brilliantly illumined by the rays that fall on it from all around the finger. Small as is the magnification, the experienced observer has no difficulty in distin. guishing the principal organs, which, under this mode of illumination, have a relief and a solidity which are very instructive.

But I must describe our little beauty in detail. Its body is nearly a cylinder, of about a hundredth of an inch in length, with a diameter of one-third the length. Sometimes it becomes almost pear-shaped, by the swelling out of the hinder half of the body, when an egg is maturing in the ovary. In the ROTIFERA generally, the eggs are of immense size in proportion to the body; and in this species they are even larger than usual, an egg just ready for discharge occupying nearly one-fourth of the entire volume of the animal. In the figure such an egg is seen occupying a large space just behind the middle of the animal.

The head is capable of considerable alteration in shape. When the animal is busy feeding, resting on its two-toed foot, with the body thrown forward horizontally, the front of the head is so oblique that, by a little bending downward, producing some strong foldings of the skin of the breast, it can be brought, as a nearly flat surface, into contact with the ground. A specimen is represented in this position, under a lateral view, in fig. b. The action of the frontal cilia is then seen, as if evenly covering this surface, producing currents, which, however, do not form whirlpools. But suddenly the little animal ceases feeding ; rears itself on its toes till it stands nearly erect; balances itself for a moment; evolves from each side of the head, by a kind of turning inside-out, a remarkable ear-like process, which is clothed with much stronger cilia. The action of these immediately begins to form vigorous circular vortices, of which the “ears” are the centres, and in the same instant the animal shoots forward with beautiful ease and precision, on its even course, revolving slowly as it goes.

Some very favourable observations on the animal, made when I could look down directly on the head in a vertical position, have revealed some organs, which are doubtless vehicles of sensation, though I can but very uncertainly conjecture their office and value. At the back of the head there are two very minute prominences, each of which is terminated by a tuft of bristles. These organs are very generally found in the wheel-animals, and are in many species much more developed, and more favourably situated for observation, than in this case. They appear to be tubular prolongations of the skin, within which a piston-like body moves to and fro, carrying at one end a tuft of bristles, which diverge when protruded from the tube, and attached by a muscular cord at the other end to the interior of the body. A nervous thread in some species is seen to pass from the brain to each tube. The organs may be considered as analogous to the antennæ of insects, though even of these we are not quite sure of the function.

In our Eosphora there is seen, seated in a central depression between these two antennæ, a little wart, of which I cannot suggest the purpose. Directly at the front of the head, at the most advanced point of the animal, whether creeping or swimming, there are placed two symmetrical round specks of opaque crimson pigment, which come out with great brilliancy when the animal is viewed under sufficient magnifying power by reflected light. Dr. Leydig refuses to allow these to be organs of vision, but from repeated examination of them I have no doubt that he is in error, and that Ehrenberg was right in considering them eyes. Besides these, however, there is another red speck, which is without dispute of a visual character. It is far remote from the two frontal specks, being situate within the transparent skin, but near it, on the dorsal aspect, at about one-fourth (following the curve of the back) of the distance from the front to the origin of the foot. To make my readers understand its relations, I must describe a highly interesting and very remarkable organ.

In most of the animals which constitute this order, but seen with peculiar distinctness in the present subject, and in one which very closely resembles it in appearance, Notommata aurita,—there is seen a great ovate sac passing from the front along the back of the head, and terminating by a rounded extremity in the neck. This is occupied by turbid matter, and seems to be indubitably a brain ; though it is quite without parallel among invertebrate animals to find any such concentration of the nervous substance in a single organ. We are accustomed to consider this character as exclusively belonging to the vertebrate classes, and to mark their superior position in the scale of being; the most sentient, the most

« 이전계속 »