Midland Naturalist/Volume 01/Freshwater Life.—1, Entomostraca (continued)
Feeshwater Life.— 1. Entomostraca.
By Edwin Smith, Esq., M.A.
Passing to the second order, which is named Copepoda, we select for description the well-known Cyclops quadricornis. A lively female specimen, let us suppose, with egg-sacs attached, is, after repeated attempts with the dipping-tube, at length safely landed in the live-box. What is she like? We observe that the carapace is made up of many parts corresponding to the segments of the body. Four segments compose the thorax, the first, with which the head is consolidated, being very large. The abdomen counts six rings, and terminates in a forked tail. Standing out conspicuously from the head are two pairs of antennas, each of the larger being made up of numerous joints, and all four armed with bristles. The mouth has a pair of strongly toothed mandibles, besides a first and second pair of foot-jaws, between which and the antennae the breathing function appears to be divided. There are five pairs of feet, the four pairs useful for locomotion springing from the four divisions of the thorax. Each foot is itself double, and all are thickly furnished with bristles. The first segment of the abdomen is small, and carries a fifth pair of feet, modified in both sexes to subserve the reproductive process. In the male the next two segments are distinct; in the female not. The larger antennæ of the male swell out about the middle, and make a hinge-joint behind the swelling, peculiarities wanting in those of the female. The adult female, in the breeding season, carries two external ovisacs, fastened one on each side, near the base of the abdomen. They must not be confounded with the true ovaries, which are internal. They are only a temporary shield, secreted by the female around each bundle of eggs at the time of laving them, and are ruptured and finally shaken off when the eggs are hatched, Lastly, one eye placed in the front of the head serves as the single organ of vision. It is often of a brilliant ruby colour, especially in young specimens.
As in most other genera so in this, the females are much mere numerous than the males. The fact is no doubt connected with another peculiarity, which has been termed parthenogenesis. One impregnation enables a female Cyclops to go on laying successive batches of eggs for life. And the female progeny are themselves fertile, though carefully isolated as soon as born; and so on for several generations. The rate of increase of some species of Entomostraca is enormous, According to Jurine it is quite possible for a single female to he the progenitor during one year of many millions of young, But, practically, the rate of increase is checked by various aquatic enemies, the vast majority of the young being simply born to be eaten. There are few more interesting spectacles under the microscope than the batching of a brood of Cyclops. When the time draws near the little things are seen all huddled together in a cluster, each in its separate pocket of the ovisac, peeping through the membranous veil with bright ruby-coloured eye. You single out one for special observation, and feel a growing interest as you watch the repeated struggles of the tiny prisoner, its final escape, and its first joys of liberty, as it darts away In the surrounding water, with the jerky, zigzag motion of its parent, Nor is if less interesting afterwards to note from day to day the changes of form and successive moultings by which the adult state is reached, Three moultings take place before the animal is perfect, and capable of producing its species.
Nearly related to Cyclops, and not much unlike it in appearance, is Canthecamptus, found abundantly in the ponds about Nottingham. As it is rather small, the best way lo secure a specimen for examination is to place a portion of the gathering in a shallow dish, and look it well over with «a pocket lens. A small dipping tube, made as follows, will be found useful:—One end must be drawn to a blunt point with moderate aperture, the other inserted into a short piece of india-rubber tubing, sealed air tight at the free extremity. The length of the glass part may be about two inches, press the india-rubber between thumb and fore-finger, dip into the water, and by removing pressure at the right moment the object is sucked up into the tube, whence it may be expelled by once more pinching the india-rubber. The two commonest species of Canthecamptus are C. minvtus and C. fareatus. In the female I have often found a curious reddish structure coming off from the sixth body segment. It is of a hard and horny nature, but its use is not known. Closely allied to the preceding is Diaptomus castor, easily recognised by its inferior antennæ, which are "fully as long as the entire body. I have found it amongst algæ in stagnant drains.
Animals belonging to the first two orders of Entomostraca have comparatively few branchia, and these attached to the appendages of the mouth. Animals belonging in the remaining two orders have many branchia, and these attached to the legs, which are often numerous. The latter are sometimes grouped together under the common designation of Branchiopoda, or the gill-footed division. To that division we now proceed.
To the third order, the Cladocera, belong the various species of Daphnia and its near relations. Let us first examine a full-grown Daphnia, preferably one of the larger species, say D. pulex or D. retula. We see a body composed of two parts, the head terminating below in a sort of beak, and a thorax and abdomen, the two last enclosed in a nearly transparent carapace. In D. pulex the carapace tapers off behind in a long dagger-shaped point. In D. macronata it bears two long spines at the posterior corners. In D. retula it is lopped off bluntly and slantingly forwards. In D. reticulata it is marked with a network of lines over its surface: and in Acroperus harpæ these lines resemble the strings of a harp. Below the beak are a pair of exceedingly small antennæ, so small that they may be easily overlooked. Springing, as it were, from the neck, we see a pair of vary large branched antennæ. These are the principal organs of locomotion. The eye consists of twenty crystalline lenses, or fewer, and is turned about on a cushion of appropriate tissue by two sets of muscles. This coarsely compound organ may be regarded as foreshadowing the highly perfect compound eye of insects. Just behind the eye may be seen the brain. At the junction of the head and body near the base of the beak, is situated the mouth, which opens into a short gullet, and that again into a roomy stomach with its two cæca; and then follows a straight intestine, which finally curves downwards towards the tail. Only the first segment of the body is adherent to the carapace, the rest moving quite freely between the valves. Ample room is thus afforded on the back of the animal for the accommodation of its eggs after they are laid; and there, accordingly, the eggs are carried about in a bundle till they are hatched. The tail, which terminates in two hooks, is used as a rudder and propeller. Its motions are very vigorous. If you look through the shell at that part of the body which succeeds the neck, you will notice an oval-shaped organ, which keeps regularly dilating and contracting. This is the heart. From its anterior extremity springs an artery, and on opposite sides of the heart are two slits which receive the blood from the surrounding cardiac chamber, and close up at each contraction while the blood is urged forwards. There are five pairs of legs, employed for other purposes than swimming. When the animal is at rest, they create currents in the water, and so bring food to the mouth. The first and second pairs are used as organs of prehension. The third and fourth pairs mainly subserve respiration, for which they are well adapted by their bronchial plates fringed with numerous filaments. I have already remarked upon the extraordinary fecundity of many of the Entomostraca. The Daphnia are no exceptions to the rule, and present similar phenomena of parthenogenesis. The development of the embryo may easily be watched through the thin carapace, and is, of course, extremely interesting. The first organ to show itself is the eye; on the fifth day from laying, the young Daphnium cone out, and then go through their series of moultings. One great advantage of the moulting process to the Entomostraca is, that they are thus able to rid themselves of troublesome infusorial parasites, which often lodge upon the carapace in great numbers, and seriously retard the movements and depress the vitality of their host. Bunches of Epistylis are very common upon Cyclops and Daphnia. Towards the approach of winter, the Daphnia have the remarkable habit of enveloping eggs in a special casing between the outer and inner layer of the carapace. This casing, which is developed on the back of the mother, has been called the ephippium, from its fancied resemblance to a saddle. It will easily be recognised by the microscopist. When the skin is cast, this structure is cast along with it; and, floating on the water, preserves the eggs through the cold season, till they are hatched by the returning warmth of spring.
In July, 1870, I found in a shallow pool on the turfy soil of Lindow Common}, Cheshire, a good many examples of one of the Daphnia family, which, from its long bristles, bears the name of Macrothrix. My specimens corresponded to Baird's description, except that a particular bristle mentioned by him was wanting in my captive. I had possibly chanced upon a new variety. The superior antennæ are of considerable size, hanging from the beak like two flat swords, with a broad, straight-edged extremity. While watching an example in the live-box, what was my astonishment to observe some of the parts suddenly became double, One after the other, the hinder segment, its hooked spines, the beak, superior antennæ, and so on, appeared double; and it became evident that I was the fortunate witness of the moulting process. I at once mounted the whole affair in glycerine, and my Macrothrix now lies side by side with its cast-off skin.
One other family of the Cladocera should be mentioned, if only because it contains Chydores sphæricus, a very common species in our stagnant ponds. The Lynceidæ (for so the family is called) may he recognised by a black spot situated in front of the eye, and looking not much unlike a second eye, which, however, it is not. The intestine, moreover, makes one complete turn and a half. To the same family belong Eurycercus lamellatus and Acroperus herpæ, both of which occur in this neighbourhood. The latter is fond of resting on the top of the water, moored by its antennæ to a bit of weed; or a cluster of them will collect round some floating leaf or sprig, and he motionless in the warm sunshine as if asleep.
Of the last order, Phyllopoda, we shall cite only one example, Cheirocephalus diaphanus, or the Fairy Shrimp. I have not yet seen it alive, but those who have speak with enthusiasm of its singular beauty. The male is especially gorgeous. With regard to anatomy, the body is destitute of a carapace, and is divided into many segments, affording great freedom of movement. The male has a remarkable pair of inferior antennæ, employed essentially as clasping organs. Those of the female are simpler, being shaped like a broad sickle, There are two stalked eyes, each composed of an immense number of lenses, showing a further advance towards the insect type. Its young, however, has but one simple eye, placed centrally in front of the head. This is represented in the adult by a dark blind spot. No fewer than eleven segments compose the thorax, to each of which is attached a pair of branchial feet, of a broad and leaf-like form. The tail consists of two broad appendages; and both feet and tail are thickly beset with plumose hairs. Extending from the head to near the tail may be discerned about eighteen or nineteen small hearts, or quasi-hearts, placed end to end and all moving together. The female possesses a single external ovary, from which, at the proper time, from 100 to 400 eggs are jerked out in succession during twenty-four hours. When hatched the young undergo a series of moultings and changes of form before they finally resemble their parent.
We have now completed our illustrations of freshwater Entomostraca. I shall not at present enter into any description of the marine kinds. Specimens of their empty carapaces, particularly of the Ostracoda, may be found in the sand of our coasts, and will often occur to the searcher after foraminifera. I have thus obtained examples of Cythereis and Cythere. I have also taken, near Penmaenmawr, a good many Cetochilus, a rather striking form resembling Cyclops.
The Entomostraca have played an important part in the life-history of the globe from the earliest epoch to the most recent. They have been well represented through untold ages from Cambrian times to the present day. Their remains, especially of the Ostracoda and Phyllopoda, occur in all formations, and in some are so abundant as to give a peculiar foliated character to the rock containing them.
A few books of reference may not be unwelcome to the student. Before all the very complete manual of the British Entomostraca by Dr. Baird, published by the Ray Society; then the portions bearing upon the subject in Professor Huxley’s "Anatomy of the Invertebrate Animals," and Professor Nicholson's "Zoology and Palæontology;" lastly, the splendid monographs on the fossils of the group by Professor Rupert Jones, the Rev. H. W. Crosskey, and ethers, published by the Palæontotogical Society. With such help, the systematic study of our old friends, the so-called water-fleas, will be found replete with interest.
This work was published before January 1, 1930, and is in the public domain worldwide because the author died at least 100 years ago.
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