This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1070). In the genus Apus, another plan is resorted to for the protection of the ova: - the eleventh pair of legs, called by Schaffer "womb-legs," have their first joints expanded into two circular valves, which shut together like a bivalve shell, and thus form a receptacle in which the eggs are contained until they arrive at maturity.
(1071). In Daphnia (fig. 212), the ovaria are easily distinguished through the exquisitely transparent shell, especially when in a gravid state; and the eggs, after extrusion, are lodged in a cavity situated between the shell and the exterior of the body, where they remain until the embryo attains its full growth.
(1072). One fact connected with the reproduction of the Entomostraca is so remarkable, that, had we not already had an instance of the occurrence of a similar phenomenon in the insect world (Aphides), the enunciation of it would cause no little surprise to the reader; and had its reality been less firmly substantiated by the concurrent testimony of numerous observers who have witnessed it in many different genera (Cyclops, Daphnia, &c), it might still be admitted with suspicion. In the genera above mentioned, it has been ascertained, by careful experiments, that a single intercourse between the sexes is sufficient to render fertile the eggs of several (at least six, according to Jurine) distinct and successive generations.
* Histoire des Monocles. 1 vol. 4to. Geneve, 1820.
1 Materiaux pour l'Histoire de quelques Monocles allemands. 4to. 1805.
(1073). In many species there is a double mode of reproduction, the sexual and the non-sexual. The former takes place at certain seasons only, the males disappearing entirely at other times; while the latter continues at all periods of the year, so long as warmth and food are supplied, and is repeated many times, so as to give origin to many successive broods. Further, a single act of impregnation serves to fertilize not merely the ova which are then mature, or nearly so, but all those subsequently deposited by the same female, even at considerable intervals. In these two modes the multiplication of these little creatures is carried on with great rapidity, the young animal speedily coming to maturity and beginning to propagate, so that, according to the computation of Jurine, founded upon data ascertained by actual observation, a single fertilized female of the common Cyclops might be the progenitor of 4,442,189,120 young.
(1074). The eggs of some Entomostraca are deposited free in the water, or are carefully affixed in clusters to aquatic plants; but they are more frequently carried for some time in special receptacles developed from the posterior part of the body, and in many instances they are retained there until the young are ready to come forth. In Daphnia the eggs are received into a large cavity between the back of the animal and the shell, and there the young undergo almost their whole development. Soon after their birth a moult or exuviation takes place, and the egg-coverings are got rid of with the cast shell. In a very short time afterwards another brood of eggs is seen in the cavity, and the same process is repeated. At certain times, however, the Daphnia may be seen with a dark opake substance within the back of the shell, which has been called the ephippium, from its resemblance to a saddle. This, when carefully examined, is found to be of dense texture, and to be composed of a mass of hexagonal cells; and it contains two oval bodies, each consisting of an ovum covered with a dense horny casing, enveloped in a capsule, which opens like a bivalve shell.
From the recent observations of Mr. Lubbock*, it appears that the ephippium is really only an altered part of the carapace, its outer walls being a part of the outer layer of the epidermis, and its inner valve the corresponding part of the inner layer. The development of the ephippial eggs takes place at the posterior part of the ovaries, and is accompanied by the formation of a greenish-brown mass of granules; from this situation the eggs pass into the receptacle formed by the new carapace, where they become included between the two layers of the ephippium. This is cast, in process of time, with the rest of the skin, from which, however, it soon becomes detached, and continues to envelope the eggs, generally floating on the surface of the water until they are hatched with the returning warmth of spring. This curious provision is obviously destined to afford protection to the eggs which are to endure the severity of the winter's cold. There seems a strong probability, from the observations of Mr. Lubbock, that the ephippial eggs are true sexual products, since males are to be found at the time when the ephippia are developed, whilst it is certain that the ordinary eggs can be produced non-sexually, and that the young that spring from them can reproduce their race in like manner.
It has been ascertained by Dr. Baird that the young produced from the ephippial eggs have the same power of continuing the race by non-sexual reproduction as the young developed under ordinary circumstances. In most Entomostraca, the young, when first hatched, have only the thoracic portion of the body as yet developed, and possess but a small number of locomotive organs. The eyes, too, are at first frequently wanting. The process of development goes on with great rapidity, the animal at each successive moult (which process takes place at intervals of a day or two) presenting some new parts and becoming more and more like its parent, - the females laying eggs before they have acquired their full size.
* Proc. of Roy. Soc, Jan. 29, 1857.
(1075). The cast shell carries with it the sheaths not only of the limbs and plumes, but of the most delicate hairs and setse connected with them. If the animal have previously sustained the loss of a member, it is gradually renewed at the next moult, as in the higher Crustacea.