The winter-eggs are fertilised in the same part of the carapace of the female in which the parthenogenetic eggs develop, but after fertilisation they are thrown off from the body of the mother, either with or without a protective envelope formed from the cuticle of the carapace. The eggs of Sida are surrounded by a thin layer of a sticky substance, and when cast out of the maternal carapace they adhere to foreign objects, such as water-weeds; those of Polyphemus have a thick, gelatinous coat; in Leptodora and Bythotrephes the egg secretes a two-layered chitinous shell. In these forms the cuticle of the parent is not used as a protection for the winter-eggs, although it is generally, if not invariably, thrown off when the eggs are laid. In the Lynceidae the cuticle is moulted in such a way that the winter-eggs remain within it, at least for a time; the cuticle is occasionally modified before it is thrown off; thus in Camptocercus macrurus the cuticle of the carapace, in the region of the brood-pouch, becomes thickened and darkly coloured, forming a fairly strong case round the eggs. The modification of the cuticle round the brood-pouch is much more pronounced in the Daphniidae, where it leads to the formation of a saddle-shaped cuticular box, the “ephippium,” in which the winter-eggs are enclosed. The ripening of a winter-egg in the ovary of a Daphnia is accompanied by a great thickening of the cuticle of the carapace (cf. Fig. [18]), so that a strong case is formed in the position of the brood-pouch. The winter-eggs are laid between the two valves of this case, and shortly afterwards the parent moults. The eggs are retained within the ephippium, from which the rest of the cuticle breaks away (Fig. [18]). After separation, the ephippium, which contains a single egg (Moina rectirostris) or usually two (Daphnia, etc.), either sinks to the bottom, as in Moina, or floats.
The winter-eggs usually go through the early stages of segmentation within a short time after they are laid, but after this a longer or shorter period of quiescence occurs, during which the eggs may be dried or frozen without injury. The sides and floor of a dried-up pond are often crowded with ephippia, containing winter-eggs which develop quickly when replaced in water; and the resting-stage of winter-eggs produced in aquaria can often be materially shortened by drying the ephippia which contain them, though such desiccation does not appear to be necessary for development. Under normal conditions large numbers of winter-eggs remain quiescent through the winter and hatch in the following spring.
The individual developed from a sexually fertilised winter-egg is invariably a parthenogenetic female: the characters of the succeeding generations differ in different cases.
In a few forms, of which Moina is the best known, the parthenogenetic female, produced from a winter-egg, may give rise to males, to sexual females, and to parthenogenetic females, so that the cycle of forms which intervene between one winter-egg and the next is short. A sexual female produces one or two winter-eggs, and if these are fertilised they are enclosed in an ephippium and cast off; if, however, the eggs when ripe are not fertilised, they atrophy, and the female produces parthenogenetic eggs, being thenceforward incapable of forming sexual “winter” eggs. An accidental absence of males may thus lead to the occurrence of parthenogenesis in the whole of the second generation. The regular production of sexual individuals in the second generation from the winter-egg appears to depend on a variety of circumstances not yet understood. Mr. G. H. Grosvenor tells me that Moina from the neighbourhood of Oxford may give rise to several successive generations of parthenogenetic individuals, when grown in small aquaria.
In the greater number of Daphniidae, the parthenogenetic female, produced from a winter-egg, gives rise only to parthenogenetic forms, and it is not until after half a dozen parthenogenetic generations have been produced that a few sexual forms appear, mixed with the others. Such sexual forms are fairly common in April or May in this country; they produce “winter” eggs and then die, the generations which succeed them through the summer being entirely parthenogenetic. In late autumn sexual individuals are again produced, giving rise to a plentiful crop of winter-eggs, but many parthenogenetic females are still found, and some of these appear to live and to reproduce through the winter.
In Sida, in the Polyphemidae and Leptodoridae, and in most of the Lynceidae, sexual individuals are produced only once in every year, while in a few forms which inhabit great lakes the sexual condition occurs so rarely that it is still unknown.
Weismann[[33]] has pointed out that the sexual forms, with their property of producing eggs which can endure desiccation, recur most frequently in species such as Moina, which inhabit small pools liable to be dried up at frequent intervals, while the species which produce sexual forms only once a year are all inhabitants either of great lakes which are never dry, or of the sea. Many suggestions have been made as to the environmental stimulus which induces the production of sexual individuals, but nothing is definitely known upon the subject.
We have said that even in those generations which contain sexual males and females there are always some parthenogenetic individuals; there is therefore nothing in the behaviour of Daphniidae, either under natural conditions or when observed in aquaria, to suggest that there is any natural or necessary limit to the number of generations which may be parthenogenetically produced.
The parthenogenetic Daphniidae are extremely sensitive to changes in their surroundings; small variations in the character and amount of substances dissolved in the water are often followed by changes in the length of the posterior spine, in the shape and size of crests on the head, and in other characters affecting the appearance of the creatures, so that the determination of species is often a matter of great difficulty. It is remarkable that the green light which has passed through the leaves of water-plants appears to have a prejudicial effect upon some species. Warren has shown that Daphnia magna reproduces more slowly when exposed to green light, and that individuals grown in this way are more readily susceptible to injury from the presence of small quantities of salt (sodium chloride) in the water than individuals which have been exposed to white light.
The majority of the Cladocera belong to the floating fauna of the fresh waters and seas; a few are littoral in their habits, clinging to water-weeds near the shore, a very few live near the bottom at considerable depths, but the majority belong to that floating fauna to which Haeckel gave the name of “plankton.” The Crustacea are an important element in the plankton, whether in fresh waters or in the sea, the two great groups which contribute most largely to it being the Cladocera and the Copepoda. For this reason it will be more convenient to discuss the habits and distribution of individual Cladocera and Copepoda together in a chapter specially devoted to the characters of pelagic faunas (cf. Chap. VII.). We will only add to the present chapter a table of the families with a diagnosis of the British genera.