The smaller Branchiopoda known as "Water-fleas," forming the order Cladocera, are abundant everywhere in fresh water. Daphnia pulex and other species of the genus, and the little Lynceidæ, of which Chydorus sphæricus ([Fig. 56]) is the commonest species, are to be found in ponds and ditches, and often swarm in farmyard ponds where the water is foul with decaying matter. In most gatherings from such localities only female specimens will be found, and nearly all of these will be seen to carry a cluster of eggs or of developing embryos in the "brood-chamber" between the back part of the body and the shell. In Daphnia pulex (see [Fig. 12], p. 37) a single brood may consist of thirty young, and occasionally of more than twice that number. As the broods may succeed each other at intervals of two or three days, it will be seen that the multiplication of the species in favourable circumstances may be exceedingly rapid. It has been calculated that in sixty days the progeny of a single female might amount to about 13,000,000,000. In addition to these parthenogenetic eggs, which hatch at once while still within the brood-chamber, the Cladocera produce, at certain seasons, another kind of egg which requires to be fertilized by the male before it will develop. These eggs are dark in colour and are enclosed in a thick shell, and they do not hatch at once, but are cast off when the shell of the female is moulted. Very commonly these "resting eggs," as they are called, are produced in the autumn and lie dormant until the following spring, and they can survive drying or freezing without injury, while the thin-shelled parthenogenetic eggs within the brood-chamber of the mother are easily killed. In addition to having thick shells, the resting eggs are further protected in most, but not in all, cases by the moulted carapace of the parent, which is specially thickened for the purpose. This modification of the carapace is most highly developed in the family Daphniidæ ([Fig. 57]), where a saddle-shaped area on the dorsal side, known as the "ephippium," becomes thickened, and on moulting separates from the rest of the carapace to form a compact case enclosing the two resting eggs. The outer wall of the ephippium is divided up into small hexagonal cells, which become filled with air, causing the ephippium to float at the surface of the water. In this position the ephippia readily become entangled in the feathers of birds, and in some cases the shell is provided with spines or hooks, which facilitate transport to other localities by such means.
The appearance of males and the production of ephippial eggs—in other words, the "sexual period"—is generally more or less restricted to one season of the year. In most species, particularly in those which live in lakes, the sexual period occurs in the late autumn, and the ephippial eggs lie dormant during the winter, and hatch in the spring. In species living in small ponds exposed to the risk of overheating or of drying up during summer, there is often a distinct sexual period in the spring, when ephippial eggs are produced to tide over the unfavourable conditions of the warmer months of the year. Although no species is known to be exclusively parthenogenetic, yet it appears that purely parthenogenetic colonies of certain species may be found in favourable localities, where they may reproduce from year to year without males ever being found.
Certain species of Cladocera belong to the plankton of lakes and large ponds, and show modifications which adapt them for a floating life. Some of these belong to the genus Daphnia, and differ from the species found in other situations by their glassy transparency. As in the case of many marine plankton Crustacea, this transparency is probably due to the thinness of the shell and to the general watery condition of the body, giving the necessary buoyancy to enable the animal to remain constantly afloat. The same effect is no doubt produced by the long terminal spine of the carapace and by the great helmet-shaped crest into which the upper part of the head is often produced. A form very characteristic of the plankton of large lakes is Bythotrephes ([Fig. 58]), which is found in the lakes of Scotland, Ireland, Wales, and the Lake District of England. In Bythotrephes the carapace does not enclose the body, but is reduced to a small brood-sac; the abdomen, however, is drawn out into a long spine, which may be two or three times as long as the body. A further point of interest is the division of the eye into a dorsal and a ventral portion, differing in structure in much the same way as do the two divisions of the eyes in certain marine plankton Crustacea (see [p. 152]). Another very remarkable lacustrine form is Leptodora, the largest of all the Cladocera, being sometimes more than half an inch in length. In this case also the carapace is very small, and does not enclose the body. The swimming antennæ are very large, and the abdomen is long and divided into several segments.
Fig. 58—Bythotrephes longimanus, Female, with Embryos in the Brood-sac. × 12. (After Lilljeborg.)
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Leptodora is further remarkable on account of its mode of development. The parthenogenetic eggs, as in other Cladocera, develop directly, but the resting eggs give rise to larvæ of the nauplius type.
Holopedium, which is found in similar situations, surrounds itself with a mass of a jelly-like substance which it secretes. A similar envelope of jelly is found in some marine plankton animals, though not, so far as is known, in any Crustacea, and it no doubt serves to give buoyancy to the animal.
The Copepoda of fresh water are as abundant and universally distributed as the Cladocera. Species of the genus Cyclops (see [Fig. 14], p. 39), easily recognized by the pear-shaped body and the two egg-packets carried by the female, are to be found in almost every pond and ditch. The genus Canthocamptus comprises species of smaller size, with slender, flexible body, and carrying only a single egg-packet. The plankton of lakes and ponds includes species of Diaptomus ([Fig. 59]), which have a narrow body and very long antennules. The latter are held out stiffly while the animal swims by rapid movements of the antennæ and mouth parts, making occasional sudden leaps by means of its oar-like feet. In this genus also the egg-packet is single. The development can easily be studied by keeping egg-carrying females of Cyclops in a jar of water, when the nauplius larvæ will soon hatch out.