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. [Illustration: FIG. 58--_Bythotrephes longimanus_, FEMALE, WITH EMBRYOS IN THE BROOD-SAC. × 12. (After Lilljeborg.)] _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.