_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. [Illustration: FIG. 59--_Diaptomus coeruleus_, FEMALE. × 25. (After Schmeil.)] Although the Copepoda, unlike the Cladocera, are not parthenogenetic, it has been found that certain species of _Diaptomus_ produce resting eggs capable of surviving freezing or drying. In the early part of the breeding season the eggs have thin shells, and they hatch after a short time. In the autumn, however, thick-shelled eggs are produced, which lie dormant in the mud until the following spring. It has recently been discovered that species of _Cyclops_ and _Canthocamptus_ pass through a resting stage, in which the animal surrounds itself with a cocoon-like capsule of mud held together by a glutinous secretion produced by glands on the surface of the body and limbs. The encapsuled animals, in the cases observed, lie dormant in the mud during the summer, to resume active life in the colder months of the year. It is very probable that they can also be dried without injury, and that the "cocoons" serve the same purpose as the resting eggs of other species. Numerous species of Ostracods, belonging to the genus _Cypris_ (see Fig. 13, B, p. 38), and other closely related genera, occur in fresh water. Like the Cladocera, they reproduce largely by parthenogenesis, and the males of many species are rarely found, while in some species they have not yet been discovered. In Professor Weismann's laboratory at Freiburg a colony of _Cypris_ was kept in an aquarium for eight years, and during the whole of that time no males made their appearance, the colony reproducing exclusively by parthenogenesis. Probably in all species the eggs survive drying.