Fig. 59—Diaptomus cœruleus, Female. × 25. (After Schmeil.)

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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.

The common "Freshwater Shrimp" (Gammarus pulex), which has already been described, may be taken as a type of a large number of Amphipoda, for the most part closely allied, which are widely distributed in most regions of the world, with the exception of the tropics. G. pulex itself ranges from the British Islands to Mongolia. As the eggs are carried, till they hatch, in the brood-pouch of the parent, and are not known to survive drying, it is difficult to understand in what way Gammarus and its allies contrive to spread from one locality to another.

The little fresh-water Isopod Asellus aquaticus ([Fig. 60]) is common in ponds and canals in this country. It may be recognized by its general resemblance to a Woodlouse, with very long antennæ, and with a pair of long, slender, forked uropods projecting behind. The species is widely distributed in Europe, and other species of the same and closely related genera are found in North America.