The subclass COPEPODA comprises animals, for the most part of microscopic size, which are abundant in fresh water and in the sea. The common fresh-water genus _Cyclops_ (Fig. 14) furnishes a good example of the type of structure characteristic of the class. The body is somewhat pear-shaped, with a narrow abdomen ending in a "caudal fork." The body is divided into somites, and there is no overlapping carapace, although the head and the first two thoracic somites are coalesced. There are four pairs of two-branched, oar-like, swimming feet, and a fifth pair, found in some other Copepoda, is represented in _Cyclops_ by minute vestiges on the first segment of the narrow posterior part of the body. The antennules are very large, unbranched and composed of numerous segments; the antennæ are much smaller. In addition to the usual mandibles, maxillulæ, and maxillæ, there is a pair of maxillipeds which really represent the first pair of trunk limbs. There is a single red eye in the middle of the front of the head. This eye is not formed, like the single eye of the Cladocera, by fusion of a pair of eyes, but it corresponds to a median eye of simple structure which is found in the Branchiopoda, Ostracoda, and many other Crustacea, in addition to the paired compound eyes. From the fact that this median eye is the only one present in the earliest larval stage of Crustacea, the Nauplius (see Chapter IV.), it is sometimes known as the "nauplius eye." The female _Cyclops_ carries her eggs until they hatch, in two oval packets attached to the sides of the body. Forming a separate order (BRANCHIURA) apart from the more normal Copepoda (order EUCOPEPODA) is the little group of the Carp-lice, one of which, _Argulus foliaceus_, is common in England, living as a parasite on different species of fresh-water fish, and often found swimming free in ponds and rivers. It has a broad, flat, and very transparent body, about three-sixteenths of an inch in length. It differs from _Cyclops_ in a great many points, of which, perhaps, the most conspicuous is the possession of a pair of true compound eyes in addition to the median eye. On the under-side of the head are a pair of large round suckers, by means of which the animal fixes itself on to its prey. A study of their development shows that these suckers are really the maxillæ, which in the young animal are jointed limbs ending in a strong claw, but later become changed into the suckers of the adult. A sharp spine, which can be protruded in front of the mouth, is connected with what is believed to be a poison-gland. The eggs are not carried in packets by the female as in _Cyclops_, but are deposited on stones or water-weeds.