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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.

The fourth subclass, Cirripedia, comprises the Barnacles and Acorn-shells. These are very unlike any of the other Crustacea, and, in fact, they were long classed by naturalists with the Mollusca. It was not until their larval development was made known that they were recognized as Crustacea. The common Goose Barnacle (Lepas anatifera—[Plate III].) is found adhering to the bottoms of ships and to floating timber. It has a fleshy stalk or peduncle which is fixed at one end to the supporting object, and bears at the other end a shell, made up of five separate plates, enclosing the body of the animal. The stalk corresponds to the front part of the head, and careful examination may discover at its end, among the hardened cement which fixes it to the support, the remains of the antennules by which the attachment of the young animal was first effected. The body of the animal within the carapace or shell bears the usual mandibles, maxillulæ, and maxillæ, close to the mouth, and six pairs of long, tendril-like feet. These feet have each two branches, composed of numerous short segments and fringed with long hairs. They can be protruded from the slit-like opening of the shell, forming a sort of "casting-net" for the capture of minute floating prey.

The Acorn-shells, of which one species (Balanus balanoides—[Plate III].) is abundant everywhere on our coasts, covering rocks and stones just below high-water mark, differ from Lepas and its allies in having no peduncle. The shell is cemented directly to the rock, and is conical in shape, like a small limpet, with a hole at the top which is closed by four movable valves.

PLATE III

GROUP OF SPECIMENS OF THE GOOSE-BARNACLE (Lepas anatifera), ONE SHOWING THE CIRRI EXTENDED AS IN LIFE. (NATURAL SIZE)

(From Brit. Mus. Guide)