Cyclops.—Numerous species, split up according to segmentation of rudimentary fifth pair of legs, number of joints in antennae, etc.
Fam. 2. Harpacticidae.—The cephalothorax is not clearly separated from the abdomen. The first antennae are short in both sexes, both being clasping organs in the male. The second antennae have a rudimentary exopodite. The fifth pair of limbs are rudimentary and plate-shaped; a heart is absent, and the egg-sacs of the female may be one or two in number.
1. Ophiocamptus (Moraria).—Body worm-shaped; first antennae of female 7–jointed, rostrum forming a broad plate.
2. Body not worm-shaped; first antennae of female 8–jointed, rostrum short and sharp.
(a) Endopodites of all thoracic limbs 3–jointed. The first antennae in female distinctly bent after the second joint. Nitocra.
(b) Endopodite of at least the fourth limb 2–jointed; first antennae in female not bent. Canthocamptus.
3. Ectinosoma.—Body as in 2, but first antennae are very short, and the maxillipede does not carry a terminal hooked seta as in 1 and 2.
Fam. 3. Peltiidae.[[44]]—This is an interesting family, allied to the Harpacticidae, and includes species with flattened bodies somewhat resembling Isopods, and a similar habit of rolling themselves up into balls. No parasitic forms are known, though Sunaristes paguri on the French and Scottish coasts is said to live commensally with hermit-crabs.
We have now enumerated the chief families of free-living Copepods; the rest are either true parasites or else spend a part of their lives as such. A number of the semi-parasitic and parasitic Copepods can be placed in the tribe Ampharthrandria owing to the characters of their antennae; but it must be remembered that many parasitic forms have given up using the antennae as clasping organs; however, the sexual differences in the antennae, and the fact that many of the species which have lost the prehensile antennae in the male have near relations which preserve it, enable us to proceed with some certainty. The adoption of this classification necessitates our separating many families which superficially may seem to resemble one another, e.g. the semi-parasitic families Lichomolgidae and Ascidicolidae, and the Dichelestiidae from the other fish-parasites; it also necessitates our treating the presence of a sucking mouth as of secondary importance. This characteristic must certainly, however, have been acquired more than once in the history of the Copepods, for instance in the Asterocheridae and in the fish-parasites, while it sometimes happens that genera belonging to a typically Siphonostomatous group possess a gnathostome, or biting mouth, e.g. Ratania among the Asterocheridae. Again, it is impossible even if we use the character of the mouth as a criterion to place together all the true parasites on fishes in one natural group, because the Bomolochidae and Chondracanthidae, which are otherwise closely similar to the rest of the fish-parasites, possess no siphon. It seems plain, therefore, that the parasitic habit has been acquired several times separately by diverging stocks of free-swimming Copepods, and that it has resulted in the formation of convergent structures.
