=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. [Illustration: FIG. 30.—_Haemocera danae_, × 40. =A=, Side view ♀; =B=, ventral view ♂. _Ant.1_, 1st antenna; _e_, eye; _ov_, ovary; _ovd_, oviduct; _St_, stomach; _Th.1_, 1st thoracic appendage; _Th.5_, 5th thoracic segment; _vd_, vas deferens. (After Malaquin.) ] [Illustration: FIG. 31.—Free-swimming Nauplius larva of _Haemocera danae_; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _e_, remains of eye; _Md_, mandible. (After Malaquin.) ]