FAMS. ONCAEIDAE, CORYCAEIDAE, LICHOMOLGIDAE, ERGASILIDAE, BOMOLOCHIDAE, CHONDRACANTHIDAE, PHILICHTHYIDAE, NEREICOLIDAE, HERSILIIDAE, CALIGIDAE, LERNAEIDAE, LERNAEOPODIDAE, CHONIOSTOMATIDAE. The families Oncaeidae and Corycaeidae contain pelagic forms of flattened shape and great swimming powers, but the structure of the mouth-parts in the Corycaeidae points to a semi-parasitic habit. =Fam. 1. Oncaeidae.=—This family, including the genera _Oncaea_, _Pachysoma_, etc., does not possess the elaborate eyes of the next family, nor is the sexual dimorphism so marked. =Fam. 2. Corycaeidae.=—These are distinguished from the Oncaeidae, not only by their greater beauty, but also by the possession of very elaborate eyes, which are furnished with two lenses, one at each end of a fairly long tube. The females of _Sapphirina_ are occasionally found in the branchial cavity of Salps, and their alimentary canal never contains solid particles, but is filled with a fluid substance perhaps derived by suction from their prey. _S. opalina_ may occur in large shoals, when the wonderful iridescent blue colour of the males makes the water sparkle as it were with a sort of diurnal phosphorescence. The animal, however, despite the opinion of the older observers, is not truly phosphorescent. It may be that the ornamental nature of some of the males is correlated with the presence of the curious visual organs, which are on the whole better developed in the females than in the males. As in so many pelagic Copepods, the body and limbs may bear plumed setae of great elaboration and beautiful colour, _e.g._ _Copilia vitrea_ (Fig. 37). We now pass on to the rest of the parasitic Copepods,[51] which probably belong to the tribe Isokerandria, and we meet with the same variety of degrees of parasitism as in the Ampharthrandria, often leading to very similar results. [Illustration: FIG. 37.—_Copilia vitrea_ (Corycaeidae), ♀, × 20. (After Giesbrecht.) ] In the first seven families mentioned below there is no siphon. The Lichomolgidae and Ergasilidae have not much departed from the free-living forms just considered, retaining their segmentation, though in the Ergasilidae the body may be somewhat distorted (Fig. 39). In both families the thoracic swimming feet are of normal constitution. [Illustration: FIG. 38.—_Lichomolgus agilis_, × 10. _Abd. 1_, 1st abdominal segment; _cpth_, cephalothorax; _Th.1_, 1st thoracic segment; _Th.5_, 5th thoracic appendage. (After Canu.) ] =Fam. 3. Lichomolgidae.=[52]—These are semi-parasitic in a number of animals living on the sea-bottom, such as Actinians, Echinoderms, Annelids, Molluscs, and Tunicates. _Lichomolgus agilis_ (Fig. 38) occurs in the North Sea, Atlantic, and Mediterranean, on the gills of large species of the Nudibranch, _Doris_, while _L. albeus_ is found in the peribranchial cavity and cloaca of various Ascidians. _Sabelliphilus_ may infect the gills of Annelids such as _Sabella_, and is common at Liverpool.