=Fam. 4. Ergasilidae.=—_Thersites_ (Fig. 39) is parasitic on the gills of various fishes, _e.g._ _T. gasterostei_, which is common on _Gasterosteus aculeatus_ on the French and North Sea coasts, and may even be found on specimens of the fish that have run up the River Forth into fresh water. The animal possesses claw-like second antennae by which it clings to its host. [Illustration: FIG. 39.—_Thersites gasterostei._ =A=, ♀, × 10; =B=, ♂, × 20. _Abd. 1 & 2_, Fused 1st and 2nd abdominal segments; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _e.s_, egg-sac; _Th_, thoracic appendages. (After Gerstaecker.) ] Similarly characterised by the absence of a siphon are three other families of fish-parasites, the Bomolochidae, Chondracanthidae, and Philichthyidae. =Fam. 5. Bomolochidae.=—_Bomolochus_ (Fig. 40), parasitic on the skin of the Sole (_Solea_) and in the nostrils of Cod (_Gadus_), is held to be related to the Ergasilidae. The first thoracic limb is remarkably modified. Were it not for the absence of a siphon, it would be hard to separate this family from the Caligidae. [Illustration: FIG. 40.—_Bomolochus_, sp. (Bomolochidae), × 8. _Abd. 1_, 1st abdominal segment; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Mx.1_, _Mx.2_, 1st and 2nd maxillae; _Mxp_, maxillipede; _Th.1_, 1st thoracic appendage. (After Gerstaecker.) ] [Illustration: FIG. 41.—_Chondracanthus zei_, ♀, × 4. ] [Illustration: FIG. 42.—Dwarf male of _Lernentoma cornuta_ (Chondracanthidae), × 10. _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Th.1_, 1st thoracic segment. (After Gerstaecker.) ] =Fam. 6. Chondracanthidae.=—These Copepods infest the gills and even the mouth of various marine fish, such as the Gurnard, Plaice, Skate, Sole, and many others. The sexual dimorphism is very marked, the female being large, indistinctly segmented, and with irregular paired processes protruding from the sides of the body, giving the animal a monstrous form (Fig. 41); while the male (Fig. 42) is very small, has a completely segmented thorax, and lives clinging on to the female by its prehensile second antennae—_Chondracanthus_, _Lernentoma_. =Fam. 7. Philichthyidae.=—These parasites, which are hardly known to occur in British waters,[53] are mucus-feeders and infest the skin of Teleosts, _e.g._ the Sole; often taking up a position in the lateral line or in a slime canal. They show a similar sexual dimorphism to the foregoing family, the adult female being extraordinarily drawn out into finger-like processes (_e.g._ _Philichthys_)[54] or else long, slender, and Nematode-like, with much reduced appendages (_Lernaeascus_), while the male retains a more normal structure. As in all the foregoing forms there is no siphon.