Fig. [41].—Chondracanthus zei, ♀, × 4.
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.
We now return to two semi-parasitic families, Fam. 8, Nereicolidae, and Fam. 9, Hersiliidae, in which there is certainly no well-developed siphon, but the upper and under lips protrude, forming a hollow between them in which the mouth-parts work. Both families are ectoparasites which frequently leave their hosts, and they retain their segmentation and powers of swimming. Perhaps the best-known form is the Hersiliid, Giardella callianassae, which lives in the adult state in the galleries excavated in the sand by Callianassa subterranea, gaining its nourishment as an ectoparasite on the Decapod. The larvae are pelagic, and are said by Thomson[[55]] to occur in Liverpool Bay.
List[[56]] describes Gastrodelphys, a parasite of doubtful position, from the gills of tubicolous worms, such as Myxicola and Sabella, which possesses a perfectly siphonostomatous mouth.
The remaining families to be dealt with are those containing all the fish-parasites which possess a true siphonostome, as well as the siphonostomatous family Choniostomatidae, which is parasitic on other Crustacea. In all these forms the mouth is prolonged into a tube in which the styliform mandibles work.
Fam. 10. Caligidae.—Ectoparasites on fish, lodging most frequently in the gill-chamber. In most of the genera the segmentation and power of swimming are retained in both sexes, the sexual dimorphism not being very well marked, though the males are smaller than the females, and were in some cases originally described as belonging to a special genus Nogagus. The females carry two long egg-sacs; the general structure may be made out from the ventral view of Caligus nanus (Fig. [43]).