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). Some of the Caligidae are distinguished by the terga of the thoracic segments being expanded to form large chitinous elytra, e.g. _Cecrops_, found parasitic on the gills of the Tunny and on the Sun-fish (_Orthagoriscus mola_). _Caligus rapax_ is parasitic on the skin and in the gills of Sea-Trout, Pollan, etc.; and _C. lacustris_ is common in fresh-water lakes and streams on Pike and Carp. [Illustration: FIG. 43.—_Caligus nanus_, × 10. _Abd.1_, 1st abdominal segment; _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Mx.1_, _Mx.2_, 1st and 2nd maxillae; _Mxp_, maxillipede; _s_, siphon; _Th.1_, _Th.5_ 1st and 5th thoracic appendages. (After Gerstaecker.) ] [Illustration: FIG. 44.—_Lernaea branchialis_ from the Haddock, ♀, × 1. _Ceph_, cephalothorax; _e.s_, egg-sacs. (After Scott.) ]