PODOPLEA ISOKERANDRIA 



73 



completely segmented thorax, and lives clinging on to tlie female 

 by its prehensile second antennae — Chondracanthiis, Zernentoma. 



Fam. 7. Philichthyidae. — These parasites, which are hardly 

 known to occur in British waters,^ 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. Philich- 

 thysY 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, Nereico- 

 lidae, 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 swim- 

 ming. Perhaps the best-known form is the Hersiliid, Giardella 

 callianassae, which lives in the adult state in the galleries ex- 

 cavated in the sand by Callianassa subterranea, gaining its 

 nourishment as an ectoparasite on the Decapod. The larvae are 

 pelagic, and are said by Thomson^ to occur in Liverpool Bay. 



List* describes Gastrodelpliys, 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 para- 

 sitic on other Crustacea. In all these forms the mouth is pro- 

 longed 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 Camliridge Museum possesses two specimens of Philichthys xiphiae, from 

 the frontal bones of a Swordfish {Xiphias gladius) taken off Lowestoft in 1892. 

 - Glaus, Arb. Zool. Inst. Wicn, vii., 1888, p. 281. 

 ' Proc. Biol. Soc. Liverpool, i., 1887. * Zeitschr. 'onss. Zool. xlix., 1890, p. 71. 



