=Fam. 11. Lernaeidae.=—These parasites burrow with their heads deep into the skin, or even into the blood-vessels or body-cavity, of various marine fish. The body of the adult female _Lernaea_ is extraordinarily deformed, consisting of a mere shapeless sac with irregular branched processes on the head, and two egg-sacs attached behind (Fig. 44). _Pennella sagitta_[57] bores so deeply into the flesh of its host, _Chironectes marmoratus_, that only the egg-sacs and some remarkable branchial processes attached to its abdomen protrude outside the host to the exterior. _Peroderma cylindricum_ bores similarly into the flesh of the Sardine, and where it is common, inflicts considerable damage. The males of these curious animals are of more normal structure (Fig. 45). Claus[58] states that fertilisation takes place when both sexes are free-swimming, and of a more or less similar structure, and that subsequently the female becomes fixed to her host and degenerates into the shapeless mass shown in Fig. 44. [Illustration: FIG. 45.—_Lernaea branchialis_, ♂, × 10. _Ant.1_, _Ant.2_, 1st and 2nd antennae; _Br_, brain; _e_, eye; _g_, stomach; _t_, testis; _vd_, vas deferens; _ves. sem_, vesicula seminalis. (After Claus.) ] [Illustration: FIG. 46.—_Achtheres percarum_. =A=, ♀, × 4; =B=, ♂, × 4. _Ant.2_, 2nd antenna; _g_, stomach; _Mx.2_, 2nd maxilla; _Mxp_, maxillipede; _ov_, ovary; _ovd_, oviduct. (After Gerstaecker.) ] =Fam. 12. Lernaeopodidae.=—This family may be illustrated by the common gill-parasite of Perch and Trout, known as _Achtheres percarum_. The female (Fig. 46), which is much larger than the male, and is not clearly segmented, is attached to the host by means of the maxillipedes, which are fused distally into a pad armed with chitinous hooks. In the male the maxillipedes are prehensile, but are not so fused. Besides _Achtheres_ there are other fresh-water forms, _e.g._ _Lernaeopoda salmonea_ on Salmon, and a number of marine genera. It appears that the larvae fix themselves to their hosts by means of a long glandular thread, which proceeds from the middle of the forehead.[59] [Illustration: FIG. 47.—Ventral view of _Stenochotheres egregius_ (Choniostomatidae), ♂. _A_, _A′_, 1st and 2nd antennae; _M_, mouth; _Mx_, 2nd maxilla; _T_, 1st thoracic leg. (After Hansen.) ]