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.
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.)
Fig. [44].—Lernaea branchialis from the Haddock, ♀, × 1. Ceph, cephalothorax; e.s, egg-sacs. (After Scott.)
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].
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.)
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.)