Fig. [38].—Lichomolgus agilis, × 10. Abd. 1, 1st abdominal segment; cpth, cephalothorax; Th.1, 1st thoracic segment; Th.5, 5th thoracic appendage. (After Canu.)
Fam. 3. Lichomolgidae.[[52]]—These are semi-parasitic in a number of animals living on the sea-bottom, such as Actinians, Echinoderms, Annelids, Molluscs, and Tunicates. Lichomolgus agilis (Fig. [38]) occurs in the North Sea, Atlantic, and Mediterranean, on the gills of large species of the Nudibranch, Doris, while L. albeus is found in the peribranchial cavity and cloaca of various Ascidians. Sabelliphilus may infect the gills of Annelids such as Sabella, and is common at Liverpool.
Fam. 4. Ergasilidae.—Thersites (Fig. [39]) is parasitic on the gills of various fishes, e.g. T. gasterostei, which is common on Gasterosteus aculeatus on the French and North Sea coasts, and may even be found on specimens of the fish that have run up the River Forth into fresh water. The animal possesses claw-like second antennae by which it clings to its host.
Fig. [39].—Thersites gasterostei. A, ♀, × 10; B, ♂, × 20. Abd. 1 & 2, Fused 1st and 2nd abdominal segments; Ant.1, Ant.2, 1st and 2nd antennae; e.s, egg-sac; Th, thoracic appendages. (After Gerstaecker.)
Similarly characterised by the absence of a siphon are three other families of fish-parasites, the Bomolochidae, Chondracanthidae, and Philichthyidae.
Fam. 5. Bomolochidae.—Bomolochus (Fig. [40]), parasitic on the skin of the Sole (Solea) and in the nostrils of Cod (Gadus), is held to be related to the Ergasilidae. The first thoracic limb is remarkably modified. Were it not for the absence of a siphon, it would be hard to separate this family from the Caligidae.
Fig. [40].—Bomolochus, sp. (Bomolochidae), × 8. Abd. 1, 1st abdominal segment; Ant.1, Ant.2, 1st and 2nd antennae; Mx.1, Mx.2, 1st and 2nd maxillae; Mxp, maxillipede; Th.1, 1st thoracic appendage. (After Gerstaecker.)