Fish-louse

Fish-louse, a name widely applied to any of the Copepod crustaceans which occur as external parasites both on fresh-water and marine fishes. Some have also been found on amphibians. As they live on the juices of their hosts, they undoubtedly do damage, and the fishes on which they are abundant are often sickly. They may occur on the skin itself, but are even more common about the gills. To the zoologist they have a special interest on account of the degeneration which they often exhibit, when contrasted with their free-living relatives or even with their own young stages. They are often curiously misshapen, and swollen out with abundant feeding; the males are sometimes free and the females alone parasitic, or the males may be pigmies borne on their mates. In other cases the females become truly parasitic only after sexual union, when about to bear eggs, and the habit of parasitism has doubtless been acquired in some instances for the safety and convenience of the mothers. Some (e.g. Penella) pass from being external hangers-on to become true internal para- sites. The following are examples of common 'fish-lice' and of the hosts which they infest: Chondracanthus cornutus, on flat-fish (frequent); Caligus rapax, dog-fish, gurnard, lumpsucker, dab, whiting, &c. (frequent); Ergasilus sieboldii, carp, pike, &c. (frequent); Dichelestium sturionis, sturgeon (on gills); Lernæocera cyprinacea, carp, &c. (boring in flesh); Lernæa branchialis, flounder, &c. (on gills); Penella sagitta, fishing-frog (in flesh); Achtheres percarum, perch, &c. (on gills); Lernæopoda elongata, dog-fish (on eyes); Anchorella uncinata, cod species (on fins and gills); Argulus foliaceus, carp, perch, pike, stickleback, trout, &c. (frequent). The above are all Copepods—i.e. low down in the crustacean series; but parasitic Cirripedes (q.v.) occur in the skin of whales and fishes, and on other crustaceans; a little family of Amphipods (Cyamidae) also infest cetaceans; among Isopods, many Cymothoidæ live on the skin and gills of fishes, while Bopyridæ and Entoniscidæ frequent other crustaceans. See CRUSTACEA, PARASITIC ANIMALS.

Fish River Caves. See BLUE MOUNTAINS.