The New International Encyclopædia/Fish-Louse

FISH-LOUSE, or SEA-LOUSE. Any of various small crustaceans (copepods) which live parasitically on the outside or in the branchial chambers of marine animals, especially fishes and whales. All are of small size, and attached either temporarily or permanently to the hosts on the juices of which they live, although many species have also the power of swimming freely in the water, some of their legs being adapted to this purpose. They do not begin life as parasites, the females depositing their eggs on stones, plants, etc. They are animals of singular form and appearance. In the genus Aragulus there is a curious sucking disk on each side of the beak, or proboscis, although there are also jointed members terminated by prehensile hooks. In the genus Caligus the hooks of the anterior pairs of feet are the principal organs of adhesion to the slippery bodies of the fishes from which food is to be drawn, and the abdomen of the female is furnished with two remarkably long tubes, the functions of which are not perfectly ascertained. The bodies of all of them are transparent, or nearly so. Consult United States Fish Commission Annual Reports (Washington, 1871 et seq.) for particulars as to the extensive list of species catalogued in American waters. See Copepoda.