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]]
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.)
Fam. 13. Choniostomatidae.[[60]]—The members of this family are all parasitic on other Crustacea. The majority live parasitically in the marsupial pouches of female Amphipods, Isopods, Mysidae, and Cumacea, e.g. Sphaeronella and Stenochotheres in the marsupia of Gammarids; but Choniostoma occurs in the branchial cavity of Hippolyte, Homoeoscelis is common in the branchial cavity of Diastylis and Iphinoe, and Aspidoecia on the outside of the body of the Mysid Erythrops. The males and females live together in the same marsupium, but the adult males retain the power of roving about, and do not feed so much as the females, though their mouth-parts are similarly constructed (Fig. [47]). Representatives occur all over the world, but the majority of species known at present are from the North Sea, the most abundant being Stenochotheres egregius, parasitic on the Gammarid Metopa bruzelii, Goës.
The male bears a median glandular thread on the forehead by which it attaches itself to the females or to the host. Hansen considers that the family is most closely allied to the Lernaeopodidae.
Fig. [48].—Argulus foliaceus, young ♂, × 15. a1, a2, First and second antennae; ab, abdomen; E, compound eye; l, liver; m, mandibles and first maxillae; mx, second maxilla (the median eye is seen between the two second maxillae); mxp, maxillipede; s.g, shell-gland; sp, spine; t, testis; 1, 4, first and fourth swimming appendages. (After Claus.)
BRANCH II. BRANCHIURA.
Fam. Argulidae.[[61]]—We have yet to mention this group of fish-parasites, related to the Copepoda, but occupying an isolated position. They are ectoparasites upon various species of fish, Argulus foliaceus being common in the fresh waters of Europe, infesting the branchial chamber or the skin of fresh-water fish, but being frequently taken swimming freely in the water. Both males and females can swim with great agility, and they leave their hosts regularly at the breeding season in spring and autumn; fertilisation is internal, and the female deposits the eggs on stones and other objects. After leaving its host, an Argulus, if it cannot find a fish of the same species, can live on almost any other species, and may even attack Frog tadpoles; while the kinds that infest migratory fish can change with their hosts from salt to fresh water, or the reverse. America appears to be the home of the Argulidae.[[62]]
The structure of an Argulid is exhibited in Fig. [48]. In front of the siphon, within which the styliform mandibles and first maxillae work, there is a poison-spine (sp); the appendages which correspond to the second maxillae (mx) are modified into sucking discs, but in the genus Dolops they terminate in normal claws. The next pair of appendages, usually spoken of as maxillipedes (mxp), are clasping organs, and behind follow four pairs of thoracic swimming feet (1–4). The body is foliaceous, and they always apply themselves to their hosts with the long axis pointing forwards and parallel to that of the host, while on various parts of the under surface of the body are spines pointing backward which prevent the parasite being brushed off by the passage of the host through the water. These animals, alone among the Copepoda, possess compound eyes.