=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. [Illustration: FIG. 48.—_Argulus foliaceus_, young ♂, × 15. _a^1_, _a^2_, 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]