FAMILY I. ECHINORHYNCHIDAE.—The body is elongated and smooth. The proboscis-sheath has a double wall, and the proboscis is invaginated into it. The central nerve-ganglion lies in the middle line, as a rule on the posterior blind end of the proboscis-sheath. The papillae which bear the hooks are only covered with a chitinous cap at their apex, and the hooks have a process below. This family is by far the largest; a few species only can be mentioned. _Echinorhynchus proteus_ lives in its mature form in fishes; the young forms, up to a centimetre in length, are found living freely in the intestine of numerous fresh-water fishes. Those found in _Gobio fluviatilis_, the gudgeon; _Leuciscus virgo_; _Lota vulgaris_, the burbot or eel-pout; young trout; _Thymallus vulgaris_, the grayling, seldom surpass this size, but those found in _Acerina cernua_, the pope fish; in _Abramis bipunctatus_; in _Esox lucius_,the pike, and in older trout, attain or surpass double the length. As the parasites grow older they bury their proboscis and neck in the wall of the intestine, the inner surface of which is studded with the orange-coloured bodies of the parasites. The proboscis is so deeply sunk in the wall of the alimentary canal as to form a papilla on its outer surface (Fig. 92). The larvae of _E. proteus_ are found in the body-cavity of _Gammarus pulex_, one of the Amphipod Crustacea, and also in the same position in numerous fresh-water fishes; they must have passed into this first host by the {183}mouth and alimentary canal. If the liver of an infested minnow, _Leuciscus phoxinus_, be examined, it will be found to contain on its surface numerous spherical or egg-shaped capsules of an orange colour, 2 to 2.5 mm. in length; these contain the larval forms of the parasite. They develop into the adult form when the first host is eaten by a carnivorous fish, but a complication may take place when the larval form is found in _Gammarus_, as the latter, the first host, may be eaten by a fish (intermediate host) in which the larva does not become mature, and only develops sexual organs when eaten by a carnivorous fish (second host). The larval form is also found in _Nemachilus barbatulus_, _Gobio fluviatilis_, and the sticklebacks _Gasterosteus aculeatus_ and _G. pungitius_.