This small class of entoparasitic Vermes contains the single genus Echinorhynchus with many species very variable in size. The adult worm is found in the alimentary canal of some Vertebrate, the immature in the body of a Non-vertebrate; e. g. Ech. Proteus of the Pike and other freshwater fish is found in the body cavity of the Amphipod Crustacean Gam-marus pulex; E. angustatus of the Perch in the Isopod Asellus aquaticus.

The body is divisible into three regions, a proboscis, neck, and body proper, which is cylindrical, and when contracted, transversely wrinkled. The proboscis carries a variable number of chitinoid recurved hooks, arranged in longitudinal rows, and is plunged into the tissues of the host. The body-walls consist of an external delicate cuticula, a striated sub-cuticula, a layer of circular and most internally of longitudinal muscle-cells. The subcuticula has an outer layer composed of a complicated tissue of circular and longitudinal fibres and an inner layer of radial fibres, amid which lies a system of canals. The subcuticula of the body proper, and consequently its canal system, are separated from the corresponding structures in the neck by a fold of cuticula. Two remarkable semilunar masses of subcuticula, the lemnisci, depend from the neck into the body cavity. They vary in length in different species. The canal system of the body proper consists of two longitudinal vessels, right and left in most instances, dorsal and ventral in Ech. clavaeceps, with lateral vessels which branch and anastomose.

There is a circular canal at the base of the neck connected with an irregular network, but in the proboscis longitudinal vessels alone are found, one between each series of hooks, united laterally between each pair of hooks. The circular vessel also sends into each lemniscus two principal vessels, each of which divides into two with a number of secondary branches. There is no opening into the system of canals. Their contents are a granular liquid. The granules appear to be of a fatty nature, and are orange coloured, or in the lemnisci of Ech. Proteus brownish-yellow. Nuclei of large size are also found in the canals as well as in the intervening fibres, and differ in shape, etc. m the body proper and the neck. The muscular layers of the body are continued into the neck, and the longitudinal layer forms a sheath for each lemniscus. A longitudinal muscle layer is absent in the proboscis. The muscular tissue is peculiar and somewhat like that of Nematoda, the striated fibrillar substance being developed only on the outer surface of the cell.

A double circular layer of muscular cells forms a proboscis sheath, which hangs down towards the coelome and is attached to the base of the proboscis. It contains four longitudinal retractor muscles for the proboscis, by means of which that structure can be invaginated. These retractors are continued on (?) through the sheath into the coelome as two retractor muscles of the proboscis-sheath and are attached to the body-walls.

A mass of ganglion cells lies at the bottom of the proboscis-sheath. It sends two sets of nerves forwards and two nerves (posterior lateral) backwards. The latter pass to the sides of the body inclosed each within a muscle-cell, the retinaculum. In the male two ganglia, connected together, lie anteriorly upon the genital sheath, and not only supply the sexual apparatus, but are connected to the two nerves just mentioned. There are no organs of special sense. Mouth and alimentary tract are also absent. There is a coelome. An axial muscular sheath, the suspensory ligament, is attached to the base of the proboscis-sheath. It is tubular, and incloses the testes, the first part of their ducts, and the cement glands in the male; the ovaries in the female. The two testes are ovular, one placed in front of the other. The two vasa deferentia which unite into a single ejaculatory duct are furnished each with 3-4 vesiculae seminales. There are 6-8 cement glands. Their ducts eventually open into the ejaculatory duct, and are surrounded by a muscular genital sheath derived from the longitudinal body-muscles. There is a papilliform penis situated at the base of a copulatory bursa formed by the invagination of the posterior end of the body, but capable of eversion by a compressor muscle.

The two ovaries are broken up at an early period into masses of egg-cells. These masses grow and multiply, eventually rupturing the suspensory ligament and filling the body cavity. The hind end of the ligament either passes into or is inserted (Ech. clavaeceps) round the edge of a 'bell.' This structure consists, as do the oviducts and uterus, of muscle cells with circularly disposed fibrillae. It opens and shuts rhythmically, and swallows the ova. The long oval ripe ova (really embryoes in their envelopes) pass onwards through the two oviducts into the uterus, but the unripe ova, which are rounder, are rejected by a dorsal aperture at the base of the bell. A vagina opens at the posterior end of the body. Its walls are composed externally of two sphincter muscles, internally of four or eight peculiar hour-glass shaped cells.

The ovum is impregnated in the coelome, undergoes unequal segmentation, and surrounds itself with three envelopes. The larva has a tapering body and an anterior disc bearing a circle of spines. It has a delicate cuticle, an underlying semifluid syncytium inclosing three structures, a rudimentary pharynx (?), a muscle which depresses the centre of the disc, and a granular mass or 'embryonic nucleus' formed by the central cells of the ovum. The larva inclosed within its envelopes passes out of the Vertebrate, and is swallowed by the Non-vertebrate host. It breaks through its envelopes, and enters the coelome. The adult worm is formed entirely from the 'embryonic nucleus' (supra) with the exception of the cuticula, which is the only larval structure that persists. It only becomes sexually mature when transferred to the alimentary canal of its Vertebrate host.

Saefftigen, M. J. x. 1884; Baltzer, A. N. 46, 1880 (contains litt). Development, Leuckart, Die Menschlichen Parasiten, ii. Leipzig, 1876. Female sexual organs of Ech. gigas, Andres, M. J. iv. 1878. Larva of Ech. clavaeceps, Villot, Z. A. viii. 1885.