690 THE ANIMAL KINGDOM. 



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) back- 

 wards. 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 segmen- 

 tation, 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 Verte- 

 brate, 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. 



