THE CESTOIDEA. 



183 



ranged in longitudinal series. The openings of the genital 

 glands are situated in the middle line of the bod}'. These 

 parasites inhabit fishes and amphibians, as well as water- 

 birds, but they attain their sexual state only in the latter. 



Fig. 46.— Diagram of the structure of a cestoid worm, with only one joint. The posi- 

 tion of the hooks of a Ttenia and of one of the proboscides of a Tetrarhynchus 

 is indicated. A, head and neck; B, segment of the body corresponding with a 

 proglottis: a, rostellum; J, rostella spines (Tamid); c, c', o", spinose eversible 

 proboscis {Tetrarhynchus) ; d, sucker; e, ganglion (?) ; /; lateral, and g, circular 

 water-vessel ; h, ramifications of the water-vessels ; k, anastomosing trunk ; *, 

 contractile vacuole ; I, genital vestibule ; m, penis and vas deferens ; n, vagina ; 

 o, common cavity and vesicula seminalis interior ; p, ovary; q, uterus ; r, vitel- 

 larian duct. 



In the more typical Cestoidea the body is elongated, and 

 presents, at one end, a head provided with suckers, and very 

 generally with chitinous hooks, either disposed circularly 

 around the summit of the head, or upon proboscidiform ten- 

 tacles, which can be retracted into, or protruded from, the 

 head. Sometimes the head is produced into lobes ; and very 

 generally, when lobes or tentacles exist, they are four in 

 number, and are disposed symmetrically round the head. A 

 short distance beyond the latter, the slender body widens and 

 becomes transversely grooved, so as to be marked out into 

 segments. Longitudinal water-vessels run parallel with one 

 another through the body, and are connected by transverse 

 trunks in each segment, and by a circular vessel in the head. 

 In Bothriocephalus latus, the principal trunks are occupied 

 by a spongy reticulated tissue. 



In most of the tape-worms, innumerable, solid, strongly- 



