THE CESTOIDEA. 



183 



ranged in longitudinal series. The openings of the genital 

 glands are situated in the middle line of the body. 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 liooks of a Ttenia and of one of the proboscides of a Jetrarhynchus 

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

 proglottis: a. rosteUum; b, rostella spines (Ttenia); c, c'. c". spinose eversible 

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

 water-vessel ; h, ramifications of the water-vessels ; £, anastomosing trunk ; i, 

 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- 



