Till-; CHSTOIDEA. Is;} 



ranged in longitudinal series. The openings of the genital 

 glands ;ire situated in the middle line of tin- body. T 

 parasites inhabit fishes and amphibians, as well as \% 

 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 poei- 

 tion of the hooks of n Ttenia. and of one of the proboscides of a Tttntrhynrhu* 

 is indicated. -4, head and neck; B, seijment of the body corresponding uiili a 

 proy'ottiK: , rostelluin ; !>, mstella spines (Tania); c, </, o", epiuoee avertible 

 proboscis (TNJrar&ffno&tM) ; c/, sucker; e, ganglion (?) ; ./, lateral, and g, circular 

 wat'-r -vf*-cl ; A, ramifications of the water-vessels ; fc, anastomosing trunk ; ', 

 contractile vacuole ; /, genital vestibule; m. penis and vas deferene ; n, va^rinn; 

 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 

 p -Morally, when lobes or tentacles exist, they are four in 

 number, and are disposed symmetrically round tin- head. A 

 short distance beyond the latter, the slender body \vidrns 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 Sothrioeephalits /atus, the principal trunks are occupied 

 by a spongy reticulated tissue. 



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



