244 PLATYHELMINTHES. 



segmentation of the generative organs ; while in other cases the 

 segments of the body are clearly differentiated, and each is provided 

 with a set of generative organs, but they do not attain individual 

 independence (Ligula). The proglottides, however, usually become 

 separated off, and after their separation from the body of the 

 tape-worm continue to live for some time independently, and in 

 some cases even increase considerably in size, if they remain within 

 the intestine of their host (Echeneibotliriuin, Calliobothrium, etc.). 

 The proglottis after separation rrfHy remain for a certain time in 

 the intestine, but eventually makes its way either passively in the 

 faeces (e.g. T. solium), or actively by its own movement ( T. saginata) 

 to the exterior per anum. Here it retains its vital power for a 

 short time, and crawling away from the faeces ascends the stalks 

 of plants. It soon dies, and the body decomposes and the eggs 

 are scattered; or in some cases the eggs escape through a rupture 

 in the body-wall, and are left as a trail on the objects over which 

 the proglottis crawls. The eggs soon lose their vitality in a dry 

 atmosphere. The proglottides of T. saginata, which have consider- 

 able powers of movement, have been found on the wall a yard above 

 the bed of their quondam host, and they frequently creep over his 

 warm body. 



These facts seem to be sufficient to justify the view that the 

 tape-worm is a colonial or polyzoic animal, the individual members 

 of which have the power of separate and independent life. At 

 the same time the existence of monozoic forms like Archigetes 

 (Fig. 211) and Caryopliyllaeus (Fig. 212) must not be forgotten: 

 in these the Cestodariidae there is only one set of generative 

 organs, and the body is unsegmented ; the head and body not being 

 sharply distinguished. It would appear that these monozoic forms 

 of Cestoda have the same relation to the proglottis that Lucernaria 

 has to Aurelia in the Acalephae. Just as the Lucernaria may be 

 compared to a Scyphistoma, which develops generative organs and 

 does not strobilate, so an Archigetes may be looked upon as a scolex, 

 which becomes sexual but does not bud off proglottides (i.e. does 

 not develop into a strobila). 



This is the only satisfactory mode of regarding the Cestoda; 

 especially as the entire tape-worm, and not the proglottis alone, 

 corresponds to the Trematode, and is to be regarded as being 

 derived from the latter by a simplification of organization and loss 

 of the alimentary canal. 



The anterior part of the tape-worm is narrow, and presents a 



