FLAT- WORMS. 



461 



PILIDIUM LARVA, WITH NEMERTINE WORM 

 DEVELOPING INSIDE. 



shorten its body to a very considerable extent. The chief centre of the nervous 



system lies in the head, and from this portion, which may be called the brain, 



nerves pass forwards to supply the suckers, while a single stout cord runs back- 

 wards on each side to the end of the body, 



lying close to the edge of the segments. As 



already pointed out, there is no trace of a 



mouth nor intestinal canal, although there is 



an excretory organ, consisting of a ring-shaped 



vessel in the head, from which four tubes, 



corresponding in position with the sucker, are 



prolonged backwards. Two of these soon 



vanish, but the others lying near the edges of 



the segments, close to the inner side of the 



nerve-chords and the longitudinal muscular 



band, extend to the hinder end of the body, where 



they unite and communicate with the exterior 



by a common aperture. At the hinder end 



of each of the segments these two ducts are 



united by a third, which runs across from side 



to side. In addition to these structures, each 



fully -grown segment contains a complete set of organs for the formation and 



fertilisation of eggs, of which an immense number are developed. The mature 

 segments begin at a distance of about a foot from the head, 

 and those at the hinder end of the body are the first to become 

 distended with eggs ready to hatch. The eggs, however, are 

 not laid within the human body, but the ripe segments break 

 off one by one, beginning with the last, and pass out of the 

 e intestine. The rup- 



ture is effected by 

 the contraction of 

 the muscular fibres, 

 which acts upon the 

 transverse vessel of 

 the excretory system 

 of the segment in 

 front, in such a way 

 that a fresh terminal 

 pore is formed. The 

 ripe proglottis, or 

 segment ready for 

 separation, is little 

 more than a sac 

 that is crammed with 



minute spherical eggs, set free by the bursting of its walls. In this way the 



eggs, which, on account of their thick protecting shell, are able to withstand the 



most unfavourable conditions, are disseminated abroad ; and, owing to their vast 



r 



HUMAN TAPE-WORMS. 



a Taenia solium (nat. size) ; b, c, d, Head enlarged ; e, Segments : 

 /, Segment of Taenia saginata (enlarged). 



