234 



ZOOLOGY. 



of a suitable host, the joint and the egg-shells, or the 

 latter only, as the case may be, are digested in the 

 stomach or intestine, so that the larvse are set free. 

 These (Fig. 137) are spherical, glass-like, of micro- 

 scopic size, as might be expected, and bear three pairs 



Fig. 136. — The Common Tapeworm (TfEwia officindlis) : a, head and neck, strongly 

 magnified ; J}, joints, natural size ; c, eggs, strongly magnified. 



of booklets, by means of which they quickly perforate 

 the wall of the gut, and make a way through the 

 tissues of the host. They may also be carried further 

 by the blood-stream. The larva of 

 a particular species settles down 

 finally, not only in some particular 

 kind of host, but also in some defi- 

 nite organ or tissue. It then loses 

 its hooks, and is rapidly trans- 

 formed into a hollow bladder-worm, 

 (measle), which grows till it reaches a definite size, 

 depending on the species. Meanwhile it gradually 

 develops one (Crjsticercus) or several (Gcenurus) in- 



FiG. 137. — Tapeworm 

 Larva (of TiEnia solium), 

 mucli enlarged. 



