3^4 



ELEMENTS OF BIOLOGY 



adult stage in the human intestine only. Such strictly limited adap- 

 tations are frequently associated with degenerative structural 

 changes. The tapeworm, one of the phylum Platyhelminthes, does 



EMERGENT 

 FORM 



PROGLOTTID 



TESTES 



GENITAL PORE 

 OVARY 



CYSTICERCUS 



UTERUS 



DISTENDED 



WITH RIPE 



EGGS 



FREE EMBRYO 



FERTILIZED EGG 



Fig. 2 1 8. — Diao^rammatic representation of the life cycle of the human tape worm, 

 Tcenia solium. (Partly after Kiihn: Grundriss der allgemeinen Zoologie, published 

 by Georg Theime, Leipzig.) 



not possess a functional digestive canal, absorbing its food already 

 digested from the food in the intestine of the host. 



The Tapeworm as an Example of Limited Habitat. The 

 human tapeworm, Tcenia solium (Fig. 218) is one of the class 

 Cestoda of the phylum Platyhelminthes. In its adult stage it is 

 found only in the human intestine. The adult body consists of a 

 modified head termed the scolex, equipped with hooks by which it 



