sporulation. The quotidian form, however, may be the result of 

 double or triple infections having different times of sporulation. 



PLATYHELMINTHES. 



Organisms with leaf-like or ribbon-like form of body, associated 

 with low organization of the mesoderm, which consists of primitive 

 mesenchyme and inter-cellular spaces but no continuous 

 coelom. A high grade of specialization of the reproductive organs 

 associated in most cases with hermaphroditism. The group 

 shows a range of specialization extending from free-living to highly 

 modified parasitic types, the latter showing progressive develop- 

 ment of life history adaptations to the hosts, including structural 

 devices, sucking discs and hooks, for attachment. There is corre- 

 sponding degeneration of nervous system, sense organs, and in- 

 testine. Three classes are distinguished: 



Turbellaria. For the most part free-living flatworms, with 

 ciliated epithelium forming the surface layer of the body. 

 Simple or sacculated intestine with oral but no anal aperture. 

 Organisms of marine and fresh water, sometimes, but seldom, 

 parasitic. Type Planaria. 



Trematoda. Flatworms with cuticle and one, two (distom) 

 or more sucking discs. Digestive tube somewhat as in turbel- 

 larians, but often reduced. Organisms ectoparasitic or endo- 

 parasitic, i.e., living on the surface or in the interior of various 

 vertebrate hosts. Ciliated only in larval stage. Alternation of 

 generations with change of host. Type Fasciola hepatica; 

 Primary host, Sheep; secondary host, Pond-snail. 



Cestoda. Flatworms without alimentary canal, usually seg- 

 mented into proglottides with anterior scolex. Tapeworms. 

 An intermediate stage in the life-history and change of hosts. 

 Type Taenia solium; primary host, Man; intermediate host, Pig. 

 Convenient for laboratory study is Dipylidium caninum from in- 

 testine of dog; intermediate host, dog-flea (Ctenocephalus canis). 



Trematoda. The ectoparasitic forms are less specialized than 

 the endoparasitic ones, and have also a direct development (mono- 

 genetic). The endoparasitic or digenetic forms present in this 

 way a more advanced phase of the parasitic development. 



Fasciola hepatica is a lanceolate worm of 20 to 30 mm. in length, 

 living in the smaller bile-ducts of the sheep. The body is covered 

 by a resistant cuticular membrane of uncertain derivation. There 

 are two suckers, one anterior, encircling the mouth, the other 

 central. Older individuals show the coils of the uterus gorged 

 with fertilized eggs. The latter develop only in water into which 



8 



