252 ZOOLOGY SECT. 



it is never provided with an anal aperture. The completed egg 

 contains, in addition to the oosperm, a quantity of yolk-matter, 

 usually in the form of definite yolk-cells, and usually produced by 

 a special set of yolk-glands. Development is sometimes direct, 

 sometimes accompanied by a metamorphosis. 



CLASS I. TURBELLARIA, 



Mostly non-parasitic Platyhelminthes with a ciliated cellular 

 epidermis; with a digestive cavity (except in the sub-division 

 Ac&la). 



ORDEK 1. POLYCLADIDA. 



Flattened leaf-shaped Turbellaria, without separate yolk-glande : 

 testes and ovaries numerous ; male and female genital apertures 

 usually separate ; intestine complexly branched. 



ORDER 2. TRICLADIDA. 



Turbellaria with elongate depressed body ; with numerous yolk- 

 glands, two ovaries, numerous testes ; a single genital aperture ; 

 intestine consisting of a median anterior division and two lateral 

 posterior limbs which are provided with side branches. 



ORDER 3. RHABDOCXELIDA, incl ACCELA. 



Comparatively small Turbellaria, with the body usually elongate 

 and cylindrical or compressed : with simple, or nearly simple, 

 sac-like intestine ; with or without yolk-glands ; with one or two 

 ovaries and two or many testes. 



CLASS II.-TREMATODA, 



Ecto- or endo-parasitic Platyhelminthes devoid of cilia, 1 or of a 

 cellular epidermis ; 2 with a well-developed digestive apparatus. 



ORDER 1. MONOGENETICA (HETEROCOTYLEA). 

 Mostly ectoparasitic Trematodes ; with direct development. 



ORDER 2. DIGENETICA (MALACOCOTYLEA). 

 Endoparasitic Trematodes with complicated life-history. 



1 Except in certain species of Temnocephala. 



2 Except in the Temnocephalea and Actinodactylella. 



