252 ZOOLOGY 



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 Platy helminth es with a ciliated cellular 

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



Arodo). 



Order 1.— Polygladida. 



Flattened leaf-shaped Turbellaria, without separate yolk-glands ; 

 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. — Rhabdoccelida, 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,^ or of a 

 cellular epidermis ; ^ 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. 



' Except in certain species of Temnocephnla. 



^ Except in the Temnocephalea and Actinodactylella. 



