252 ZOOLOGY sE"ct. 



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 byra metamorphosis. 



CLASS I— TURBELLARIA. 



Mostly non-parasitic Platyhelminthes with a ciliated cellular 

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

 Acada). 



Order 1. — Polycladida. 



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. Acxela. 



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 TemnocephcUea and Actinodactylella. 



