248 ZOOLOGY . SECT. 



of Man, the bladder is thrown off, the scolex attaches itself to the 

 wall of the intestine by its hooks and suckers, and develops the 

 series of proglottides of the adult Tape- Worm. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Platyhelminthes are bilaterally symmetrical, usually dorso- 

 ventrally compressed animals, devoid t)f hard supporting skeleton 

 either external or internal^and also of metameric segmentation ; 

 with three embryonic layers ectoderm, mesoderm, and endoderm 

 entering into the formation of the body. A body-cavity is not 

 present. There is a system of excretory vessels, communicating in 

 the majority of cases with the exterior, and furnished with ciliary 

 flames. There is no blood- vascular system. An enteric cavity 

 may be absent, may be rudimentary, or may be highly developed ; 

 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. 



ORDER 1. POLYCLADDIA. 



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 c}dindrical 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. 



1 Except in certain species of Temnoccphala. 



2 Except in the Tetnnocephalea and Actinodactylella. 



