ZOOLOGY SKOT. 



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 of 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 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. 



1 Except in cert aii i spo-irs of T&mnocephaln. 



2 Except in the Temnocephalm ami Aftinoddctyletta, 



