v PHYLUM PLATYHELMINTHES 237 



muscles. Here they increase greatly in size, and develop into 

 rounded cysts with a large cavity rilled with watery fluid the 

 proscolex stage (B). On the wall of the proscolex, at one side, is 

 formed a hollow ingrowth, or invagination (C) ; and on the inner 

 surface of this are developed the hooks and suckers characteristic 

 of the head or scolex of the adult (D). When these are fully formed 

 the hollow ingrowth becomes everted (E), the suckers and hooks thus 

 coming to be situated on the outer surface (F). The whole embryo 

 has now the form of a bladder or vesicle, with which is connected 

 at one point a process having all the characters of the head and 

 neck of the mature Ta?nia solium ; this is the bladder-worm stage, 

 or Cysticercus. If a portion of Pig's muscle containing Cysticerci 

 which have not been killed by cooking, is taken into the stomach 

 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 devoid 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 oosperin, 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 

 Aarla}. 



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 : 



