PHYLUM PLATYHELMINTHES 



251 



or cysticercius. If a portion of Pig's muscle containing cysticerci 

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



FIG. 196. Development of Tapeworm. A, hexacanth embryo; B, proscolex of Tce-nia 

 saginata ; CE, stages in the formation of the scolex of the same ; C, the invagination before 

 the hooks and suckers have become developed; D, after the appearance of the hooks and 

 suckers ; E, partly evaginated ; F, fully evaginated scolex of T. solium with caudal vesicle ; 

 G, scolex of T. serrata with remains of the vesicle ; H, young tapeworm of T. serrata. (After 

 Leuckart.) 



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 ; 



