PHYLUM PLAT YH ELM INTHES 



137 



In 



are no cilia on the surface, and a well-developed enteric 

 canal is always present. 

 A remarkable series of 

 metamorphoses, such as 

 that which has been de- 

 scribed in the liver-fluke, 

 is characteristic of the 

 internally parasitic forms ; 

 in the ectoparasitic or 

 externally parasitic Tre- 

 matodes development is 

 direct, the young animal 

 when it escapes from the 

 egg differing little from 

 the adult except in size. 



2. THE TTJRBELLARIA 



The Turbellaria are a 

 class of flat-worms which, 

 though for the most part 

 non-parasitic, resemble the 

 Trematodes ver^^J^sely, 

 the chief differerBBbeing 

 the presence of a coating 

 of vibrating cilia, and the 

 absence, in the majority, 

 of suckers. The leaf form 

 is the prevailing one (Fig. 

 72), but in many the body 

 is elongated and ribbon- 

 like, or subcylindrical. In 

 some the anterior end is 



FIG. 72. General plan of the structure of a 

 Triclad Turbellarian. en, brain; e, eye; 

 g, ovary; t\, median limb of the intestine; 

 / 2 , left limb; z' 3 , right limb; In, longitudinal 

 nerve-cord; m, mouth; od, oviduct; ph, 

 pharynx; t, testes; te, tentacles; vd, vas 

 deferens; n, uterus; $ , ejaculatory duct; 

 5 , vagina; tf $ , common genital aperture. 

 (After Von Graff.) 



retractile, and may be everted as 



