PHYLUM PLATYHELMINTHES 



137 



and a well-developed enteric 



In 



no cilia on the surface, 

 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 ex- 

 ternally parasitic Trema- 

 todes development is 

 direct, the young animal 

 when it escapes from the 

 egg differing little from 

 the adult except in size. 



2. THE TURBELLARIA 



The Turbellaria are a 

 class of Flat-worms which, 

 though for the most part 

 non-parasitic, resemble the 

 Trematodes very closely, 

 the chief difference being 

 the presence of a coating 

 of vibratile cilia, and the 

 absence, in the majority, 

 of suckers. The leaf 

 form is the prevailing one 

 (Fig. 67), but in many the 

 body is elongated and rib- 

 bon-like, or subcylindrical. 

 In some the anterior end is retractile, and may be everted as 



FIG 67. General plan of the structure of a 

 Triclad Turbellarian. en. brain ; e. eye ; 

 g. ovary; z'l. median limb of the intestine ; 

 ?2- right limb ; i%. left limb ; In. longi- 

 tudinal nerve-cord ; in. mouth ; od. ovi- 

 duct ; ph. pharynx ; /. testes ; te. ten- 

 tacles ; iid. vas deferens ; u. uterus ; 

 ct ejaculatory duct ; 9 vagina ; $ 9 

 common genital aperture. (After Von 

 Graff.) 



