PHYLUM PLATYHELMINTHES 



257 



Turbellaria multiply by huiding, and these form long chains, having 

 something in common with the string of proglottides of a Cestode, 

 but differing radically, as will be shown later, in the mode of 

 development. Colour is very general in the Turbellarian, though 

 some are transparent and colourless. The most vivid coloration 

 characterises some of the marine Polyclads, the Rhabdocceles being 

 comparatively obscure. The surface is covered with a coating of 

 fine vibratile cilia, the vibration of which subserves respiration as 

 well as (in the smaller forms) locomotion. Among the ordinary 

 cilia are frequently disposed longer whip-like cilia or flagella, like- 

 wise motile ; and sometimes non-motile (sensory) cilia may occur 

 here and there. 



The Trematodes (Figs. 186, 199, 200, 201), nearly related to the 

 Turbellarians in internal organisation, resemble them also in 





Fio. 190. — Digenetic Trematodes. A, Amphistomum ; B, Homaloga»te>: g. p. genital 

 aperture ; in. mouth ; ». posterior sucker ; te. testes ; vit. vitelline glands. (After M. Braun.) 



external form, with certain modifications connected with a parasitic 

 mode of life. As in the latter class, the leaf-shape prevails ; an 

 elongated form also occurs, though more rarely. The body is 

 usually thicker and more solid than in most Turbellaria. The 

 anterior end is distinguished from the posterior by its shape, by 

 the arrangement of the suckers, and, in many of those Trematodes 

 that are external parasites, by the presence of eyes. Suckers, 

 present in the Turbellaria only in some of the Polycladida and a 

 few Tricladida, are universal in their occurrence. They are always 

 ventrally placed, their chief function being to fix the parasite to 

 the surface of its host in such a way as to facilitate the taking in 

 by the mouth of animal juices and epithelial debris ; their 

 number and arrangement vary considerably. There are nearly 

 vol. I s 



