242 ZOOLOGY SECT, 



colourless. The most vivid coloration characterises some of the 

 marine Planarians, the Rhabdocoeles 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 dis- 

 posed longer whip-like cilia or flagella, likewise motile ; and some- 

 times non-motile (sensory) cilia may occur here and there. 



The Trematodes, nearly related to the Turbellarians in internal 

 organisation, resemble them also in 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, 



B 



Fio. 188. Digenetic Trematodes. A, Amphistomum ; B, Homalogaeter. <i. p. genital 



aperture; m. mouth; *. posterior sucker ; te. testes ; vit. vitelliue glands. (After M. Brauii.) 



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, 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 always present an anterior set of suckers (or a single 

 anterior sucker surrounding the mouth) and a posterior set, or a 

 single large posterior sucker. The arrangement already described 

 as characterising the Liver-fluke is that which is typical in the 

 digenetic forms : a single anterior and a single posterior sucker. 



