PHYLUM PLATYHELMINTHES 



253 



>'' 



B 



., 



- 



-.0"' 



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 



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 ; but 



in some of the Digenetica the 



posterior sucker is wanting. 



Adhesive papillae on the dorsal 



or ventral surface may supple- 



ment the adhesive action of the 



suckers (Fig. 202, B). In the 



Monogenetica the suckers are 



often more numerous ; in the 



family Gyrodactylidce (Fig. 203, 



A) there is no anterior sucker, 



but at the posterior end one or 



two discs armed with hooks ; in 



the PplystomecB (Fig. 203, B) 



there is also a posterior disc on 



which are six suckers with several 



hooks ; in the Temnocephalea 



(Fig. 204) there is no anterior 



sucker, but the anterior end FIQ . 202 Digenetic Trematodes. A, 



TOW (two Onlv in Amphistomum; B, Homalogaster. g.p. 



row ^WO Olliy genital aperture; m. mouth; s. posterior 



of adhesive ten- sucker; te. testes; if. vitelline glands. 



i , T . A . . 7 . 7 77 (After M. Braun.) 



tacles, while m ActinoaactyleUa 



(Fig. 205) a series of marginal tentacles is present in addition to 



both anterior and posterior suckers. In the Aspidocotylea there is 



only a single sucker ; but it extends over nearly the whole of the 



ventral surface, and is complicated in structure owing to its cavity 



being divided into a number of compartments by a system of 



partitions. 



Save in two exceptional cases (Temnocephala) vibratile cilia are 

 not known to occur on the surface in the adult condition ; in some 

 there are groups of non-motile cilia, situated on little conical 

 elevations the tactile cones. Pigment is rare in the endoparasitic 

 Digenetica, save in a. few that live in the interior of transparent 

 animals ; though many appear coloured variously by the internal 

 organs shining through the translucent body-wall, or are stained 



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a 

 d, 



