PHYLUM PLATYHELMINTHES 



253 



B 



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 Polystomece (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 FIG . 

 develops a row (two only in 

 Scutariella) of adhesive ten- 

 tacles, while in Actinodactylella 

 (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 tqctile cones. Pigment is rare in the endoparasitic 

 Digenetica, save in a few that five in the interior of transparent 

 animals ; though many appear coloured variously by the internal 

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



202. Digenetic Trematodes. A, 

 Amphistomum ; B, Homalogaster. ff.p. 

 genital aperture ; m. mouth ; s. posterior 

 sucker ; te. testes ; vit. vitelline glands. 

 (After M. Braun.) 



