PHYLUM PLATYHELMINTHES 



ventral surfaces, right and left sides or borders, and anterior 



and posterior ends. The anterior end is that which is directed 



forwards in ordinary locomotion : it usually has sonje of the 



features which distinguish a head-end ; but a distinct head is 



rarely developed, and the mouth, when present, is usually placed 



some distance back on the ventral surface. 



In the Turhellaria (Fig. 197) the leaf-form is the prevailing one, 



a shape resembling that described for Planaria being very common. 



In many, however, the body is 



greatly elongated, and it may 



assume the shape of a thin 



ribbon with piuckered edges, as 



in some marine forms ; or may 



be thickened and band-like, as 



in the Land Planarians ; or it 



may approach the shape of a 



cylinder, as in some Rhabdo- 



coeles. A head-region is not 



usually distinct; but there is 



always something to mark off 

 the anterior from the posterior 

 end — a difference in shape, the 

 presence of eyes, and, sometimes, 

 of a pair of short tentacles ; in 

 some a slight constriction sepa- 

 rates off an anterior lobe, on 

 which the eyes are borne, from 

 the rest of the body. In others 

 the anterior end is retractile, 

 and may be everted as a pro- 

 boscis. The mouth is never at 

 the extreme anterior end, but 

 always ventrally placed, some- 

 times behind the middle. In 

 some Polycladida there is a small 

 ventral sucker, probably with a copulatory function ; and in 

 some Rhabdocoeles both the anterior and posterior ends, though 

 not provided with suckers, are adhesive, so that the animal 

 can loop along like a Hydra or a Caterpillar. There is never 

 any external appearance of segmentation, though in at least 

 one exceptional instance {Gtmda segmentata, Fig. 198) the internal 

 parts may be so disposed as to approximate to the metameric 

 arrangement {pseudo-metamerism). In such a case a numbsr 

 of transverse muscular septa are present, imperfectly dividing the 

 body internally into a series of segments ; and various internal 

 organs — intestinal caeca, gonads, transverse commissures of the 

 nervous system — are arranged in pairs following this division. A few 



Fig. 197.— Various Flanarians. A, Con- 

 ■voluta ; B, Vurtex ; C, Moiiotus ; D, 

 Thysanozoon ; E, Khynchodeinus ; F, 

 Bipalium ; G, Polycelis. All natural size. 

 (After Von Graff.) 



