PHYLUM PLATYHELMTNTHES 



255 



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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 some 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 Turbellaria (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 puckered 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, 

 r 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 (Gunda segmentate/,, Fig. 198) the internal 

 parts may be so disposed as to approximate to the metameric 

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

 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 



f 



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

 voluta ; B, Vortex ; C, Monotus ; D, 

 Thysanozoon ; B, Rhynchodemus ; F, 

 Bipalium ; (J, Polycelis. All natural size. 

 (After Von Graff.) 



