PHYLUM PLATYHELMINTHES 



255 



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 Turlellaria (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- 

 cceles. 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 Poly clad id a there is a small 



ventral sucker, probably with a copulatory function ; and 

 some Rhabdocceles 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 segmentata, 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 



Fir.. 197. Various Planarians A, Con- 

 voluta ; J>, Vortex ; C, Monotus ; D, 

 Thysanozoon ; E, Rhyuchodemus ; F, 

 Bipalium ; Cf, Polycelis. All natural size. 

 (After Von Graff.) 



Ill 



