PHYLUM PLATYHELMINTHES 



251 



condition, never being observable. A Flat- Worm has dorsal and 

 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. 200) 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 

 Rhabdocceles. 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 pre- 

 sence of eyes, and, sometimes, of 

 a pair of short tentacles ; in some 

 a slight constriction separates 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, proboscis. 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 FIG. 200. various Planarians. A, 



emfL-<vr rvivVKaKhr wi+Ti a nnrvnla voluta ; B, Vortex; C, Monotus ; D, 



SUCKer, prODaDly Wltn a COpUia- Thysanozoon; E, Rhynchodemus ; F, 



torv function ; and in SOme Bipalium ; O, Polycelis. All natural size. 



-T^T i n 1 1 ,1 ,1 (After Von Graff.) 



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 segmentata, Fig. 201) the 

 internal parts may be so disposed as to approximate to the meta- 

 meric 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 

 Turbellaria multiply by budding, and these form long chains, having 



Con- 



