240 



ZOOLOGY 



SKCT. 



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 pro- 

 boscis. The mouth is never at the extreme anterior end, but 

 always ventrally placed, sometimes behind the middle. In some 

 Polycladida there is a large ventral sucker serving for temporary 

 fixation ; 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 



FIG. ISO. Various Planarians. A, Convoluta ; B, Vortex; C, Moiiotus ; D, Thysanozoon ; 

 E, Rhynchodemus ; F, Bipalium ; G, Polycelis. All natural size. (After Von Graff.) 



any external appearance of segmentation, though in at least one 

 exceptional instance (Gunda segment at a, Fig. 187) the internal parts 

 may be so disposed as to approximate to the metameric arrange- 

 ment (pseudo-metamerism). A number of transverse muscular septa 

 are present, imperfectly dividing the body internally into a series of 

 segments ; and various internal organs intestinal cceca, gonads, 

 transverse commissures of the nervous system are arranged in 

 pairs following this division. A few multiply b}* budding, and 

 these form long chains, having something in 'common with the 

 string of proglottides of a Cestode, but differing radically, as will 

 be shown later, in the mode of development. Colour is very 

 general in the Turbellarian, though some are transparent and 



