PHYLUM PLATYHELMINTHES 



269 



abundantly provided with 

 stinging-capsules ; sometimes 

 it is beset with glandular 

 adhesive papilla;. Posteriorly, 

 this non-eversible part of 

 the proboscis passes into a 

 retractor muscle, by means of 

 which the whole organ is 

 capable of being retracted 

 within the interior of an in- 

 vesting sheath, the proboscis 

 sheath (Figs. 216 and 219, 

 p. s.). It is by the contraction 

 of the muscular walls of this 

 sheath that the proboscis is 

 everted ; sometimes the con- 

 traction takes place with 

 such force that the proboscis 

 is broken off entirely from 

 the body. The abundant 

 nerve-supply of the proboscis 

 points to its use being 

 mainly as a tactile organ. 



The alimentary canal is 

 a simple tube distinguish- 

 able into oesophagus (Fig. 214) 

 with longitudinally folded 

 walls, and intestine with 

 lateral cceca (div.). It ends in 

 an anal opening (a) situated 

 near the posterior extremity 

 of the body. 



The outermost layer of 

 the integument is an epi- 

 ' dermis of ciliated cells, with 

 enclosed unicellular glands 

 containing bodies similar to 

 the rhaldites of the Turbel- 

 laria. Beneath this is some- 

 times a thin homogeneous 

 basement membrane ; then a 

 layer of longitudinally ar- 

 ranged muscular fibres, 

 among which are unicel- 

 lular glands with long ducts 

 that perforate the epidermis, 

 together with pigment. From 

 this layer there is a gradual 



f>ro I afo 



proV- 



retr.mus 



an 



FIG. 215. Tetrastemma. General view of the 

 internal organs. an. anus ; ac. st. accessory 

 stylet ; cer. g. brain ; cil. gr. ciliated groove ; 

 dors. res. dorsal vessel ; lat. ne. lateral nerve ; 

 tat.ves. lateral vessel ; neph. nephridium ; op.neph. 

 nephridial aperture ; probl. eversible part of 

 proboscis ; proV*. non-eversible part of proboscis ; 

 prob. ap. aperture for the protrusion of the pro. 

 boscis ; re.tr. mus. retractor muscle of the pro. 

 boscis ; st. stylet. (From Hatschek's Lehrbuch.) 



