264 



NEMERTEA. 



lies in a sheath (Fig. 219 bis) which is placed on the dorsal side 

 of the gut, and extends in most forms along the whole length of 

 the body. It is a hollow tubular organ, opening nearly always at the 

 front end* of the body, but closed behind. In the retracted state 



rsh. 



FIG. 219 bis. Diagram showing the proboscis of 

 an armed Nemertine in its relation to its sheath 

 (from Lang). A, in the retracted condition ; 

 B, in the protruded state, r proboscis ; rs pro- 

 boscis sheath ; rsh rhyncocoelom ; st spine ; 

 gd cavity of the posterior non-eversible part 01 

 the proboscis ; rm retractor muscle. 



FIG. 219. Tetrastemma obscurum 

 (after M. Schulze). Young 

 specimen about 3 lines in 

 length ; mouth ; D intestine ; 

 A anus ; Bg blood-vessels ; R 

 proboscis armed with stylet; 

 Ex lateral trunks of the excre- 

 tory system ; P excretory pore ; 

 G lateral organ ; Nc nerve 

 centre ; Ss lateral nerve cords ; 

 Oc eyes. 



the proboscis lies in its sheath, to the hind end of which its blind 

 end is fastened by a muscular band (Fig. 219 bis, A}; its walls 

 contain both transverse and longitudinal muscles, the latter of 

 which are continued into the band. The proboscis sheath also 



* In Akrostomum, Malacobdella, etc., the opening of the proboscis is within 

 the mouth. 



