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 



FIG. 219. Tetrastemma obscwrwm 

 (after M. Schulze). Young 

 specimen about 3 lines in 

 length ; mouth ; D intestine ; 

 A anus ; Sg blood-vessels ; R 

 proboscis armed with stylet ; 

 jc lateral trunks of the excre- 

 tory system ; P excretory pore ; 

 G lateral organ ; N<: nerve 

 centre ; Ss lateral nerve cords ; 

 Oc eyes. 



si 



FIG. 210 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 ; rin retractor muscle. 



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, Malacobdclla, etc., the opening of the proboscis is within 

 the mouth. 



