290 



PL A THELMINTHES. 



f 



nepA 



proboscis is a muscular tube closed at 

 one end and at rest is infolded like 

 the finger of a glove inside a closed sac, 

 the proboscis sheath, which extends far 

 back in the body. Its tip is bound to 

 the posterior end of the sheath by a 

 retractor muscle. By contraction of 

 the sheath the proboscis is everted, 

 while it may be retracted again by the 

 muscle. Nettle cells are not uncom- 

 mon in the proboscis wall, while in some 

 forms (the older Enopla) the effective- 

 ness of the organ is increased by the 

 presence of a dart-like stylet at the 

 tip (reserve stylets occur on either side, 

 fig. 255), and at the base of the stylet 

 is the opening of a poison sac. 



The blood-vascular system consists 

 of a pair of lateral tubes connected by 

 transverse loops, and in most forms a 

 third tube is present lying between the 

 intestine and the proboscis sheath. 

 The excretory system consists of two 

 tubes lying close beside the lateral blood- 



FIG. 255. 1 10. 256. 



FIG. 255. Young Tetrastemma obscurum. (From Hatschek, after M. Schultze.) a, anus; 

 cc, dorsal commissure; eg, cerebral ganglia; /, ciliated grooves; i, digestive tract: 

 Iv, lateral, mt>, dorsal blood-vessel; nep/i, water-vascular tubes; nZ, lateral nerve; 

 oc, eye spot; or, proboscis pore; r, proboscis; r,, glandular hinder portion of pro- 

 boscis; rm, retractor of proboscis; sf, stylets: *, opening of excretory system. 



FIG. 256. Pilidium larva. (From Lang, after Salensky.) eg, invaginations which 

 later give rise to the nemertine skin; w, oral lobes; rad, archenteron; r?i, ring 

 nerve; sp, apical plate; st, eesophagus; wTc, ciliated band. 



