290 



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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 sheatli, which extends far 

 baclc in the bod_y. Its tip is bound to 

 the posterior end of the sheath by a 

 retractor muscle. P)y contraction of 

 the sheath the proboscis is everted, 

 while it may be retracted again by the 

 muscle. Xettlc cells are not uncom- 

 mon in the proboscis wall, while in some 

 forms (the older Enopla) the effective- 

 ness of tlie 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 i^air of lateral tubes connected by 

 transverse loops, and in most forms a 

 third tnl)e is present lying between the 

 intestine and the proljoscis sheath. 

 The excretory system consists of two 

 tubes lying close beside the lateral blood- 



FiG. 25.5. i'lu ■:M- 



Fia. S'l.').— YouTii:; Titra.itewma (Htsrvrum. (From Ilatschok, after II. Schultzc.1 a, anus 

 (7, dor^^al conimissuro; f(/. ci'ii'bral txan^lia ; /, ciliatotl .L;roo\-es; i, digestive tract 

 ;i', lateral, liiv, dorsal liloiid-vcsstd; iii')ili, water-vasrular tiilH'S ; »i/, lateral nerve 

 n<\ eye spot; or, jiroliowclM pore; r, proboseis; 7',, ^ilainUilar hinder portion of pro- 

 hoMuis; 7'Ml, retractor of proboscis; si, stylets; *, opening of excretory system. 

 Fio. aw.— Pilidiuni larva. (From Lanu', after 8alensky.) <■.«, invaginations which 

 later give rise to the neinertine skin; hi, i>ral lobes; ?n(/, archenteron ; rn, ring 

 nerve; .^p, apical plate; .s(, ti'soithagus; iiih, ciliated band. 



