COE: NEMEETEANS OF WEST AND NOETHWEST COASTS. 



27 



Proboscis and Proboscis Sheath. 



The proboscis is attached to the tissues of the head in the region 

 of the brain and extends backward in an outer closed tube, the pro- 

 boscis sheath, situated in tlie median line 

 above the digestive tract. This sheath is 

 filled with a lluid in which the proboscis is 

 freely suspended. 



The proboscis opens subterminally on or 

 very near the tip of the snout except in 

 the genus Valej^cinia, "where the opening 

 is situated back nearly to the brain. 



In all except certain species of hetero- 

 nemerteans (Zygeupolia, Cerebratulus) 

 the proboscis is likewise attached at its 

 posterior end to the wall of the proboscis 

 sheath. This attachment is accomplished 

 by means of two or ruore strands of longi- 

 tudinal muscles which become interwoven 

 with the musculature of the sheath. 



In the parasitic Carcinonemertes 

 (Text-tigs. 7, 8, 9) the proboscis sheath 

 is reduced to the merest rudiments, ihe 

 posterior chamber of the proboscis being 

 imbedded in the adjacent connective tissue 

 of the body. In this form the proboscis 

 extends but a little behind the brain and 

 is so intimately connected with the esopha- 

 gus that the single stylet with which it is 

 provided can be everted only as far as the 

 opening of the rhynchodaeum, as (Jescribed 

 elsewhere. With the rhynchodaeum open- 

 ing pressed against the tissues of the gills 

 of the crab on which this nemertean lives, 

 the stylet can thus puncture the gills. The 

 exuding blood and othei- fluids are then 

 drawn into the nemertean's stomach where 

 they serve as food. 



Fig. 7. — Carcinonemertes epi- 

 alti. Optical section of 

 proboscis removed from 

 the worm; ac, mc, pc, an- 

 terior, posterior, and mid- 

 dle chamber respectively; 

 g, gland cells ; c, canal con- 

 necting- anterior and mid- 

 dle chambers ; ps, remnants 

 of proboscis sheath at- 

 tached to posterior cham- 

 ber ; ct, connective tissue 

 in which posterior cham- 

 ber is imbedded ; b, basis 

 of central stylet. 



