. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 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 th
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 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 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 posterio
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1905
