1901.] NATURAL scii:ncks OF riiir,Ai)i::i.piiiA. 691 



when especially contracted. The basement layer is a homoj^e- 

 neous, gelatinous-looking, fibrous connective tissue layer, resem- 

 bling that of the proboscis and body epithelia. The epithelial cells 

 in the anterior part of the rhynchoccid, Plate XLllI, fig. 50, are 

 not in close contact with each other and do not form a flattened 

 endothelium. They are small, slender, pear-shaped cells, attached 

 by their proximal ends to the basement membrane, with the nuclei 

 at their distal ends, which project freely into the rhynchocoel. 

 Farther back these cells become a flattened endothelium. 



The dorsal blood vessel in this region is bordered on its ventral 

 surface by numerous bundles of longitudinal muscle, evidently 

 derived from the longitudinal muscle of the proboscis sheath. 



From the end of the (esophageal region to the beginning of the 

 middle intestine the rhynchocoel is usually greatly dilated, the pro - 

 boscis is intricately coiled and much of the rhynchocoelomic fluid is 

 centred here. Frequently the entire proboscis is drawn forward 

 into this region, leaving the posterior part of the rhynchoco?l quite 

 empty. 



The proboscis sheath in the expanded region is stretched to its 

 greatest extent, so that it appears in cross section as an extremely 

 thin sheet of tissue, and its component layers are scarcely distin- 

 guishable. 



In the posterior part of the rhynchocrel — t.e., in the part lying 

 above the middle intestine — the circular muscle is the predominating 

 layer in the proboscis sheath, the longitudinal muscle being repre- 

 sented by a very small number of fibres. 



A word may be said here in regard to the comparative extent 

 and character of the rhynchocad and proboscis sheath in the differ- 

 ent groups of Nemerteaus, and of its position in respect to other 

 organs. 



In the Protonemerteans the rhynchocal is short, its extent being 

 only about one-third that of the body. It is widest in the ne- 

 phridial region, then l)ecomes constricted, owing to the thickening 

 of the inner circular muscle, but again widens somewhat before its 

 termination, just in front of the beginning of the middle intestine. 

 In the words of Burger (1895), p. 95 : ** Das Rhynchocolom 

 ist vor der Xephridialregion am geriiumigsten, in derselben wird 

 es durch die miichtig angeschwollene innere Kingmuskelschicht 

 sehr beti-iichtlich eingeengt und erweitert sich wieder etwas, nacli- 



