W. M. Coe — Anatomy of Cerehratiilus lacteiis. 487 



In a worm a meter or more in length the proboscis-sheath reaches 

 within a few millimeters of the posterior end of the body. Such 

 extreme length of the proboscis-sheath is more characteristic of the 

 Enopla than of Cerebratulus. This organ lies in the dorsal portion 

 of the body (Plate XI, figs". 3, 4) above the alimentary canal. In 

 the head (Plate X, fig. 3) it lies between the dorsal ganglia and 

 between the side-organs. Its wall is very thick and strong in the 

 oesophagal region, but farther back it diminishes in strength and 

 towards its posterior extremity becomes merely a delicate membrane 

 surrounding a lumen much smaller than that of the dorsal blood- 

 vessel beneath it. 



At its extreme anterior end its muscular wall is made up of a thin 

 layer of circular muscular fibres. The longitudinal layer of muscles 

 arises between the circular layer and the endothelial lining. It 

 increases in strength further back and in the intestinal region it is 

 equal in size with the circular layer. Besides this compact layer 

 (Plate XII, fig. 7, Im) which lies next the endothelium we find in the 

 oesophagal region a second, but incomplete, layer of longitudinal 

 muscles [Im') outside the circular layer. On the dorsal side of 

 the sheath this outer longitudinal musculature is wanting, but beneath 

 the dorsal vessel it becomes very strong and is made up of several 

 layers separated by bundles of circular fibres. In the mouth-region 

 all the layers are very thick and more or less intermixed. The cir- 

 cular layer sends off fibres to the circular muscular layer of the body- 

 wall as well as to the wall of the oesophagus. The sheath is bor- 

 dered on the outside by the body-parenchyma. 



Towards the posterior end of the sheath the layers of muscles dis- 

 appear and there is merely a thin sheath of connective tissue surround- 

 ing the endothelial lining. This endothelium (Plate XII, fig. 7, en) is 

 made up of flattened cells with spherical nuclei imbedded in a granular 

 protoplasm,but the cell-walls are not readily distinguished. The endo- 

 thelium rests on a thickened and much folded layer of connective tissue, 

 the basement membrane (5m). The space within the proboscis-sheath 

 which is not occupied by the proboscis is filled with a corpusculated 

 fluid, the rhynchocoelomic fluid (Plate XII, fig. 9), which bears a 

 close resemblance to blood. The flattened or rounded corpuscles 

 contain spherical nuclei which are often eccentricall}^ placed. The 

 dorsal blood-vessel, as described below, enters the proboscis-sheath 

 near its origin and runs in the rhynchocoelom nearly to the intestinal 

 region, where it passes through the walls of the proboscis-sheath 

 beneath which it runs to the posterior end of the body. The pro- 



