6^8 PROCEEDINGS OF THE ACADEMY OF [DeC, 



At the extreme tip of the proboscis the muscular layers ahnost 

 disappear, the couuective tissue cells become even more abundant, 

 and the gland cells of the inner epithelium are replaced by low, 

 flattened endothelial cells. 



7. The Blood Vascular Svstem. — The anterior part of the 

 head of Zijgeujjolla differs from that of most Nemerteans in being 

 quite devoid of blood vessels, and even of blood spaces that are 

 large enough to be seen, although there is no doubt a network of 

 capillaries too fine to be distinguishable either in life or in fixed 

 preparations. 



Tlie blood system makes its first appearance in the brain region, 

 just behind the insertion of the proboscis, Plaie XL, fig. 16. 



In the blood system the following parts may be found (see figs. 

 16, 17): 



(1) The fine paired head vessels, H.V.,* which unite to form 

 (2) the unpaired median vessel, M. F.; (3) the paired cerebral 

 organ vessels, C. Org. V.; (4) the unpaired dorsal vessel, D. V.; 

 (5) the paired lateral vessels of the body, L. V.; (6) the ventral, 

 connectives of the lateral vessels (fig. 22), V.bl.con.; (7) the 

 paired dorso-lateral vessels at the posterior end of the body, derived 

 from the forking of the unpaired dorsal vessel, DI. V., and (8) the 

 central blood lacuna in the caudicle, Bl.L. 



In sections of the brain region, fig. 19, H. Y., the head vessels 

 appear as two irregular clefts running dorso-ventrally, one on each 

 side of the proboscis, just behind its insertion. These narrow slits 

 lie in the limited area between the proboscis sheath and the circular 

 muscle of the body wall, and in the vicinity of the veurral brain 

 commissure their ventral ends coalesce, forming an unpaired cres- 

 cent-shaped vessel, the median vessel, which encircles the lower 

 half of the proboscis sheath, fig. 20, M. Y. 



The median blood vessel extends from this point backward as far 

 as the mouth. In its course it becomes considerably deeper dorso- 

 ventrally, and is very noticeable in sections of this region. It 

 assumes a horseshoe shape, with the apex pointing ventrally and 

 the two long slender arms prolonged dorsally. In the vicinity of 

 the cerebral organs, fig. 21, the arms are cut off by a horizontal 

 band of muscle fibres, H.M., that lies beneath the cerebral organs 



' The reference line from H.V., in fig. 16, should extend in as far as the 



red lines indicAtin"; the head vessel. 



