ON THE ANCESTRAL FORM OF THE CHORDATA. 357 



The dorsal blood-vessel takes its course beneath the probos- 

 cidian sheath, between it and the intestine; in many instances 

 it is enclosed in the muscular wall of the sheath in the fore- 

 most part of the body, above the oesophagus. The possibility 

 of a comparison with the subnotochordal rod of Vertebrates 

 ought to be considered. 



The inner epithelium lining the cavity of the sheath is very 

 marked and everywhere present; it is least conspicuous in 

 Carinella, owing perhaps to the considerable extension which 

 the sheath had undergone in all specimens that have hitherto 

 been examined in view to this point. 



This being the general arrangement of the proboscidian 

 sheath, it now concerns us to examine what is known about its 

 development in the embryo. The data available are very scanty, 

 and in some respects contradictory. Barrois describes it in 

 certain species of Lineus as developing from the reticulum, 

 the mesoblastic tissue between the epi- and hypoblast, and 

 gradually extending backwards at the same rate as the develop- 

 ing proboscis pushes it in that direction. In Amphiporus the 

 development of the proboscidian sheath was studied by the 

 same observer, and according to his description there is a 

 remarkable divergence from the development in Lineus. 



In Amphiporus the proboscidian sheath is not formed gra- 

 dually, travelling slowly backwards along the median dorsal 

 line, but the sheath suddenly appears all round the 

 'whole length of the proboscis. It is here formed out 

 of the fatty mass, which also gives rise to the diges- 

 tive tube. 



Tetrastemma, another Hoplonemertine, corresponds closely, 

 according to the same observer, with Amphiporus just de- 

 scribed. 



Salensky, who has lately given a very short account (' Biolo- 

 gisches Centralblatt,' 1883) of the development of Nemertes 

 (Borlasia) vivipara, ascribes a mesoblastic origin to the probos- 

 cidian sheath. Nevertheless, he noticed what appeared to him 

 to be a connection between the first origin (Ankge) 

 of the oesophagus and that of the proboscis. As he 



