768 PROFESSOR J. STEPHENSON ON 



The Ventral Vessel. 



In ^Eolosoma hemprichi the ventral vessel appears in the anterior part of the 

 body as a distinct vessel in close anatomical relation to the alimentary canal ; in the 

 intestinal region it is represented by a median ventral sinus-like channel on the 

 intestinal wall, which loses itself behind in the vacuolar spaces of the alimentary 

 network. In all the other groups of 01igocha?ta described above the ventral vessel 

 is an independent vessel in its whole length, separate from the alimentary canal, 

 and connected with the alimentary network by a num])er of mesially situated short 

 communicating channels. 



The Ventral Vessel an Earlier Differentiation than the Dorsal. — The ventral 

 vessel thus shows throughout the series a higher grade of differentiation than the 

 dorsal vessel. In ^-Eolosoma, the lowest term of the series, it has attained a degree of 

 development approximately comparable to that which the dorsal vessel shows in the 

 Naididse. In the higher terms of the series its greater anatomical separation from the 

 alimentary canal, and its freedom from chloragogen cells, show that it is still ahead, 

 and so to speak maintaining its lead. It may be mentioned that embryologically also 

 the ventral vessel begins to be developed before the dorsal. 



The above considerations render it allowable to suppose that, as the dorsal vessel 

 has been differentiated from the alimentary plexus, so also, but at an earlier stage, has 

 the ventral. While the earlier stages in the evolution of the dorsal vessel are exemplified 

 for us in the lowest terms of our anatomical series, the final in the later ; and while the 

 evolution of the supraintestinal and subintestinal trunks has not yet begun in the lower 

 forms, though it has made considerable progress in the higher ; the ventral vessel, on 

 the other hand, began its evolution at a phylogenetic stage of which representatives are 

 no longer extant. Hence it is only the later portion of the evolution of the ventral 

 vessel that is actually exemplified in our available material. 



In another way, and even more markedly, the ventral vessel is distinguished from 

 the dorsal. The dorsal vessel, both in its earlier form as a channel in the intestinal 

 wall, and again after its separation from the gut, maintains its contractility ; the ventral 

 vessel is from its first appearance non-contractile. We may ask, therefore — (l) Why 

 should the ventral vessel be the first to be diflerentiated ? and (2) why, even while 

 still attached to the intestine [jEolosoma), should it lose its contractility ? 



Tlie answer mu§t be as follows : — The eflfect of the antiperistaltic contractions of the 

 intestine must be to drive the fluid contained in the network in a forward direction, 

 and, in a closed system, a channel must necessarily exist by which the fluid can return ; 

 the ventral vessel is such a return channel. Again, this cliannel must, to fulfil its 

 purpose, be from its first appearance to some extent at least outside the influence of 

 these contractions. If under these circumstances the ventral vessel were to be con- 

 tractile, the contractions would have to be antero-posterior ; but this is impossible, 

 since vascular contractility is a derivative of that of the intestine, which is postero- 



