68 



OLIGOCHAETA 



Fig. 16. 



indicate the persistence of an embryonic condition ; for Vejdovsky showed that 

 in Criodrilus the dorsal vessel was developed from two independent tubes, and he 



has recently extended this discovery to species of Lum- 

 bricus (s. 1.). As a general rule the dorsal vessel of the 

 Oligochaeta is of much the same diameter throughout ; 

 but in a few forms it is locally dilated ; this is the case, 

 for instance, with Microchaeta where there is heart-like 

 swelling in the ninth segment. In various Enchytraeids 

 too there is a dilatation of the dorsal vessel just where 

 it arises from the peri-intestinal sinus. 



The Supra-intestinal vessel. — In addition to the dorsal 

 vessel a good many Oligochaeta possess another vessel, also 

 running along the dorsal side of the gut, which has been 

 termed the ' Supra-intestinal ' vessel. It is not long since 

 the presence or absence of this vessel would have been held 

 to be at least partly distinctive of the two groups instituted 

 by Claparede, the ' Limicolae' and the ' Terricolae/ Until 

 the researches of Stolc the supra-intestinal vessel was 

 considered to be confined to the Terricolae, althouffh not 

 occurring in all the genera of that group. It is, however, 

 now known to exist in a number of Tubificidae as . well 

 as in the aberrant genus Phreodrilus. The supra-intestinal 

 trunk is limited to the oesophageal region, where it takes 

 the part with reference to the intestinal circulation that is 

 elsewhere played by the dorsal vessel. In the oesophageal 

 part of the alimentary canal the dorsal vessel comes to 

 be some way removed from the gut, and it gives rise to 

 a series of stout lateral branches which embrace the gut 

 and join the ventral vessel without being connected in 

 any way with the walls of the gut. 



It has been stated that the supra-intestinal trunk 

 passes back along the intestine as the typhlosolar trunk ; 

 but this appears to be very doubtful; the careful re- 

 searches of BouEyE (4) into the circulation of Megascolex 

 coeruleus do not support the existence of such a vessel ; 

 and, considering its relations to the gut and to the body -wall in the oesophageal 

 segments, it does not seem likely on a priori grounds that it does exist in the intestinal 

 region, where indeed it would seem to be de trop. 



BOTHEIONEURON VASCULAR 



SYSTEM. 



(After Stole.) 



I-IX. Segments. i-6. Lateral 

 hearts. 7. Ventral vessel. 8. Pha- 

 rynx. 9-14. Vessels supplying in- 

 testinal network. 15. Sub-intestinal 

 vessel. 16-19. Vessels supplying in- 

 testinal network. 18, 21. Intestinal 

 hearts. 22. Supra intestinal vessel. 

 23. Dorsal vessel. 



