66 OLIGOCHAETA 



The blood contains minute corpuscles suspended in it which are little more than the 

 nuclei of the lining cells. 



The blood-vessels of the Oligochaeta are often provided with valves. These, 

 however, are limited to the dorsal vessel and to the hearts. They do not appear 

 to occur in the lower Oligochaeta — unless indeed the cardiac body of the Enchy- 

 traeidae is derived from a fused series of valves such as occurs in Vertebrates — 

 with the exception of Phreoryctes, where they have been described by Leydig (6) 

 and by Timm in the dorsal vessel. In earthworms they are very general, 

 probably universal. I have myself found them wherever I have looked for them. 

 In the dorsal vessel they occur just where the vessel traverses the septa ; in 

 the hearts they may occur all along (cf. e.g. Peerier, 5, PL XV, fig. 29) from its 

 point of origin from the dorsal vessel up to the opening into the ventral vessel. 

 The valves are essentially proliferations of the lining membrane of the blood- 

 vessels, the cells forming them being large and granular. The blood gland of 

 PhreodHlus may very possibly be regarded as homologous with a fused series of 

 valves, unless the interpretation which I have suggested in describing it be accepted 

 in preference. 



§ a. Main trunks. 



The principal longitudinal trunks in the Oligochaeta are five, all of which do 

 not exist in the lower forms ; they will be now considered seriatim. 



The Dorsal vessel. — This, the principal of the longitudinal trunks, is present in 

 all Oligochaeta and is invariably contractile ; it is not, however, always equally 

 well developed; for in some forms (e.g. Aeolosoma) it is deficient posteriorly while 

 in others (e.g. Chaetogaster cristallinus, Rhinodrilus ecuadoriensis, qq. v.) it terminates 

 a little way before the anterior end of the body. In nearly all those forms where 

 the dorsal vessel is fully developed, which include the great majority, it passes from 

 one end of the body to the other upon the dorsal surface of the alimentary canal ; in 

 Branchiura, however, and in Dero this is only true of the anterior section of the 

 tube; posteriorly it comes to lie on the ventral surface of the body, near to the 

 ventral vessel. The dorsal vessel is commonly separated by a little distance from 

 the actual walls of the canal, but along the intestine it closely invests the gut ; it 

 is not, however, in the higher Oligochaeta, covered by the intestinal peritoneum, 

 but has a layer of peritoneal cells to itself. 



The way in which the dorsal vessel terminates anteriorly varies in different 

 Oligochaeta. As regards the higher Oligochaeta we have two elaborate memoirs 



