On the Paired Dorsal Vessel of Certain Earthvjorms. 427 



in mammals, birds, and Teleosteans is paired, and there is 

 therv^fore a further similarity with the dorsal vessel of Lum- 

 bricus. Balfour, however, in his " Comparative Embryology," is 

 disinclined to admit that the origin of the vertebrate heart 

 as a double tube has any phylogenetic significance ; he gives 

 reasons for believing " that the formation of the heart as two 

 cavities is a secondary mode of development, which has been 

 brought about by variations in the period of the closing in 

 of the wall of the throat." Quite recently this statement of 

 opinion has been controverted by Schimkewitzsch,^ who 

 declares that the origin of the vertebrate heart from two 

 rudiments is a primitive mode of development. 



I am unacquainted with any instance of a double dorsal 

 vessel in the Oligochceta limicola, but it is characteristic of 

 many earthworms. Two years ago I described in the Tran- 

 sactions 2 of the Koyal Society of Edinburgh the anatomy of an 

 earthworm which at that time was believed to be a new genus, 

 but which has since turned out to be identical with Temple- 

 ton's Megascolex cceruleus ; ^ in that earthworm the dorsal vessel 

 has retained to a certain extent the embryonic character ; 

 instead of being a single tube passing from end to end of the 

 body as in Lumhricus, the anterior portion of the dorsal 

 vessel is partially divided into two separate tubes ; this 

 occurs in five of the anterior segments (apparently 4-8 in- 

 clusive) ; the two halves of the dorsal vessel are not entirely 

 separate from each other, but reunite at the point where the 

 vessel traverses the mesenteries wliich are interposed between 

 the several segments ; behind the eighth segment the dorsal 

 vessel becomes a single trunk, and is continued without any 

 alteration in its character to the posterior extreme of the body. 



So far as I am aware Megascolex is the only genus of earth- 

 worms in which this condition of the dorsal vessel has been 

 recorded. I take this opportunity of bringing together a few 

 other instances in which I have recently noticed a similar 

 state of affairs. 



In a large earthworm from Natal, belonging to quite another 

 genus (Microchaeta), and differing in many essential characters 



1 Zool. Anzeig., No. 186 (1885), p. 40. ^ Vol. xxx., pt. ii., p. 481. 



3 Ann. and Mag. Nat. Hist., May 1884, p. 398. 



