770 PROFESSOR J. STEPHENSON ON 



Tubificidse ; in Pheretima it extends backwards along the intestine also. It is, 

 however, by no means invariably present in the higher Oligochseta {cf. the data given 

 by FucHS, 19). 



In the Tubificidse Beddard (3, p. 69) considers that this vessel is in the anterior part 

 of the body the homologue of the dorsal vessel in the posterior part, and that the 

 anterior part of the dorsal vessel, which occurs along with and above the supraintestinal 

 in the oesophageal region, is a new structure. But it is to be noted that even in the 

 Naididse the anterior part of the dorsal vessel is less intimately related to the alimentary 

 canal than the posterior; and that in Pheretima (and Pontodrilus, cf. Fuchs, loc. cit.) 

 the supraintestinal is not confined to the anterior part of the body, but extends the 

 whole length of the intestine. 



A more acceptable view of the relations of the supraintestinal would seem to be the 

 following : — With the progressive differentiation of the vascular system the dorsal 

 vessel separates itself from the intestine ; such diff"erentiation proceeds, as we have 

 seen, more rapidly in the anterior than in the posterior part of the body ; the same 

 causes which led to the specialisation of the dorsal vessel as a median channel in the 

 intestinal network again become active after the separation of the dorsal vessel from 

 the intestine ; and hence we have the formation of the supraintestinal, at first in the 

 oesophageal region, and later, when the dorsal vessel has attained a considerable 

 anatomical separation along its whole length (e.g. Pheretima), throughout the entire 

 extent of the intestine. 



The Suhintestinal Vessel. 



The suhintestinal vessel is well known in certain Tubificidae, where it corresponds 

 in position on the ventral side of the oesophagus to the supraintestinal on the dorsal 

 side. It is not recognised by Fuchs as occurring in any of the higher groups. If, 

 however, reference be made to the account previously given of the blood-vessels in 

 connection with the alimentary canal in Pheretim^a posthuma, it will be seen that there 

 is in that species a channel on the anterior part of the intestine to which the name 

 may be applied. A similar mid-ventral channel occurs on the "crop" in Chsetogaster 

 orientalis {vide ante) ; according to Fuchs [loc. cit., p. 454) the same vessel has been 

 described in CJisetogaster crystallinus by Leydig, and in Lumhriculus by Bulow. The 

 suhintestinal may therefore be looked on as a specialisation of the alimentary network 

 showing various degrees of development, and formed after the separation of the 

 ventral vessel in the same way as the supraintestinal after the separation of the dorsal. 



Other Specialisations of the Alimentary Network. 



These are sufficiently illustrated by the foregoing description of the intestinal 

 network of Pheretima, which, belonging to the Megascolecidae, may be looked on as 

 one of the most highly developed forms of the Order, The transverse channels of the 



