INTESTINAL RESPIRATION IN ANNELIDS. 789 



on which it would be interesting to dwell. Since, however, the purpose of this section 

 is historical, I must content myself with the following remarks. 



In viewing the accessory gut as a means of bringing respiratory water to the 

 oesophagus, Eisig, influenced probably by his previous work on the T-shaped glands of 

 Syllids, apparently considers the anterior part of the alimentary tract of Polychseta as 

 that in which the respiratory function, when it exists, is specially and necessarily 

 localised. But there is no necessity for respiratory water to reach the oesophagus in 

 order to be of use ; the posterior portion of the gut is throughout the Chsetopoda far 

 more generally respiratory than the anterior. The fact is, that one or two cases where 

 the function of respiration is discharged by the anterior part of the alimentary tube 

 have attracted special attention, while the much more widely distributed respiratory 

 activity of the hinder intestine has received comparatively little notice. 



One might, I think, ask for more evidence that this respiratory water reaches the 

 oesophagus ; the current in the intestine was followed, under favourable circumstances, 

 as far as the posterior end of the accessory gut, but I have not found any mention of a 

 current within the " Nebendarm," the cilia of which are minute and with difficulty 

 recognisable, and which, moreover, may be blocked by a structureless mass. Indeed, 

 the respiratory value of the " Nebendarm " at the present day, whatever may have been 

 its original use, would almost seem to be doubtful. 



Eisig's last point is that the "Nebendarm" has disappeared in the Oligochseta 

 because this group was originally terrestrial ; the present aquatic Oligochseta, that is, 

 are aquatic secondarily, and have no accessory gut because their terrestrial ancestors 

 lost it. That marine Oligochseta are always descended from terrestrial forms, as Eisig 

 states, is certainly not in all cases true ; Clitellio arenarius, one of the best examples 

 of a marine Oligochsete, can hardly be anything else than an originally freshwater 

 Tubificid which has become habituated to a new environment. Moreover, alimentary 

 respiration, by the introduction of water at the anus (though without an accessory gut), 

 is in fact present in most aquatic Oligochseta ; if the aquatic Oligochseta were in general 

 descended from terrestrial forms, this respiratory activity of the intestine would have 

 been lost, in the same way as Eisig supposes the accessory intestine to have been lost. 



The absence of the accessory intestine in the majority of Chsetopoda needs no special 

 explanation. The only thing that need be said is that it has never been present. We 

 may agree that the possession of intestinal respiration is a primitive feature ; we must, 

 however, recognise that an accessory intestine is not a primitive character at all, but 

 one of high specialisation. The aquatic Oligochseta, and a large number of Polychseta, 

 show the less specialised condition, diffuse respiration by means of the whole circuit of 

 the intestinal wall ; some forms show a degree of specialisation, in possessing a more 

 markedly ciliated ventral intestinal groove, along which the introduced water passes 

 upwards ; and further progress may result in the separation of an accessory gut. 



The foregoing account of Eisig's work almost exhausts the literature which falls to 

 be referred to under this head. Gamble and Ashworth (20, 20a) find that there is in 



