ON THE ANATOMY OF HISTRIOBDELLA HOMAEI. 349 



in favour, therefore, of a close relationship with Annelids. 

 The general ciliation, the caudal appendage, ciliated rings, 

 nervous system, general configuration of the head, trunk, and 

 alimentary can^l are what are fonnd in a number of Annelids, 

 and most clearly in such a form as Ophyotrocha. Nelson 

 (25) has even suggested that the pre-oral nerve commissures 

 can be satisfactorily explained by deriving them from tlie 

 nerve-ring of the Trochopore. He comes to the conclusion : 

 " On the Avhole, Di nophilus can best be considered as a very 

 young Polychget worm, retaining some of its larval features, 

 with setae and parapodia undeveloped, and whose peritoneum 

 and coelom have been transformed into a generative organ"' 

 (p. 135). 



The relationships of Histriobdella to Polygordius and 

 Protodrilus have been gone into fully by Foettinger (8), 

 Harmer (12), and Haswell (13), so I need not repeat their 

 arguments for this relationship here. It seems to me, from 

 the Archiannelid point of view, it is important to determine 

 what features of Histriobdella are primitive, and what 

 have been derived from its peculiar mode of life. Eisig (5) 

 has gone so far as to suggest that in Histriobdella we 

 have to do with a highly modified, possibly degenerate animal, 

 and not an Archiannelid at all. If Histriobdella is a 

 degenerate form then it must be a degenerate Chastopod as 

 Haswell (13) has pointed out. " If we are to take this view, 

 we must at the same time acknowledge that side by side with 

 the supposed degeneration, there must have gone on a special 

 development in certain directions; that, while the definite 

 characters of the segmentation became lost, a special set of 

 locomotor organs with an elaborate musculature became 

 evolved." "This view appears to me to involve difficulties 

 so great that they render the degeneration theory extremely 

 improbable, and it seems to me more in accordance Avith the 

 facts of the case to conclude that the Histriobdellidas are 

 reallv primitive Annulates, and that the rudiments of their 

 specialised features have been inherited from forms lower in 

 the scale" (p. 327). 



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