GENERAL CONSIDERATIONS. 215 



prove correct, this fact would furnish another Annelidan characteristic 

 for Peripatus. 



The nephridia in Peripatus, as in the Annelida, are utilised as 

 efferent ducts of the genital organs. The genital products here, as 

 there, arise in the wall of the secondary hody-cavity (which, 

 in Peripatus, is very much circumscribed), and pass from it into 

 the funnel of the nephridium. At this point, however, the develop- 

 ment of Peripatus seems once more to bring it near to the 

 Arthropoda. The ducts unite with the genital glands to form 

 one whole, a condition which, indeed, has already been met with 

 in various divisions of the Annelida. 



The structure of the eyes in Peripatus and their mode of origin 

 show no connection with the organisation of the Arthropoda, but, 

 on the contrary, agree very closely with that of the Annelida. 

 The eyes closely resemble the eyes of the Alciopinae. Further, 

 in comparing Peripatus with the Myriopoda, the absence of 

 Malpighian vessels, or, indeed, of any trace of such organs, is a 

 striking peculiarity. 



In forming a comprehensive judgment of the anatomical and 

 ontogenetic relationships of Peripatus, we have to admit that it 

 unites Annelidan with Arthropodan characters, but that the lattei 

 preponderate ; not only in its external form, but in its inner 

 organisation, does Peripatus appear far more like an Arthropod 

 than an Annelid. Phylogenetically, Peripatus may well be con- 

 sidered as an intermediate form in a series beginning with the 

 Annelida and ending with the Insecta, although this does not, of 

 course, imply that Peripatus is actually to be regarded as the 

 ancestor of the Myriopoda and the Insecta. 



Another pecidiarity, worthy of note from an ontogenetic point 

 of view, is the late appearance of the tracheae, the origin of which 

 has not so far been observed even in the oldest embryos, and the 

 interpretation of which is rendered appreciably more difficult by our 

 ignorance of their formation. "We can hardly err in tracing them 

 back to ectodermal invaginations, and it therefore seems probable 

 that they are to be derived from modified integumental glands, 

 or, still better, from respiratory portions of the body-covering. 

 Whether we may, from the absence of observations on this point, 

 conclude that the tracheae actually appear very late, or whether we 

 must consider that they have been overlooked, does not appear 

 very certain, but we are inclined to adopt the first of these views, 

 and to explain the late ontogenetic appearance of the tracheae by 



