46 



to show Arthropod affinities in some of its characters, and 

 it is possible that the group may be connected with the base 

 of the Crustacean series. On the other hand, the Rotifera 

 retain in the trochal disc an organ which is apparently the 

 ciliated prae-oral lobe of the larvae of so many of the Vermes, 

 and this seems to shew that they have arisen directly from 

 the vermean axis. Some of the groups of Rotifera have 

 degenerated to a very great extent. 



We now come to the important region where the two 

 great Arthropodan series, the Crustacea and the Tracheata, 

 diverged from the base of the higher worms at or about the 

 same point. It is probable, from a consideration both of the 

 anatomy and of the development of the two groups, that they 

 do not belong to one great series, but have been evolved 

 independently, and therefore, having no common ancestors 

 nearer than the Vermes, they must have acquired separately 

 such Arthropodan characteristics as they possess in common. 

 The Tracheata are descended from some primitive Annelidan 

 form allied to Peripatus, while the Crustacea must have had 

 as their common ancestor a primitive Phyllopod, and the 

 differences between these two types — Peripatus and the 

 Phyllopod — are so great that they cannot have had any 

 common ancestry nearer than the Vermes. The similarity of 

 some of the organs in the two series may be explained by 

 considering them as similar differentiations of parts inherited 

 from their Annelidan ancestors, while other common charac- 

 teristics may be regarded as being merely adaptive, and due, 

 in some cases, to similar modes of life and habits. 



At the point where the Crustacea and Tracheata diverge 

 the Vermes must have already acquired some of the charac- 

 teristics of their higher groups, such as true segmenta- 

 tion, or the formation of metameres ; and rudimentary 

 appendages, as simple processes developed in pairs, one from 



