546 E. RAY LANKESTER. 



characteristic of the differewt classes, whilst the parapodia 

 and somites of the body have become variously modified and 

 grouped in these different classes. The resemblances which 

 the members of one class often present to the members of 

 another class in regard to the form of the limb-branches 

 (rami) of the parapodia, and the formation of tagmata 

 (regions) are not hastily to bo ascribed to common inherit- 

 ance, but we must consider whether they are not due to 

 homoplasy — that is, to the moulding of natural selection 

 acting in the different classes upon fairly similar elements 

 under like exigencies. 



The structure of the head in Arthropods presents three 

 profoundly separated grades of structure dependent upon 

 the number of prosthomeres which have been assimilated by 

 the prseoral region. The classes presenting these distinct 

 plans of head-structure cannot be closely associated in any 

 scheme of classification professing to be natural. Peripatus, 

 the type genus of the class Onycliophora, stands at the base 

 of the series with only a single prosthomere (fig. 3). In 

 Peripatus the prostomium of the Chgetopod-like ancestor is 

 atrophied, but it is possible that two processes on the front 

 of the head (FP) represent in the embi-yo the dwindled 

 prostomial tentacles. The single prosthomere carries the 

 retractile tentacles as its "parapodia." The second somite 

 is the buccal somite (II, fig. o) ; its parapodia have horny 

 jaws on their ends, like the claws on the following legs 

 (fig. 8), and act as hemignatlis (mandibles). The study of 

 sections of the embryo establishes these facts beyond doubt. 

 It also shows US that the neuromeres, no less than the 

 embryonic coelomic cavities, point to the existence of one, 

 and only one, prosthomere in Peripatus, of which the 

 '^Protocerebrum," P, is the neuromere, whilst the Deutero- 

 cerebrum, D, is the neuromere of the second or buccal 

 somite. A brief indication of these facts is given by saying 

 that the Onychophora are " deuterognathous," — that is to 

 say, that the buccal somite carrying the mandibular hemi- 

 gnaths is the second of the whole series. 



