Palaeozoic Arachnida of North America. 11 



toward a fusion of cephalothorax with abdomen and this is brought 

 to completion in the Acari. 



In view of such difficulties in the way of building a system based 

 on comparative anatomy alone, Ray Lankester has made the attempt 

 to employ as main character of the class, the true segmentation 

 of the head. We must bear in mind that the visible segmentation is 

 not always a correct criterion for the true segmentation and that seg- 

 ments may without difficulty be traced in the internal organization 

 of an animal when all evidence of their existence has been entirely 

 obliterated from the chitinous exoskeleton ; but even the internal 

 organization of an adult arachnid maj' show no evidence of segmen- 

 tation while the embryological stages leave no doubt of its having 

 once existed. 



Lankester divides the Euarthropoda which form his Grade C in 

 the subphylum Arthropoda of the phylum Appendiculata, into 5 clas- 

 ses : Diplopoda, Arachnida, Crustacea, Chilopoda and Hexapoda. 

 He derives them from a "common ancestor resembling a Chaetopod 

 worm, but differing from it in having lost its chaetae and in having 

 a prosthomere^ in front of the mouth (instead of prostomium only) 

 and a pair of hemignaths (mandibles) on the parapodia of the buccal 

 somite. The structure of the head in Arthropods presents three pro- 

 foundly separated grades of structure dependent upon the number 

 of prosthomeres which have been assimilated by the prae-oral region." 

 His classification is as follows : 



Phylum APPENDICULATA 

 Sub-phylum I. Rotifer a 



II. Chaetopoda 

 ,, III. Arthropoda 



Grade A. Hyparthropoda (Hypothetical forms) 



B. Protarthropoda 

 Class Onychophora 



C. Euarthropoda 



Class I. Diplopoda (Head monoprosthomerous, deutero- 

 gnathous) 

 ,, 2. Arachnida (Head diprosthomerous, tritognathous) 

 ,, 3. Crustacea j 

 ,, 4. Chilopoda I Head triprosthomerous, tetarto- 



I gnathous 

 ,, 5. Hexapoda J 



2 Prosthomeres are the segments in front of the mouth opening. 



