678 



ZOOLOGY 



sKc'l'. 



deriving the latter from the former, we should probably express 

 more correctly the affinities of the various groups of Arthropods 

 by some such scheme as that expressed in the diagram (Fig. 561). 

 Here an intermediate link between Annelida and the existing 

 Arthropoda is supposed to have been constituted by hypothetical 

 primitive forms from which Peripatus, the Insecta, and the 

 Myriapoda are supposed to have been evolved in the one direction 



Air- 

 Arac h nids 



Insecra 



Mynofjoda 



Onych 



Pycnogonida 

 Crusfocea 



Tardigrade 

 Pentasfomida 



XijjKosura 

 Euryjjferida 



Primih've Arl-hrojsods 



Annelida 



FIG. 561 



and the Crustacea, Eurypterida, Xiphosura, and air- breathing 

 Arachnida in the other. 



On account mainly of general resemblances to the Spiders, th( 

 Pycnogonida have frequently been grouped with the Arachnida 

 and attempts have been made to homologise their appendages 

 with those of the Spiders and Scorpions. There is one pair more 

 in the Pycnogonida ; and either the last pair would have to be set 

 down as corresponding to the vestigial first abdominal pair of th( 

 ordinary Arachnida, or the ovigerous legs would have to be 

 reckoned, not as independent appendages, but as parts of the 

 second pair, a view for which there is some ground. A close 

 relationship with the Arachnida, however, cannot be traced, anc 

 their affinities are perhaps best expressed, as in the diagram, by 

 connecting them with the Arachnid branch of the Arthropod tree 

 at a point below that at which the air-breathing forms had become 

 developed from forms allied to the Xiphosura. 



