XI 



PHYLUM ARTHROPODA 



629 



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, 

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

 " rachnida in the other. 



On account mainly of general resemblances to the Spiders, the 

 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 



A"ir- breaking 

 Arachnids 



Insecra 



Pyenogom'da 

 Crustacea 



Tardigrade 

 Penhasfomida 



Xibriosura 

 Eurybherida 

 Trilobifa 



Primih've Arfhrobods 



Annelida 

 FIG. 523. Diagram to illustrate the affinities of the Arthropoda. 



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

 down as corresponding to the vestigial first abdominal pair of the 

 ordinary Arachnida, or the ovigerous legs would have to be 

 reckoned, riot 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, and 

 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. 



The position of the Pentastomida is a matter of uncertainty. 

 In the absence of organs of respiration and excretion, the only 



