CH. V] RANGE OF ASTACID.E. 229 



the earthworms. Huxley 1 has divided the family into 

 two families the Potamobiidse and the Parastacidae ; the 

 former are confined to the northern the latter to the 

 southern hemisphere. Of the Potamobiidse there are but 

 two genera Astacus and Cambarus. In both of these the 

 pleurobranchs are greatly reduced, indeed in Cambarus 

 they are completely absent. As in Astacus there are distinct 

 rudiments of the pleurobranchs belonging to several pairs 

 of appendages, in addition to one pair completely developed; 

 the inference appears to be that the extra pleurobranchs 

 have in the course of time been lost. Now in all the 

 Parastacidae with the exception of Astacoides there are 

 four pairs of fully developed pleurobranchs, thus indicating 

 an earlier type. Furthermore the genera of the southern 

 hemisphere, " in which the apices of the podobranchise are 

 not separated into a branchial plume and a well developed 

 lamina, present a less differentiated type of branchial 

 structure than that which obtains in Astacus and Cam- 

 barus" Prof. Huxley himself is inclined to postulate the 

 existence of a marine ancestor of world wide distribution 

 which entered the rivers of the northern and the southern 

 hemisphere and diverged in different ways in each hemi- 

 sphere ; but this leaves unexplained why the antarctic 

 genera should have retained more primitive characters 

 than the Crayfishes of the north. 



There is another essential if the polar theory of the 



origin of life is to be accepted. To establish this theory 



upon a firm basis it will have to be proved that the 



faunas of the different countries become more and more 



1 The Crayfish in International Sci. Series. 



