SECT, xii RELATION OF APUS TO CRUSTACEA 181 



existence into small fresh-water pools, where alone 

 it was able to hold its own, shut off from compe- 

 tition with almost all the rest of the animal king- 

 dom. It is in this way, as already stated, that we 

 account for the preservation of its primitive charac- 

 teristics. Now, in such a record as this, what are 

 the probabilities of its leaving any fossil remains ? 

 The marine carnivorous Annelids of palaeozoic 

 times have left only their hard, chitinous teeth, 

 so that the Apodidae of those times, with a skele- 

 ton not much harder than that of the Annelids, 

 would hardly be likely to be preserved. Their 

 comparative softness is thus one element to be 

 taken into account in discussing the probability 

 of their being preserved as fossils. But, further, 

 when once they had adopted their fresh-water life 

 in shallow pools, the chances of their preservation 

 would be smaller still. They would at this time 

 belong to the land fauna. There would thus be 

 very little chance of their remains being preserved. 

 In the first place the dead bodies would have 

 decayed before there was any chance of their being 

 covered by a deposit ; there is, as a rule, very little 

 suspended matter to fall in the isolated fresh-water 

 pools which we suppose the Apodidae to inhabit. 

 And in the second place, land surfaces are, as is well 

 known, seldom if ever preserved. There is therefore 

 very little chance of any Apus being preserved 

 excepting under very exceptional circumstances. 



Further, although there may be no true Apodidae 

 recorded from Palaeozoic strata, yet there are abun- 



