Evolution of Animals 399 



and 879 are littoral or marine, such statistics do not give a 

 correct estimate of the origin of the group, nor of the evolu- 

 tion of most decapodous marine forms since mesozoic times. 

 For the primitive Crustacea an origin in land-locked areas 

 of fresh or slightly brackish water seems practically certain. 



Palaeontologically the Arachnida can be traced back into 

 some of the oldest palaeozoic rocks. The silurian genus of 

 scorpions call Palceophomts shows close affinity with existing 

 genera, and affords definite proof that such specialized types 

 must have been preceded by numerous related, though more 

 primitive, groups that passed into the cambrian or even into 

 a more ancient period. Fossil scorpions, pedipalps, and spiders 

 have also been discovered in carboniferous rocks. We may 

 therefore safely conclude that convergent ancestors of these 

 three groups were land forms that were derived in turn from 

 a fresh- water ancestry. But in recent years the very ancient 

 and wholly extinct groups of the Trilobita and Eurypterida 

 have not only been included with the king-crabs — of which 

 there are living examples — amongst the Arachnida, these three 

 have been viewed by eminent authorities as primitive and 

 marine types from which fresh-water or land forms have de- 

 scended. This view is favored by the occurrence of the euryp- 

 terid genus Strabops in cambrian rocks. But against this it 

 should be said that remains of giant eurypterids are found 

 in fresh- water rocks of carboniferous age. 



But these three groups all exhibit decided affinities with 

 the phyllopod Crustacea that are and have been fresh-water 

 inhabitants. So a much more likely theory of descent for 

 the Crustacea and x\rachnida would be from some common 

 fresh-water or estuarine form of late-archsean age, that gave 

 rise to the main lines of fresh-water phyllopods and related 

 Entomostraca, that produced at first small estuarine or brack- 

 ish-water ancestors of the Trilobita and Eurypterida, and 

 which early started air-breathing or dry land groups that 

 became the progenitors of the scorpions, pedipalps, and spiders. 

 Except for the king-crabs — some ancient forms of which at 

 least were fresh-water — that have lingered on, the marine 



