Evolution of Animals 397 



considerable variations and adaptations to marine life. No 

 one would probably doubt, however, that the crayfish and 

 allied fresh-water or brackish species are migrants from sea- 

 shores.* Ecologically they resemble the salmon, the lamprey, 

 and other vertebrates in showing considerable adaptability 

 to changed environment within a comparatively short period. 

 The total genera of the Crustacea according to the authors in 

 Bronn's Tierreich are about 1092, of which at the present day 

 213 are fresh-water and 879 are marine. But, when it is re- 

 membered that, even amongst the divisions of the simplest and 

 most primitive group Entomostraca, the Branchiopoda are 

 fresh-water in the division Phyllopoda, largely so in the divi- 

 sion Cladocera; that the Copepoda and Ostracoda are in part 

 fresh-water, in part marine; and that the Cirripedia, which 

 represent the most modified degenerate and sedentary group, 

 are wholly marine, a key seems to be suggested to the probable 

 origin of the Crustacea as a whole. 



A comparison of living representatives of the group with 

 the fossil forms may also aid in a proper determination of the 

 origin and present distribution. The resemblance of many 

 annelidan larvae to the simpler nauplioid larvae of Crustacea 

 has often been commented on, and so the latter may have 

 represented the diverging point of the Crustacea and the 

 Annehda from the Rotifera. But, like most living and extinct 

 Entomostraca, the primitive types doubtless had a soft even 

 delicate body, and so have left few fossilized remains. During 

 the long geologic ages of the later archsean formation however, 

 they must have become very abundant, since in the early 

 Cambrian rocks, and from these into the base of the silurian, 

 representatives of most entomostracan groups have been found. 



Now the phyllopod or branchiopod and the cladoceran 

 Entomostraca are both largely fresh-water or lake dwellers, 

 while some species of Branchipiis and Artemia can even live 

 in very saline inland lakes that seem to have resembled like 

 areas of the archsean age. 



* This entire question needs revisal, for with Ortmann's paper on Patagoniau 

 Crustacea before us it seems possible that the crayfishes are fresh-water pro- 

 genitors of the marine lobsters. 



