FUNDAMENTAL RELATIONS OF ANIMALS 107 



lies of these orders might further show how close this correspondence 

 is through all ages. 



Of Gasteropoda I have nothing special to say, as every palaeontolo- 

 gist is aware how imperfectly their remains have been investigated 

 in comparison with what has been done for the fossils of other classes. 

 Yet the Pulmonata are known to be of more recent origin than the 

 Branchifera, and among these the Siphonostomata to have appeared 

 later than the Holostomata, and this exhibits already a general coin- 

 cidence between their succession in time and their respective rank. 



Our present knowledge of the anatomy of the Nautilus, for which 

 science is indebted to the skill of Owen,^^^ may satisfy everybody that 

 among Cephalopods the Tetrabranchiata are inferior to the Dibran- 

 chiata; and it is not too much to say that one of the first points a 

 collector of fossils may ascertain for himself is the exclusive preva- 

 lence of the representatives of the first of these types in the oldest 

 formations, and the later appearance, about the middle geological 

 ages, of representatives of the other type, which at present is the 

 most widely distributed. 



Of Worms nothing can be said of importance with reference to 

 our inquiry; but the Crustacea exhibit, again, the most striking coin- 

 cidence. Without entering into details, it appears from the classifica- 

 tion of Milne-Edwards^^^ that Decapods, Stomapods, Amphipods, 

 and Isopods constitute the higher orders, while Branchiopods, Ento- 

 mostraca, Trilobites, and the parasitic types, constitute, with Limu- 

 lus, the lower orders of this class. In the classification of Dana his 

 first type embraces Decapods and Stomapods, the second Amphipods 

 and Isopods, the third Entomostraca, including Branchiopods, the 

 fourth Cirripedia, and the fifth Rotatoria. Both acknowledge in the 

 main the same gradation; though they differ greatly in the combina- 

 tion of the leading groups, and also the exclusion by Milne-Edwards 

 of some types, as the Rotifera, which Burmeister first, then Dana and 

 Leydig,^**^ unite justly, as I believe, with the Crustacea. This grada- 

 tion now presents the most perfect coincidence with the order of suc- 

 cession of Crustacea in past geological ages, even down to their sub- 



^^ Memoir on the Pearly Nautilus (London, 1832). 



"* Henri Milne-Edwards, Histoire naturelle des Crustaces (3 vols., Paris, 1834-1840). 

 ""Dana, Crustacea, p. 45; Franz Leydig, "Riiderthiere . . . ," Zeitschrift f. Wissen- 

 schaften Zoologie, VI (1854), 1. 



