SECT. XV CLASSIFICATION OF CRUSTACEA 273 



the Apodidae from a bent Annelid, we have endea- 

 voured to establish them as the racial form of the 

 majority of modern Crustacea. We at first thought 

 Apus might actually be the primitive Crustacean, 

 but further investigation and comparison with such 

 forms as the Trilobites have shown us that these 

 also claim the same origin as Apus from a bent 

 Annelid. These other groups have for the most part 

 died out. Apus remains, having been isolated through 

 many geological periods in freshwater pools. While, 

 however, Apus itself was not able to hold its own in 

 the struggle for existence in the open sea, modifica- 

 tions of Apus succeeded in surviving, and in producing 

 the rich Crustacean fauna of modern seas. We have 

 already deduced some of the natural groups from 

 their Apus ancestors, and we have now the chief 

 group of all to trace back to Apus. 



The Malacostraca have, by general consent, been 

 traced back to Packard's Phyllocaridae, the only living 

 representative of which is Nebalia, which, according to 

 Packard, combines Phyllopodan and Decapodan cha- 

 racteristics. It has been placed by Claus in a special 

 order — the Leptostraca — as a transition form between 

 the Entomostraca and Malacostraca. 



Going back to the earlier members of this group, 

 we find in palaeozoic times the remains of large 

 Crustacea, which appear to be true Nebalidae. The 

 most important of these are the two forms Hymeno- 

 caris and Ceratiocaris (Figs. 63 and 64). At the 

 first sight of these fossils we are at once reminded 

 of Apus, and this is exactly what our theory demands. 



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