GENERAL CONSIDERATIONS. 311 



The idea that the Zoaea had significance as a hypothetical racial 

 form was widened and modified by Dohrn (No. 9). Relying on 

 certain features found among the Entomostraca, which were inter- 

 preted as Zoaean characteristics, and supported above all by the 

 consideration of the Nauplhis of Lejms, which was taken for 

 the ArcTihoaea because of its spinous structures, Dohrn thought 

 himself justified in chximing the Zoaea as the racial form of all 

 the Crustacea, which, proceeding from the Nmipllus, had brought 

 about the transition to a Phyllopod-like ancestral form of the 

 Crustacea. Dohrn was the principal founder of the view that 

 the most primitive Crustacean forms are retained in the central 

 groups of the Phyllopoda, and that all other Crustacean groups 

 can be derived from the Phyllopoda, a view which still prevails and 

 which we ourselves accept, although, with Claus, we do not regard 

 the hypothetical racial form as possessing exclusively the characters 

 of the living Pliyllopoda, but would construct a hypothetical racial 

 group of primitive Phyllopods, in many respects, especially in the 

 structure of the mouth-parts, more primitively constituted than 

 the existing forms. 



Dohrn's assertion of the importance of the Phyllopoda as the 

 central group from which all Crustacea can be derived was, in any 

 case, a distinct advance, in so far as it removed the contrast made 

 by F. MuLLER between the Malacostraca and the Entomostraca by 

 providing a possible common derivation for all classes of Crustacea. 

 Indeed, Dohrn's view smoothed the way for further advance, since 

 it led easily, by logical sequence, to the direct deduction of the 

 Phyllopoda from ancestral forms resembling the Annelida. 



Such a derivation of the Crustacea was, however, only very 

 gradually accepted. The Nauplius at first remained unshaken in 

 its position as racial form of all Crustacea, but the supposed phylo- 

 genetic significance of the Zoaea fell into the background. To Claus 

 (No. 8) is due the credit of having, in consequence of his compre- 

 hensive investigations, recognised and established the nature of the 

 Zoaea as a secondarily modified larval form. The ontogeny of 

 the Stomatopoda and, above all, the metamorphosis of Penaeus, 

 most distinctly show that there is no essential difference between 

 the larval development of the ]\Ialacostraca and that of the Ento- 

 mostraca, Avith regard to the order in which the new segments 

 appear, that order, in both cases, being from before backwards. 

 One of the most important peculiarities of the Zoaeae of the higher 

 Macrura and Brachyura, viz., the retarded development of the 



