xlviii INTRODUCTION. 



examine the actual results, and to endeavour to reduce the 

 facts already known to some order. 



It will be inferred from the previous account, that there 

 are considerable variations in the character of the meta- 

 morphosis of different families, and that in the case of 

 Astacus JluviatiliSy there appears at present to exist even 

 an abrupt and isolated exception to the general law. As 

 this is the only case at present in which such exception 

 has been established, I refer my readers for further in- 

 formation on this subject to the work of Mr. Rathke 

 himself,* which constitutes one of the most complete 

 and elaborate monographs in existence, illustrated in 

 the most beautiful and perfect manner; and to the 

 full and satisfactory analysis of the work by Milne 

 Edwards in the first volume of his " History of the 

 Crustacea." 



Eliminating, therefore, this exceptional case, it will be 

 found that the fact of a metamorphosis has been demon- 

 strated with more or less success in no less than seven- 

 teen genera of the Brachyurous order of the Decapoda 

 in which order the phenomenon is most decided and 

 obvious belonging to the families Leptopodiadce,Maiadce, 

 CanccridcE, PortunidcB, Pinnotheridce, Grapsidtz, and Ge- 

 carcinida. In the Anomourous order, it has been shown 

 in the genera Pagurus, Porcellana, and Galathea, and 

 amongst the Macroura in Homarus, Palinurus, Palemon, 

 and Crangon. 



The facilities which everywhere exist for procuring 

 the common shore crab, Carcinus Mcenas, have occa- 



* Untersuchengen neber die Bildung und Entwickeberg des Flusskrebses, von 

 Hcinrich Rathke. Folio. Leipzig. 182.0. 



