OF METAMORPHOSES IN THE CRUSTACEA. 321 



M. Edwards, the most celebrated of modern living crustaceologists, observes, that 

 as the Malacostraca Podophthalma are divisible from the presence or absence of 

 branchiae to the thorax, inclosed in a peculiar cavity, " on n'aura plus d'incertitude 

 sur la place que doit occuper un genre tr^s curieux, Zoea ; en efFet, un examen at- 

 tentif de ces petits animaux m'a convain9u que non seulement leurs yeux sont portes 

 sur des peduncles, mais aussi de chaque cote de leur thorax il existe sous le carapace 

 une cavite respiratoire renfermant des branchies semblables par leur structure et 

 leur position a celles d'autres Macroures. II est done evident pour moi que le Zoe 

 est r^ellement un Crustace de I'ordre des D^capodes. Mr. Thompson assure que cet 

 animal n'est autre chose que le jeune de Crabe commun. Cette opinion me ne parait 

 pas soutenable, mais neanmoins il serait possible que les Zoes observes jusqu'ici ne 

 soient pas des animaux adultes, et alors il se pourrait bien que par les progr^s de 

 I'age lis deviennent assez semblables aux Megalops ; question que nous nous pro- 

 posons de traiter plus au long dans une autre occasion*." 



The talented editor of the Zoological Journal has also, in his review of Mr. Thomp- 

 son's work, expressed his doubts as to the universality of the fact of metamorphosis 

 taking place in the Crustacea ; and in the eighteenth number of that work he has 

 stated the confirmation which his doubts had received by the publication of Dr. 

 Rathke's work, adding, that if there existed no optical delusion or other cause of 

 error in the isolated observations which Mr. Thompson has given us, the difference 

 of organization between a Macrourous and a Brachyurous Decapod is much greater 

 than either analogy or anatomy would have led him to suspect. 



And lastly, Mr. Kirby has communicated to me his conviction that the researches 

 of Mr. Thompson are to be regarded with distrust, the grounds for which opinion 

 will appear in his forthcoming Bridgewater Treatise. 



I now proceed to notice, as concisely as possible, the direct observations made by 

 various authors upon different Crustaceous animals in the young state. 



And in the foremost place are to be mentioned the elaborate researches of Rathke 

 upon the development of the ova of the common Cray-fish, a work which for minute 

 and delicate investigation is rivalled only by Lyonnet's celebrated memoir upon the 

 larva of Cossus. Some idea may be entertained of the extent of these inquiries, 

 from the fact that five large folio plates are completely filled with details of the 

 structure, internal and external, of the ova in various states of development, and of 

 the newly hatched animal. And so beautifully clear are the representations of these 

 objects, and so completely is the development of the embryo to be traced through 

 all its stages, that unless we believe the whole to be the work of a fanciful imagina- 

 tion, it is impossible to arrive at any other conclusion than that the Cray-fish does not 

 undergo any change which can in the least degree merit the name of metamorphosis. 

 A full abstract of this valuable memoir is inserted in the eighteenth number of the 

 ZoologicalJournal, and in the Annales des Sciences Naturelles for August 1831, 



* Ann. Sc. Nat., April 1830. 

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