REPORT ON ZOOLOGY. 19/ 



Gecarcinus, Thelphusa ?, Pinnotheres, and Inachus ; and in 

 seven genera of the Macroura, viz. Pagurus, Porcellana, Ga~ 

 lathea, Crangon, Palcemon, Uomarus, and Astacus. 



No direct observations have been as yet made by other natu- 

 ralists sufficient to establish the existence of any error in these 

 results at which Mr. Thompson has arrived. There is, how- 

 ever, enough on record to prove that this metamorphosis is not 

 universal ; and some excellent observers have been led by their 

 own inquiries to regard it as rather improbable altogether. 

 The researches of Rathke are decidedly opposed to it. This 

 profound anatomist is the author of an elaborate treatise on the 

 development of the young Cray- fish* , which he has traced 

 through all its stages from its earliest existence ; and so far from 

 observing any metamorphosis in this species, he particularly 

 states that the young at birth scarcely differ externally from the 

 adult except in size. M. Edwards has made some remarks upon 

 Mr. Thompson's theory, which he does not consider as tenable, 

 without the support of further and more accurate observation. 

 At the same time he thinks it very possible that none of the in- 

 dividuals of the genus Zoea hitherto observed by naturalists had 

 reached their adult statef . We are informed by Latreille J, that 

 this gentleman had it in view to institute some particular re- 

 searches under the hope of throwing light on this matter. I 

 am not aware that any decisive results have been hitherto made 

 public. The subject, however, is undergoing investigation in 

 our own country, and will probably before long be satisfactorily 

 cleared up. 



The above doubts respecting the metamorphosis of the Crus- 

 tacea relate only to its existence amongst the Decapoda. That 

 it takes place in some of the other orders in this class is quite 

 certain. Jurine long since detected it in the case of some of the 

 Entomostraca. More recently M. Edwards has observed strik- 

 ing changes of foi-m, almost, if not quite amounting to meta- 

 morphosis, taking place in several genera of the Crustacea Iso- 

 poda, in one genus (Cy amies, Latr.) of the L^modipoda, and in 

 one genus {Phrotiima, Latr.) of the Amphipoda^. At the same 

 time he has fully ascertained, that in other genera, more parti- 

 cularly Gammarus and Idotea, this kind of metamorphosis does 



• See an analysis of this memoir in ^e Ann. des Set. Nal. for 1830, torn. xx. 

 p. 442. 



+ Ann. des ScL, torn. xix. p. 459. See also Hist. Nat. des Crust., toni. i. 

 p. 199 ; and Diet. Class, d' Hist. Nat., Art. Zoe. 



X Cours d' EntomoL, p. 385. 



§ These researches are contained in a memoir, of which an analysis will be 

 found in the Ann. des Scien. for Dec. 1833, p. 360. 



