OF METAMORPHOSES IN THE CRUSTACEA. 323 



Oniscus, as De Geer long* ago remarked) ; upon Cyamus, a genus of Lcemodipoda, 

 which in the young state is of a slender and cylindric form, but which afterwards be- 

 comes much enlarged and depressed ; also upon Phronyma, a genus of Amphipoda, 

 remarkable for its large head, conical thorax, and singular construction of the fifth 

 pair of thoracic legs, but which in the young state exhibits a head of ordinary size, a 

 thorax larger in the centre than at the extremities, and the fifth pair of legs not un- 

 like the others, and not didactyle. 



From the modification in form which the existent organs undergo in the passage to 

 the adult state, M. Edwards deduces this curious theory, — that the changes of form 

 which the Malacostraca undergo constantly tend to remove the animal to a greater 

 distance from the type which is common to the greatest number of individuals in the 

 group, so as to individualize it more and more completely. Thus the form of the im- 

 mature Cymothoa or Phronyma, for instance, is referrible to the general typical form 

 of the Isopoda or Amphipoda ; but, by the gradual change of form, these animals are 

 exhibited in forms the furthest removed from the types of their respective orders. 



It is evident, however, from these remarks, that the Edriophthalma undergo no 

 change worthy of the name of metamorphosis ; and this is most fully supported by the 

 observations of Latreille upon the Isopoda in general, viz. that the progeny " nais- 

 sent avec la forme et les parties propres aleur espece, et ne font que changes de peau 

 en grandissant*="; of Mr. Montague upon Caprella Phasma, who states that he ob- 

 served ten young ones crawl from the abdominal pouch of the female, " all perfectly 

 formed" \ ; of Mr. Coldstream in an admirable paper upon Limnoria terebrans, in- 

 serted in the Edinburgh New Philosophical Journal of Professor Jameson for April 

 1834 ; and, lastly, of Professor Zencker in his memoir "De Gammari Pulicis Histo- 

 ria Naturali," 4to, 1832. 



Hitherto I have confined these observations to the Malacostraca, because it is in 

 that division of the class that the non-existence of metamorphoses has been denied. 

 The Entomostraca are admitted on all hands to undergo very material modifications 

 of form, as maybe seen from the researches of Jurine, Strauss, Prevost, &c.; whilst 

 Mr. Thompson's recent memoir upon Artemia. (not Artemis,) is an additional evidence 

 of the same fact, although the nature of the various alterations is very far from being 

 detailed in that satisfactory manner which the author seems so capable of doing. 



I have therefore now to detail, as the third portion of this essay, such circum- 

 stances as have fallen under my own observation relative to this interesting inquiry, 

 the tendency of which is precisely similar to that exhibited by the two preceding por- 

 tions of my treatise. 



We have seen that Mr. Thompson's chief argument is founded upon the supposed 

 transformations of the Zoe into a Crab. His Zoe, figured as the just-hatched larva 

 of the common Crab, is not so large as a large pin's head. Slabber's "changed Zoe" 

 is represented as three lines long ; and Mr. Thompson's Zoe, which died on the sup- 



* R^gne Animal, torn. iv. p. 131. t Linnean Transactions, vol. vii. p. 66. 



