320 MR. WESTWOOD ON THE SUPPOSED EXISTENCE 



corresponding modification of structure, it would be amongst the land Crabs ; but I 

 cannot agree with this gentleman that scarcely a stronger confirmation than this 

 very circumstance could be adduced of the universality of metamorphosis, in as much 

 as it appears to me that Mr. Thompson has arrived somewhat too suddenly at the 

 conclusion that the young must consequently be Zoes, or even that, although inca- 

 pable of living out of the water, they are necessarily furnished with natatory mem- 

 bers. Examine the sea Crab, and no material difference in the structure of its loco- 

 motive organs is to be observed from that of the land Crab whence a different kind 

 of motion can be inferred ; hence there can be no actual necessity for the existence 

 of natatory apparatus in the young land Crab, which must be just as able to support 

 itself in the water without any such as an ordinary sea Crab. If, moreover, we ex- 

 amine the structure of the branchial apparatus of the land Crabs, we find still further 

 evidence in support of this argument. MM. Audouin and M. H. Edwards, in the 

 Annales des Sciences Naturelles for September 1828, have given an account of this 

 organization : the exterior of the branchial cavity is furnished with a reservoir for 

 containing a supply of water ; and in the land Crab there is moreover a second vessel 

 destined for the like purpose, whence it is evident that in this respect the land Crabs 

 do not materially differ from the sea Crabs. I will not dwell upon this subject further 

 than to refer to the conclusive facts subsequently stated in proof that the young of 

 the land Crabs is neither a Zoe nor furnished with natatory apparatus. 



Fifthly, I will only observe with reference to Mr. Thompson's general assertions, 

 that no great weight ought to be attached to them until the necessary details shall 

 have been given to the public, more especially if, as I have shown to be the case, we 

 find cause in those already published to distrust the views of the author. 



Having thus gone through the various statements made by Mr. Thompson in sup- 

 port of his theory, and ascertained from them the apparent want of confirmation of 

 such theory, I proceed to notice the opinions of crustaceologists whose writings have 

 established for them some degree of weight as authorities upon the question. 



These observations will be confined, firstly, to such as bear directly upon Mr. Thomp- 

 son's statements, and secondly, to such as relate to facts noticed respecting the trans- 

 formations in the early stages of various animals in the class ; since it is the more 

 necessary in endeavouring to ascertain the correctness of the views of an author, to 

 reject all general assertions made by others to the contrary which have not been 

 made in reference to such opinion, or which do not rest upon direct observation. 

 And it is to be regretted that Mr. Thompson's memoirs have been far from generally 

 known ; this will account for the slight degree of attention which has been bestowed 

 upon the interesting subject upon which they treat, and for the paucity of notices 

 respecting them. 



We find Latreille *, however, stating that the opinion of Mr. Thompson " a grand 

 besoin d'etre ^tayee par des experiences positives, si toutefois elle n'est pas erron^e." 



* Cours d'Entomologie, p. 385. 



