Xlii INTRODUCTION. 



which it was passing was that of Meyalopa, to which 

 further reference will be presently made. 



Here, then, was the first decided demonstration ; but 

 any doubt which might be supposed to appertain to an 

 incomplete fact, was shortly removed by Mr. Thompson's 

 success in hatching the ova of the common crab, Cancer 

 Pagurus, the product of which were true Zoeas. 



Subsequent observations by Mr. Thompson confirmed 

 his new views, and he established the truth of a metamor- 

 photic change in several genera; the results of his re- 

 searches being given to the world in a subsequent portion 

 of his Zoological Researches, in the " Entomological Ma- 

 gazine,"* in " Jameson's Journal," f and particularly in a 

 paper read before the Royal Society in 1835, and pub- 

 lished in the " Philosophical Transactions," in which 

 details are given of the complete changes in Caret mis 

 M(cnas, the common shore crab, which establish the fur- 

 ther interesting and important fact, that while the animal 

 appears under the aspect of a Zoea on its first exclusion 

 from the egg, it undergoes a further change into a true 

 Megalopa before its final assumption of the perfect form : 

 showing that this supposed genus also, which was formed 

 by Leach, is, like Zoea, only a phase of a higher type. 

 Thus, in its progress from the egg toils final development, 

 the brachyurous crustacean was proved to pass through 

 two temporary conditions, which had previously been 

 regarded as types, not of genera only, but of different 

 families ; and both strikingly dissimilar from the group to 

 which, in its perfect state, it really belongs. 



The new doctrine was not received at once with im- 

 plicit assent. Mr. Wcstwood, in a paper read before the 



* Vol. iii. pp. 85, 275, 370, 4,V_'. t For 1U46. 



