Notices of New Books. 4145 



" 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 made by Mr. Thompson confirmed his 

 new views, and he established the truth of a metamorphotic change 

 in several genera ; the results of his researches being given to the 

 world in a subsequent portion of his " Zoological Researches," in the 

 6 Entomological Magazine,' in c Jameson's Journal/ and particularly 

 in a paper read before the Royal Society in 1835, and published in 

 the ' Philosophical Transactions,' in which details are given of the 

 complete changes in the common shore crab [Carcinus M&nas), which 

 establish the further interesting and important fact, that while the ani- 

 mal 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 to its final deve- 

 lopment, 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 dis- 

 similar from the group to which, in its perfect state, it really belongs. 



" The new doctrine was not received at once with implicit assent. 

 Mr. Westwood, in a paper read before the Royal Society in June, 

 1835, not only contests the universality of the law, which Mr. Thomp- 

 son had somewhat too hastily, perhaps, deduced from his facts, but 

 concludes that that gentleman's views are erroneous, and that ' no 

 exception occurs to the general law of development in the Crustacea 

 — namely, that they undergo no change of form sufficiently marked to 

 warrant the application to them of the term metamorphosis.' 



" This hasty, and, as the result has proved, very premature con- 

 demnation, derived some prima facie supports from the elaborate in- 

 vestigations of Rathke on the development of the embryo in the ova 

 of the river cray-fish (Asiacus Jluviatilis), and the subsequent obser- 

 vations of Mr. Brightwell on that of the lobster, which latter, however, 

 have since been only partially verified by Rathke, and are, indeed, 

 modified in some particulars by Mr. R. Couch. To these I shall have 

 occasion to refer more particularly hereafter ; it is sufficient now to 

 observe, that in both instances the animal was stated to be perfected 

 by gradual development, and not by any sudden change of form. 

 xi. 3 i 



