311 



ferent situations from the majority of the species of the genus to which 

 they belong, or even have the faculty of living in several different si- 

 tuations. Thus, although the animals inhabiting the shells belonging 

 to the genera Patella and Lottia are extremely dissimilar in many 

 essential features of their organization, the shells they form cannot be 

 distinguished from one another by any known character. In other 

 instances, when the animals are very different, the distinctive cha- 

 racters of the respective shells belonging to them are so slight as to 

 be insufficient for the purpose of classing them under separate spe- 

 cies j and this difficulty of discrimination must be much increased in 

 the cases of fossil shells, especially of those which have no strictly 

 analogous forms among recent shells. 



In support of the position advanced in the second part of the paper, 

 namely, that numerous exceptions occur to the identity of habitation 

 among all the species of the same genus of conchiferous Mollusca, the 

 author adduces examples : 1st, where the species of a genus are found 

 in more than one situation, as on land, in fresh and in salt water; 

 2ndly, where one or more species of a genus, the species of which 

 generally live in fresh water, are found in salt or in saltish water; 

 3rdly, where one or more species of a genus, which is generally found 

 in the sea, are, on the contrary, found in fresh water 3 and, 4thly, where 

 the same species of shell is found both in salt and in fresh water. 



" On the supposed Existence of Metamorphoses in the Crustacea." 

 By J. O. Westwood, Esq., F.L.S. and Secretary to the Entomolo- 

 gical Society. Communicated by J. G. Children, Esq., Sec. R.S. 



The author refers the principal modifications of form which occur 

 during the progressive development of animals to the three following 

 heads : 1st, that of an animal produced from the egg in the form 

 which it is destined to retain through life, its only change consisting 

 in a series of moultings of the outer envelope, attended merely by an 

 increase of size, and not by the acquisition of new organs 3 2ndly, 

 when the animal, at its exclusion from the egg, exhibits the form which 

 it continues to possess, subject to a series of moultings, during several 

 of the last of which certain new organs are gradually developed 3 and, 

 Srdly, when the form of the animal, at its exclusion from the egg, is 

 totally different from that under which it appears at the later periods 

 of its existence 3 such change of form taking place during two or three 

 of its general moultings, and consisting, not only in the variation of 

 the form of the body, but also in a complete change in the nutritive 

 and digestive systems, and in the acquisition of various new organs. 

 This last phenomenon peculiarly characterizes what is termed a meta- 

 morphosis. 



It is the received opinion among naturalists that the Crustacea do 

 not undergo metamorphoses, properly so called, and that the trans- 

 formations they exhibit consist merely in the periodical shedding of 

 the outer envelope. The object of the present paper is to establish 

 the correctness of this opinion, in opposition to that of Mr. J. V. 

 Thompson, who has laid claim to the discovery that the greater num- 

 ber of the animals belonging to the class Crustacea actually undergo 



