1S8 FOURTH REPORT — 1834. 



not occur. The genus in which the change of form is most con- 

 spicuous appears to be that of Cymothoa. In this instance he 

 has observed the young to be not only deficient in some parts 

 which are developed in the adult, — thus, having six instead of se- 

 ven thoracic segments, and consequently only twelve instead of 

 fourteen feet, — but possessed of others well developed, which in 

 the adult state are merely rudimentary. Thus, the adult has the 

 head extremely small, and the eyes scarcely perceptible exter- 

 nally. The young, on the contrary, have the head large, and the 

 eyes remai-kably conspicuous. A similar difference occurs in 

 the segments of the abdomen, which in the adult are very short 

 and almost linear, whereas in the young they spread out almost 

 as much as those of the thorax*. 



Naturalists who have studied this class have too frequently 

 confined their researches to the Malacostraca. The Entomo- 

 straca, although everywhere to be met with, like some other 

 equally neglected gi-oiips, have received, at least of late years, 

 but comparatively little attention. In this country they have 

 been scarcely noticed at all. The works of Miillerf and Jurine| 

 still retain their value as the great storehouses of original ob- 

 servations relating to these animals, and are indispensable to 

 those who may feel induced to study them. The latter, which 

 is of recent date compared with Miiller's, deserves especially to 

 be pointed out, as, though well known and duly appreciated on 

 the Continent, it does not appear to be familiar to our own na- 

 turalists. It embraces the history of Miiller's genera Cyclops, 

 Daphnia, Polyphemus, Lynceus, and Cyjiris, including descrip- 

 tions of such species as are found in the neighbourhood of 

 Geneva. Jurine has paid the most scrupulous attention to the 

 habits and ceconomy of these minute animals. Many of them he 

 has traced through every stage of their existence ; and, amongst 

 other valuable researches, has ascertained that the genera Amy- 

 mone and Nrnqtlius of the Danish naturalist are only young 

 states of the genus Cyclops. This work is illustrated with beau- 

 tifully coloured figures of all the species. There is also ap- 

 pended to it a detailed and valuable memoir by Benedict Prt'vost 



• M. Edwards has sought to i-efer to some general principles these and other 

 similar facts which he has observed relating to change of form in the Crustacea. 

 He has arrived at the following generalization : That " the different changes 

 of form which the Malacostraca (or higher Crustacea) may experience after 

 quitting the egg, tend always, whatever be their nature or importance, to alienate 

 the animal from the type common to the greater number of these beings, and in 

 some measure to individuate it more and more." See Ann. des Scieii., 1. c. 



f Entomostraca, seu Insecta Testacea, S^-c. 4to, Lips, et Haun. 1785. 



J Histoire des Monocles qui se trouvent aux Environs de Geneve. 4to, Geneve, 

 1820. 



