238 CRUSTACEA. 



all of tliem are parasitical, and they seem to lead to the Lernaese by 

 insensible gradations ; but the presence of eyes, the faculty of 

 changing their skin, or even of undergoing a sort of metamorphosis*, 

 and that of locomotion by means of their feet, appear to us to esta- 

 blish a positve line of demarcation between the former and the latter 

 We have consulted several erudite naturalists with respect to these 

 transformations, but none of them have observed a change of skin to 

 occur. The antennae of the Entomostraca, whose form and number 

 greatly vary, serve for natation in several. The eyes are rarely placed 

 on a pedicle, and when this is the case, that pedicle is a mere lateral 

 prolongation of the hf ad, and is never articulated at the base ; they are 

 frequently closely apin-oximated and even form but one. The organs 

 of generation are situated at the orgin of the tail ; it has been thought 

 but erronenously, that their scat was in the antennae of the male. This 

 tail f is never terminated by a fan-like fin, nor does it present those 

 false feet observed in the Malacostraca. The ova are collected under 

 the back, or are external, and covered by a common envelope, and 

 resemble one or two small clusters at the base of the tail ; it appears 

 that they can be kept in a desiccated state for a long period without 

 losing their properties. 



It is only after a third change of skin that these animals become 

 adult and capable of continuing their species. It has been proved, 

 with respect to some of them, that a single copulation fecundifies 

 several successive generations- 



ORDER I. 



BRANCHIOPODA. 



A mouth composed of a labrum, two mandibles, a Hgula, and one 

 or two pairs of jaws, and branchiae, the first of which, when there 

 are several are always anterior, characteriz, this order or the sixth 

 of the class, 



These Crustacea are always wandering and are generally protected 

 by a shell resmbling that of a bivalve, and furnished with four or two 



♦ The young of Daphnia, and of some neighbouring subgenera, and probably also 

 those of Cypris and Cytherea, with the exception of size, scarcely differ, if at all, 

 from their parents on quitting the egg ; but those of Cyclops, the Phyllopa, and the 

 Arguli, experience considerable changes while young, either as respects the form of 

 the body or the number of feet. These organs in some, the Arguli for instance, expe- 

 rience changes which modify their uses. 



t If we excepted the Pbyllopa, the last feet are thoracic, or foot.jaws (Ci/pris). 



