138 CLASS CRUSTACEA. 



end. Some, having an anterior mouth, composed of a lahrum, 

 of two mandibles (rarely furnished with palpi), of a tongue, of 

 from one to two pair of jaws at most, the external ones, naked, 

 or not covered with jaw-feet, approach the preceding Crusta- 

 cea. In the other entomostraca, and which appear in many 

 respects to border on the Arachnida, sometimes the masticat- 

 ing organs are simply formed by the haunches of the feet, 

 advanced and disposed in the manner of lobes, bristling with 

 small spines, around a large central pharynx ; sometimes they 

 compose a little siphon or bill, serving as a sucker, as in 

 many Arachnides and many insects, or do not show themselves 

 at all, or scarcely show themselves externally, whether the 

 siphon be internal, or that suction is performed after the man- 

 ner of a cupping-glass. Thus the Entomostraca are either 

 denticulated or edenticulated ; the first form our order of 

 Branchiopods, and the second that of Pcecilopods, 

 which in the first edition of this work were only a section of 

 the preceding order. 



The singular fossils called Trilobites, and of which M. 

 Brogniart, our fellow-member of the Koyal Academy of 

 Sciences, has given an excellent monograph, being considered 

 by him, as well as by other naturalists, as Crustacea which 

 border on the entomostraca, we shall treat of them succinctly 

 at the sequel of the latter. 



FIRST GENERAL DIVISION. 



THE MALACOSTRACA. 



The Malacostraca are divided naturally into those whose 

 eyes are on a moveable pedicle, and those in which these 

 organs arc sessile and immoveable. 



