154 CRUSTACEA. 



and two pairs of maxillae covered by the foot-jaws. In a great num- 

 ber each eye is placed on an articulated and movable pedicle, and the 

 branchiae are concealed under the lateral margins of the upper or 

 lower shell ; in the others they are usually placed imder the post- 

 abdomen. This section consists of five orders : the Decapoda, 

 Stomapoda, Ljemodipoda, Amphipoda, and the Isopoda. The 

 four first embrace the genus Cancer of Linnaeus, and the last 

 his Oniscus. 



The second, the Entomostraca, or "Insects with shells" of Miiller, 

 is formed of the genus Monoculus, Lin. Here the teguments are 

 horny and very thin, while a shell, resembling a buckler, composed 

 of from one to two pieces, covers or incloses the body of the greater 

 number. Tlie eyes arc almost always sessile, and frequently there i.s 

 but one. The feet, the number of Avhich varies, are mostly fitted for 

 natation, and without a terminal tail. Some of them, having an 

 anterior mouth composed of a labrum, tAvo mandibles — rarely fur- 

 nished with palpi, a tongue, and one, or at most two pairs of jaws, of 

 which the external ones are naked or are not covered by the foot-jaws, 

 approximate to the preceding Crustacea. In the other Entomostraca, 

 wliich seem to api)proach the Arachnidcs in several particulars, the 

 organs of manducation are sometimes simply formed by tlic coxse of 

 the feet, projecting and arranged like lobes bristling \Wth small spines 

 round a large central pliarynx. At others, they cither compose a 

 little siphon or beak, used for suction, as in several Arachnides and 

 Insects, or they are wholly (or nearly so) invisible externally, either 

 because the siphon is internal, or because the suction is produced in 

 the manner of a cup. 



The Entomostraca are thus dentated or edentatcul. The first will 

 form our order of the Branchiopoda *, and the second that of the 

 P^ciLOPODA, which, in the first edition of this work, were a mere 

 section of the preceding order. 



The singular fossils called Trilobites, of wliicli M. Brongniart 

 has given an excellent Monograph, being considered by him, as well 

 as by many other naturalists, as Crustacea allied to the Entomos- 

 traca, Ave will Inicfly speak of them after we liave done with the 

 latter. 



* In my work entitled Families Nat. du Regne Animal, the Entomostraca are 

 divided into four orders : the Lophyrofoda, Phyllopoda, Xiphosura, and the 



SiPHOXOSTOMA. 



