10 CRUSTACEA. 



place, is composed of a labrum, tongue, two mandibles (fre- 

 quently furnished with palpi), and two pairs of maxillae co- 

 vered by the foot-jaws. In a great number 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 under the post- 

 abdomen. This section consists of five orders : the Dec apod a, 

 Stomapoda, L^modipoda, Amphipoda, and the Isopoda. 

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

 last his Oniscus. 



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

 Muller, is formed of the genus Monoculus, Lin. Here 

 the teguments are horny and very thin, while a shell, resem- 

 bling a buckler, composed of from one to two pieces, covers 

 or incloses the body of the greater number. The eyes are 

 almost always sessile, and frequently there is but one. The 

 feet, the number of which varies, are mostly fitted for nata- 

 tion, and without a terminal nail. Some of them, having an 

 anterior mouth composed of a labrum, two mandibles — rarely 

 furnished with palpi, a tongue, and one, or at most two pairs 

 of jaws, of which the external ones are naked or are not co- 

 vered by the foot-jaws, approximate to the preceding Crusta- 

 cea. In the other Entomostraca, which seem to approach the 

 Arachnides in several particulars, the organs of manducation 

 are sometimes simply formed by the coxae of the feet, pro- 

 jecting and arranged like lobes bristling with small spines 

 round a large central pharynx. At others they either com- 

 pose a little siphon or beak, used for suction, as in several 

 Arachnides and Insects, or they are wholly (or nearly so) in- 

 visible externally, either because the siphon is internal, or be- 

 cause the suction is produced in the manner of a cup. 



The Entomostraca are thus dentated or edentated. The 

 first will form our order of the Branchiopoda(I), and the 



(1) In my work entitled Families Nat. du Regne Animal, the Entomostraca are 

 divided into four orders; the Lophtropoda, Phyllopoda, Xipiiosura, and the Si- 



PHONOSTOMA. 



