ORDER DECAPODA. 93 



ORDER IV. L.EMODIFODA. 



Abdomen rudimental, with only the rudiments of one or 

 two pairs of appendages. 



ORDER V. ISOPODA. 



Body depressed; abdominal appendages flat and plate- 

 like ; mandibles not palpigerous. 



SECTION II. ENTOMOSTRACA. 



Shell not solid; legs variable in number; mouth very 

 variable. 



ORDER VI. BRANCHIOPODA. 



Mouth with a labrum, two mandibles, a tongue, one or 

 two pairs of maxillae ; legs natatory ; branchiae situated an- 

 teriorly. 



ORDER VII. P.ECILOPODA. 



Mouth suctorial, or destitute of mandibles and maxillae, 

 but defended by the ambulatory legs which perform this 

 office. 



ORDER VIII. TRILOBITA. 



Body 13-23-jointed, divided above into three lobes by two 

 longitudinal impressions. 



ORDER I. DECAPODA. 



This order comprises all the larger species of the class, 

 and is well characterized in having ten legs ; the head inti- 

 mately united with the thorax, and covered by a large shell, 

 carapax, or shield ; a mouth, consisting of numerous pairs of 

 organs, of which the outer pairs, in some of the species, are 

 elongated ; but the most characteristic trait consists in the 

 existence of gills or branchiae, more or less numerous, fixed 

 in a peculiar cavity beneath the sides of the shell. 

 This order is divided into two sub-orders, namely, 

 1. Brachyura, having the abdomen shorter than the tho- 



