SUBDIVISIONS OF CRUSTACEA. 245 



B. The second Section, EDRIOPHTHALMA, consists of those 

 Crustacea, whose eyes are sessile (that is, not mounted on a foot- 

 stalk), and whose branchia3 are not distinct organs, but are 

 united with the extremities, which are commonly seven on each 

 side, and adapted for walking. This Section contains the three 

 succeeding Orders : 



III. AMPHIPODA, in which the thoracic members are subser- 

 vient to respiration ; and which have the abdomen well developed, 

 and furnished with six pairs of appendages. 



IV. L^MODIPODA, in which there is the same provision for 

 respiration, but which have the abdomen undeveloped. 



V. ISOPODA, in which the abdomen is well developed, and 

 has members, similar in form to the rest, subservient to 

 respiration. 



C. The third Section, BRANCHIOPODA, is characterised by the 

 absence of any special organs for respiration ; their place being 

 supplied by the flattening of the anterior pair of legs into thin 

 plates, which are subservient to that function. This Section 

 contains two Orders : 



VI. PHYLLOPODA, in which the body is either shell-less, or 

 enclosed in a simple carapace, and furnished with a large number 

 of appendages. 



VII. CLADOCERA, in which there is a carapace in the form of 

 a bivalve shell. 



D. The fourth Section, ENTOMOSTRACA, is composed of Crus- 

 tacea in which there are no branchiae, nor any organs destined 

 to supply their place. The eyes are sessile, and are commonly 

 united into a single mass. This Section also contains two 

 Orders : 



VIII. OSTRAPODA, which has the body inclosed in a sort of 

 shield resembling a bivalve shell. 



IX. COPEPODA, in which there is no such envelope. 



It is in the Order Isopoda, that we find, in the equality of 

 the segments and of their appendages, as well as in the aerial 



