244 SUBDIVISIONS OF CRUSTACEA. 



under edges of the thorax ; the margins of which are soon pro- 

 longed so as to meet each other and enclose the gills, openings 

 being left for the entrance and exit of water. 



857. The undoubted members of the class of Crustacea may be 

 divided into five great Sections or sub-classes, including thirteen 

 Orders; and to these we must add a sixth Section and a fourteenth 

 Order for the curious forms already referred to ( 822 note), 

 which appear to unite the Crustacea with the Arachnida. 



A. The first Section, PODOPHTHALMA, includes all those 

 having the eyes mounted upon foot-stalks, and moveable. They 

 are almost always furnished with distinct branchiae ; their feet 

 are partly formed for walking, and partly for prehension ; and 

 the thorax is generally covered with a carapace. This Section 

 includes the first two Orders : 



I. DECAPODA, possessing five pairs of thoracic extremities, 

 and having the gills enclosed in a special respiratory cavity. 



II. STOMAPODA, having the gills external, and a variable 

 number of extremities. 



B. The second Section. EDRIOPHTHALMA, consists of those 

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

 stalk), and whose branchiae are not distinct organs, but are united 

 with the extremities, which are commonly seven on each side. 

 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 the abdomen is 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, XYPHOSURA, includes a few species in 

 which the body is incased in a strong calcareous carapace, and the 

 tail converted into a long, sword-shaped spine. The jaws are 

 wanting, mastication being performed by the first joints of the 

 feet. This Section contains only a single Order 



VI. XYPHOSURA. 



D. The fourth Section, ENTOMOSTRACA, presents a great va- 

 riety of form and structure, and in many cases a retrograde de- 



