244 SUBDIVISIONS OF CRUSTACEA. 



under edges of the thorax ; the margins of which, after no long 

 period, are prolonged so as to meet each other and enclose the 

 gills, openings being left for the entrance and exit of water, 

 which are at first large, but which subsequently become contracted 

 to the proper size. 



782. The Class of Crustacea may be divided into three natural 

 groups or Sub-Classes, characterised by differences in the conform- 

 ation of the mouth, as follows : 



I. MAXILLOSA, or Masticating Crustacea, whose mouth is 

 furnished with mandibles and maxillse adapted for mastication. 



II. EDENTATA, Toothless or Suctorial Crustacea, whose mouth 

 is composed of a tubular beak armed with suckers. 



III. XYPHOSURA (so named from the Sword-like appendage 

 with which they are furnished, Fig. 505), whose mouth has no 

 appendages peculiar to it, but is surrounded by legs, whose bases 

 perform the office of jaws. 



783. The group of MAXILLOSA comprehends the greatest part 

 of the class ; and includes all those, whose organisation is most 

 complicated and perfect. These Crustacea do not live on the 

 juices of other animals, as do those constituting the suctorial 

 group ; but they are habitually nourished on solid food. They 

 vary greatly in external form, in their number of legs, and in the 

 structure of their respiratory apparatus ; and they may be 

 divided by these characters into four Sections, containing nine 

 Orders. 



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 covered with a carapace^ formed by the great develop- 

 ment of one of the rings. This Section includes the two first 

 Orders : 



I. DECAPOD A, possessing five pairs of thoracic extremities, and 

 having the gills enclosed in a special respiratory cavity, on each 

 side of the thorax. 



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

 number of extremities. 



