INTRODUCTION. XXI 



they are slender, elongated, moveable, and multiarticulate. 

 They are, however, subject, in some forms even of the 

 higher orders, to extraordinary modifications ; thus in the 

 genera Scyllarus and Ihacus, the external pair are de- 

 veloped into broad, flat organs of natation, and probably 

 also constitute a pair of shovels for the purpose of burrow- 

 ing : and in some Amphipoda, they are much elongated, 

 serving as a pair of swimming or sustaining arms. The 

 fourth pair always appertain to the mouth, and form man- 

 ducating organs : these are the mandibles. The two 

 pairs of jaws, or maxilla?, follow, and are also employed 

 in the comminution of the food. Theoretically speaking, 

 the next pair ought to be considered as belonging to the 

 cephalic division of the body ; these, as well as the pre- 

 vious and two following pairs, are, in the Decapoda, sub- 

 servient to nutrition. The eighth and ninth pairs are, 

 therefore, properly speaking, the first and second thoracic 

 members, and, with the seventh, constitute the three pairs 

 of footjaws or pedipalps, leaving, in this particular class, 

 the five remaining thoracic appendages to serve the office 

 of ambulatory locomotion, or of claws for the apprehen- 

 sion and tearing of the food, or of weapons of defence. 

 In most of the Edriophthalma the normal arrangement 

 obtains, and the thorax bears seven pairs of ambulatory 

 members. The remaining appendages, which seldom ex- 

 ceed six pairs, belong to the abdominal portion of the 

 body, and in the higher forms are very small and slightly 

 developed, in comparison with those of the thoracic di- 

 vision. In the female Decapoda they constitute the sup- 

 port of the eggs, after their exclusion, and as long as they 

 continue attached to the parent. 



In their full development, each of these organs consists 



