APPENDAGES. 441 



together with the jaws proper, the prehension and mastication 

 of the food. 



The first thoracic leg is thus modified in Anaspides by the 

 development of jaw-like endites on the coxopodite (Fig. 287^4), 

 in the Holotropha by the shortening of the limb, and in the 

 Isopoda and Amphipoda by the shortening and more or less 

 complete fusion of the pair of limbs to form a labium-like lower 

 lip (Figs. 296 and 306). In the Cumacea and Decapoda three 

 pairs of thoracic legs are modified as maxillipeds and in the 

 Stomatopoda five pairs may be so regarded (Fig. 309). When 

 more than one pair of maxillipeds exist the anterior are gener- 

 ally the most jaw-like, the posterior being transitional in char- 

 acter to the limbs behind them. 



The coxopodites of the thoracic legs of the Isopoda and 

 Amphipoda may be expanded and so closely united with the 

 thoracic segments that bear them as to be immovable, and the 

 joint between the coxopodite and basipodite acquires a corre- 

 sponding increase in mobility. * In some cases all trace of their 

 separation from the body segment is lost. 



Of the abdominal appendages the five anterior pairs (pleo- 

 pcda) are often the main agents in gentle swimming movements, 

 while the terminal pair, the uropods form with the telson the 

 powerful fanlike tail-fin of several groups of Malacostraca. The 

 abdominal appendages are biramous limbs in which the two 

 branches spring from a frequently 2-segmented protopodite. 

 Their various modifications are described under the several 

 orders, but it may here be mentioned that they sometimes bear 

 the chief respiratory organs of the body, either (Isopoda) by the 

 transformation of the endopoclites alone, or of both rami into 

 lamellar gills, or (the Isopod Bathynomus) by the growth of long 

 respiratory filaments fringing the endopodite, or (Stomatopoda) 

 by the development of a peculiar complex gill on the exopodite. 

 The presence of tubular air-passages in the exopodites of two or 

 more of the anterior abdominal limbs of some wood-lice (Fig. 

 302) is of interest by analogy with the tracheae of Peripatus, 

 some Arachnids, the Antennata and the Siphonophoran Velella. 

 A process from the base of the inner margin of the endopo- 

 dites of the pleopods is present in Leptostraca, Hemitropha 



* See footnote p. 495. 



