522 CRUSTACEA MALACOSTBACA. 



pairs of locomotory legs on each side of the thorax. These are 

 known as the trunk-legs or pereiopods and in reference to their 

 number the group is named. 



The biramous character is retained by the maxillipeds through- 

 out life, and in the Penaeidae and some Caridea a reduced 

 exopodite persists on the trunk-legs, but in other Decapods 

 exopodites are wanting from the trunk legs in the adult, though 

 appearing in the ' Mysis ' stage of many Macrura. 



The endopodites of the third maxillipeds retain something of 

 a leg-like character in the Macrura (Fig. 314, mxf 3 ), although 

 they are no longer used in locomotion, but in the Brachyura 

 these appendages become broad and opercular, folding together 

 over the mouth parts anterior to them, like doors (Fig. 316). 



The exopodites of the maxillipeds of the Brachyura end in 

 whip-like appendages directed backwards over the gills, and the 

 bases of those of the first and third are expanded into plates 

 which control the current of the respiratory water. The 

 characters of the endopodites of the first maxillipeds of the 

 Oxystomatous Brachyura are mentioned on p. 544. 



The first maxilliped of the Caridea is peculiar in the possession 

 of an unsegmented lobe on the outer border of the exopodite. 

 The first trunk-legs (chelipeds) are generally chelate, but this 

 character is often shared by their successors. 



In connexion with the problem of segmentation attention 

 may be drawn to the remarkable multiarticulate character of the 

 carpus (antepenultimate segment) of the second trunk-leg in 

 the Hippolytidae, and other families of the Caridea. 



Of the abdominal appendages the last pair (uropods) form 

 with the telson the powerful swimming tail fin of the Macrura. 

 The pleopods are best developed in the Macrura Natantia, where 

 they are the locomotory organs in ordinary swimming. In 

 the Caridea a small lobe known as the appendix interna and 

 projecting on the inner margin of the endopodite is constantly 

 present (the " stylamblis " of Spence Bate's terminology). We 

 have already met with this appendage in the Leptostraca and 

 Euphausiidae (pp. 458 and 467). Among Decapods it is absent 

 in the Penaeidea, Nephropsidae, and Brachyura, but appears 

 in the Caridea, Eryontidae and Scyllaridae, most Thalassinidea 

 (Fig. 319) and some other Anomura. 



The first and second abdominal limbs of the male are modified 



