THE DECAPODA 



269 



of the basipodite and ischiopodite. The exopodite is rarely absent 

 (Sergestidae, Pasiphaeidae), and is often divided into a peduncle and 

 a multiarticulate Hagellum. 



The ihinl inaxilliped may, in the Natantia, even exceed in 

 length the next succeeding pair of appendages. The coxo- 

 podite and basipodite are almost always connected by an im- 

 movable articulation. In the Caridea the ischiopodite is ([uite 

 coalesced with the meropodite, and the dactylopodite is obsolete 

 or coalesced with the preceding segment. A serrate ridge or 

 '^crista denfata" (Fig. 147, A, mxj)^) is commonly present on the 

 third segment, but no endites are developed from the first and 

 second segments. Among the Brachyura (Fig. 147, B, C, mxi^^) 

 the third maxillipeds become greatly modified to form an oper- 



FiG. 15'\ 



A, tirst, B, tliiril inaxilliped ot Neptunvs pdaghms {Tira.ch.yms.). (After de Haaii.) 

 CD, er.dopodite ; ep, epipodite (mastigobranchia) ; tx, exopodite. 



culum to the buccal frame and entirel}' lose their pediform char- 

 acter. The ischiopodite and meropodite become broad plates and 

 the terminal three segments are often hidden behind the meropodite. 

 The peduncle of the exopodite may also l)e expanded and share 

 in forming the operculum. Its terminal flagellum is either folded 

 out of sight or may be entirely lost. The epipodite forms a long 

 curved blade in most Brachyura (Fig. 159, B, ep). 



The remaining five pairs of thoracic appendages (peraeopods) are 

 typically ambulatory legs, composed of the usual seven segments. 

 Exopodites may be present on some or all of them in some Penaeidea 

 and Caridea (Pasiphaeidae, Fig. 160, Hoplophoridae, some Atyidae, 

 and Crangonidae), but elsewhere they are wanting. As a rule one 

 or more pairs are chelate or sub-chelate, except in the Scyllaridea 

 (where, however, the last pair are imperfectly chelate in the female 

 sex) and in some Hippidea. The first three pairs are chelate in 



