268 THE CRUSTACEA 



to the second and third segments. The scaphognathite has been 

 variously interpreted as an epipodite or as consisting of epipodite 

 and exopodite together. A comparison with the maxilla of the 

 Euphausiacea shows, however, that it must be regarded as an 

 extreme development of the plate which in the latter case is 

 identified as the exopodite. 



The modifications which this typical form undergoes within the 

 Order are not very striking nor do they afford much material of 

 systematic value. An undivided proximal endite is characteristic 

 of the Caridea, in which group (with some few exceptions) it is 

 also greatly reduced in size (Fig. 157, C). In the Pasiphaeidae 

 both endites disappear. 



It is characteristic of the Decapoda that the first three pairs 

 of thoracic limbs are more or less distinctly differentiated from the 

 others as maxillipcds. It must be noted, however, that the line of 

 demarcation between the two groups of appendages is not always 

 sharply drawn, and that in the Penaeidea and Caridea the third 

 maxillipcds are often distinctly pediform. 



In all Decapods, however, the first maxilUped (unlike the 

 corresponding appendage of the E\iphausiacea) has completely lost 

 its pediform character. The endopodite is greatly reduced in size, 

 and the coxopodite and basipodite are produced inwards as broad 

 endites of which the proximal is often divided by an incision. 

 The most primitive condition is found in certain Penaeidae (Fig. 

 158, B), where the endopodite presents the full number of five 

 segments. In other Decapoda the number of segments is never 

 more than two and the endopodite is often unsegmented. The 

 exopodite is always present; in the Caridea (Fig. 157, D) it 

 presents a characteristic lamellar expansion of its outer margin 

 (lobe a of Boas), the narrow distal part corresponding apparently 

 to the flagellum, which in the higher forms is segmented off from 

 the peduncle and may be divided into numerous articulations. 

 The epipodite is rarely absent {e.g. in many Anomura) and is 

 especially large in the Brachyura (Fig. 159, A). 



The second maxilliped departs less from the general type of the 

 thoracic limbs than does the first. The proximal segments are not 

 produced inwards as distinct endites. The endo{)odite is relatively 

 short, permanently flexed inwards, and its distal part is commonly 

 more or less flattened. 



In the family Stylodactylidae (Caridea) the second maxillipcds 

 appear to present an anomalous structure, two terminal segments 

 articulating side by side on the end of the fifth segment. In the 

 great majority of the Caridea (Fig. 157, E) the terminal segment 

 articulates, not with the distal end but with the inwardly turned 

 (morphologically the outer) margin of the preceding segment. The 

 number of distinct joints is not infrequently reduced by the fusion 



