266 THE CRUSTACEA 



third, and the exopodite, when present, is often reduced to a spine- 

 like "acicle." In the Scyllaridea the number of free segments is 

 further reduced by the coalescence of the proximal segment with 

 the epistome. In the Brachyura, the proximal segment is only 

 distinct in the Dromiacea; in the other groups it is either fused 

 with the epistome or, perhaps, represented by a small operculum 

 (Fig. 147, B and C, /) which covers the external opening of the 

 antennal gland. The exopodite is absent in all the Brachyura ex- 

 cept possibly in certain Dromiacea (Homolodromiidae), where an 

 immovable spiniform process is supposed to represent it. The 

 flagellum is very short in most Brachyura and may. disappear 

 altogether. In some Corystidae the two flagella form a long 

 respiratory siphon in much the same way as the antermules do in 

 Albunea. In the Palinuridae not only the peduncle but also the 

 flagellum is very stout, and in the Scyllaridae the whole appendage 

 is expanded and flattened, and the flagellum is represented by a 

 broad, shovel-like plate. 



The mandibles never have a distinct lacinia mobilis, although, 

 in some of the lower types (Atyidae.), they may have a group of 

 setae or spines on the inner edge. The incisor is widely separated 

 from the molar process in many Caridea (Fig. 157, A), but in the 

 other groups the two cannot be distinguished or are separated only 

 by a groove. In some Caridea the incisor process is wanting. A 

 palp of three segments is usually present, but the number of seg- 

 ments is sometimes reduced, and among the Caridea the palp is not 

 unfrequently entirely absent either in isolated genera (Hippolyte, 

 Palaemonetes) or throughout whole families (Crangonidae, Atyidae). 

 In the Penaeidae the palp is expanded and lamellar, and apparently 

 takes part in enclosing the respiratory passages. 



The maxillulae (Fig. 157, B, arid Fig. 9, B, p. 13) have two in- 

 wardly turned endites, and a palp which is sometimes divided into 

 two, and even, in some species of Penaeus (Fig. 158, A), into three or 

 four, segments. An outwardly turned plate (ex) directly connected 

 with the proximal endite, and having the same relations as the 

 large external plate of the maxillula of Euphausiacea, can some- 

 times be observed, but only exceptionally (e.g. Caridina) is it of 

 considerable size. The chief difference from the maxillulae of the 

 Euphausiacea consists in the absence of a distinct second segment, 

 which here appears to be fused with the first. 



The maxillae are closely comparable to those of Euphausiacea, 

 though the relative proportions of the parts are very different. In 

 the typical form such as we find in the Crayfish (Fig. 9, C, p. 13) 

 the two endites are each divided into two by a deep incision, there 

 is an unsegmented palp, and a very large lamellar expansion on the 

 outer side to which the name scaphognathite is given. According 

 to Hansen, the two bifid endites arise here, as in the Euphausiacea, 



