16 Guide to Crustacea. 



Wall- the protopodite, exopodite, and endopodite respectively. In the 



Nos^l-3 ^'"^^''"^f'. on the other hand, there is little difficulty in recognising 



the two segments of the protopodite, the exopodite reduced to a 



small movable plate or scale, and the endopodite drawn out into 



a long lash or flagellum of very numerous small segments. 



The mouth-parts will be best understood by comparing them 

 in order from behind forwards, beginning with the third ynaxilliped 

 (Fig. 3). In this appendage it will be seen that the second 

 segment of the protopodite carries an exopodite which ends in a 

 lash or flagellum of numerous segments, and an endopodite of 

 five segments which forms the main part of the limb. In addition 

 to these divisions, however, there is another part not present in the 

 swimmeret which we have taken as the type. This is the " epi- 

 podite," a membranous plate attached to the outer side of the first 



segment (coxa) of the protopodite, 



and bearing one of the gills (to be 



described later) attached to it. 



'^ Exopodite The second maxiUiped is not dis- 



op.0 ites^^ 1^ ^ ^ similar in structure, though much 



smaller than the third, but the 



first maxiUipcd differs considerably 

 Proropodite \J£/ N^ "f^.Qj^^ l^Qtlj Tj^g g^j_^^g p^^^.^g ^^^^ 



Ejiinodlte^ be recognised in it, but the 



Fia. .3. endopodite is shorter than the 



Third maxillipecl of Lobster. exopodite and has only two seg- 

 [Wall-case No. 1.] ments ; and the two segments of 



the protopodite grow out on their 

 inner side into two large plates, fringed with bristles and 

 serving as jaws. In tlie maxilla {second maxilla), these jaw- 

 plates (" gnathobases ") are still more developed and each is 

 slit into two. The endopodite is small and unsegmented, 

 while on the outer side is a large plate which is probably the 

 exopodite, although some have regarded it as the epipodite. 

 Whatever its nature, this plate has an important function, since it 

 lies in a channel leading forwards from the gill- chamber and 

 serves by its continual movements to keep a current of water 

 flowing over the gills. The maxillula {first maxilla) consists of 

 little else than the two gnathobases, here undivided, and a small 

 endopodite. The strong mandibles are clearly the chief instru- 

 ments in the mastication of the food, to which the other mouth- 

 parts are only accessory. Each consists of a massive " body " 

 which seems to represent the first segment of the protopodite 



