16 
Guide to Crustacea. 
Wall- 
cases 
Nos. 1-i 
the protopodite, exopodite, and endopodite respectively. In the 
, antenna , 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-parfcs will be best understood by comparing them 
in order from behind forwards, beginning with the third maxilliped 
(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. 
The second maxilliped is not dis- 
similar in structure, though much 
smaller than the third, but the 
first maxilliped differs considerably 
from both. The same parts can 
be recognised in it, but the 
endopodite is shorter than the 
exopodite and has only two seg- 
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 the 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 i) 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 
Third maxilliped of Lobster. 
[Wall-case No. 1.] 
