MOUTH-PARTS OF THE SHORE CRAB. . 129 
external to that of the third maxiUiped. Other authors (Bell (1), Giard (7), 
Bohn (2, 4)) have stated that the water enters above the legs along the whole 
length of the carapace behind the third maxilliped. Pearson, however (13), 
finds that in Cancer entry is made principally by an anterior opening, which 
is that of Milne-Edwards, and secondarily by a posterior opening above the 
last leg, but not between these apertures. An examination of the structures 
which adjoin the edge of the carapace in Carcinus shows (1) a relatively 
large opening in the position indicated by Mdne-Edwards, (2) a smaller 
opening above each leg, the cheliped included. Tlie openings above the first 
three walking-legs are separated from one another by meetings between the 
prominences upon the flank with which the legs articulate and corresponding- 
prominences of the branchiostegid edge. They are longitudinal, and slit-like 
but well formed. That above the last leg is" very narrowband imperfectly 
separated from the one in front of it *, and that above the cheliped is separated 
from the opening of Milne-Edwards only by a close fitting of the branchio- 
stegite to the coxopodite when the limb is turned forwards. Access to each of 
the openings above the legs is obtnijed principally between its coxopodite and 
that behind it, and by moving the leg backward and upward the crab can 
almost close it, though at the same time the approach to the orifice above the 
leg next in front is more widely opened. A large hairj' tract on the under 
face of the branchiostegite has no doubt the function of filtering the water 
which is drawn through it towards the inhalent openings, and especially that 
of Milne-Edwards. It rises into a long fringe around a bare patch where the 
cheliped lies against the brancliiostegite, and is there met by fringes on the 
borders of the cheliped so as to form what is probabl}' a very efiicient guard 
for Milne-Edwards's opening. Along the edge of the branchioslegite another 
long fringe forms a similar protection for the openings above the legs. By 
placing with a pipette against each opening a little carmine suspended in 
sea-water, it may be seen that water enters ut all of them, even when the 
third maxillipeds are opposed so as partly to close with their epipodites 
Milne-Edwards's opening, and that the water from any opening takes a little 
longer to reach the exhalent orifice than that from the openings in front of 
it. AVhen the third maxilliped is divaricated from its fellow, so as to open 
widelj' the aperture of Milne-Edwards, water enters there more freel}', 
passing forwards as well as backwards, since the anterior side of tlio opening 
is now uncovered, and bathing very copiously the podobranch of the third 
maxilliped, which has been drawn backward into the full stream. That 
* -The crevice above the last leg is continuous with that between the hinder edge of the 
carapace and the first abdominal segment. Narrow though this opening is, the state of the 
hairs which line it shows that a little water enters it. No doubt this water flows both 
ways towards the two gill-chambers. The occasional presence of a-little mud in the middle 
chamber which is connected witli the hinder crevice of the carapace proves that water must 
enter it also, though probably it does not there perform any important function. 
LINN. JOURN. ZOOLOGY, VOL. XXXV. 9 
