MOUTH-PARTS OV THF, SHORE CRAB. 127 
roof, SO that it lias only roof and floor. Shallow tliough this channel is, the 
eifective depth o£ its entrance is lessened by a double barrier. One com- 
ponent of this is the epipodite of the first maxilliped, which, sweeping round 
on its spiral course, roughly parallel with the epipodite of the third maxilliped 
but starting in front of the gills and lying upon their outer surface, crosses 
the portal of the exhalent passage in such a way as to bar it from behind and 
from outside and leave access to it only from above and from within. Since 
the wide base of the epipodite does not merely touch the floor with its edge 
but lies flat against it, the movements by which the tail is caused to travel 
over the gills probably never separate the base from the wall of the passage. 
The other component of the barrier is the "branchial ridge" of Pearson, 
a fold of the membranous layer of the floor, parallel with the anterior side of 
the principal inhalent opening but at a short distance from it, which fits 
between the podobranch of the second maxilliped and the epipodite of the 
first and helps to enable the latter to make effective contact with the floor. 
On the inner side of the exhalent passage, the edge of the branchiostegite 
does not meet the body closely at the bases of the limbs, but leaves there a 
long, narrow gap. This is normally filk-d by the endopodite and exopodite 
of the first maxilliped and the exopodite of the second, which are moulded 
longitudinally to fit together and against the mandible and to receive the 
branchiostegite. Further forward, in front of the mandible, the body-wall, 
which is here the hinder part (endostome) of the epistome, is concave, and 
so falls away dorsalwards from the branchiostegite and widens the gap. 
Thus is formed the exhalent opening. It is bordered behind, and contracted, 
by the expanded end of the endopodite of the first maxilliped, and discharges 
forwards and towards the middle line, so as to direct the current of its side 
to the antenna and eye of the opposite side. 
The gills of Carcinus are nine on each side. The second maxilliped has a 
podobranch and an arthrobranch, the third a podobranch and two arthro- 
branchs, the cheliped two arthrobranchs, and the first and second walking- 
logs each a pleurobranch. Each gill is a tapering structure. Except the 
podobranch of the second maxilliped, wliich lies horizontally and is directed 
backwards at the bases of the maxillipeds in front of the principal inhalent 
opening, the gills are turned upwards and inwards, and lie against the inner 
walls of tl'.e chamber, converging to its highest point, just before the roof 
falls in front. They are closely applied to one another and separate a 
shallow " hypobranchial space " against the inner wall from an " epi- 
branchial space " under the branchiostegites. The gills are phyllobranchs. 
Each of them, save the arthrobranch of the second maxilliped, is heart- 
shaped in transverse section, owing to the fact that the leaflets project as 
lobes above the axis but die into it below. Thus it comes about that where 
two gills lie side by side there is between them on the under side a " hypo- 
branchial channel," and above each of them is an " epibranchial channel" 
