MOrXH-PARTS OF THE SHORE CRAB. 133 
influence on the current. The aperture is smallest wlien the tliird maxillipeds 
are closely opposed. At such times a steady stream issues from the opening 
which they leave in the middle o£ the epistome and flows forwards under the 
antennules. When a wider opening is necessarj^ either to provide for a 
greater flow or to allow the current to be directed to the sides of the body, 
the operculum may be opened in varying degrees by lowering either the meri 
only or also the basi-ischia. At such times the form of the exhalent opening 
proper may be modified by alterations in the position of the expanded end of 
the endopodite of the first maxiUiped. 
The function of the epipodites* is the cleaning of the gills. Moving to and 
fro over the gill-surface they brush it, drag over it their long, flexible, barbed 
hairs, and thus prevent particles that are brought in by the gill-stream from 
settling there and closing the minule passages between the leaflets. The 
importance of this function is shown by an observation of Pearson, who 
found that in a crab in which the epipodite of one of the first maxillipeds 
had been destroyed, the outer surface of the gills of its side was covered with 
a layer of fine mud. In one of my crabs there. was a similar deposit of fine 
sandy particles. In this case the epipodite was intact, but I have no doubt 
that it was in some way paralyzed f. 
By an admirable mechanism the three epipodites, between them, reach 
almost every part of the gills. The epipodite of the first maxilliped lies 
above the gills and sweeps their outer surface. It is probably moved more 
by the action of its own powerful muscles than by the excui'sions of the 
m;ixilliped as a whole, which are not extensive, and if they were so would 
interfere with the other functions of the limb. This epipodite is very flexible, 
and, doubtless by the action of its muscles, it is kept closely applied to the 
rounded surface of the gill-mound while it swings upwards and downwards 
over the gills, describing an arc with its tip. In an almost vertical position it 
stands against the forward face of the mound. The epipodites of the second 
and third maxillipeds lie below the gills and sweep the inner surface of the 
latter. Both are stiffer than that of the first maxilliped. The principal 
movements of each are probably those which it undergoes passively with the 
coxa of its limb. These movements can easily be imitated upon a dead 
specimen^ and they must occur with each of the excursions which the limbs 
are constantly making in life. The epipodite of the second maxilliped lies 
on the anterior, forward ly-facing region of the inner wall of the gill-chamber. 
When the maxilliped swings outwards, pivoting on its attachment, the 
epipodite makes a corresponding inward movement over the face of the 
thoracic wall, sweeping the inner surfaces of the gills that stand there, with 
* The epipodites of the first and third luaxillipeds have also the passive function of 
directing with their bases the course of the currents in the way that I have already described. 
t Since the foreign matter was above the gills, it had presumably been brought in during 
reversals of the current. 
