138 DR. L. A. BOREADAILE ON THE 
By brushing their various hairy surFaces against one another they detach 
the small particles of food that are clinging to them, and these are swept 
away by strong currents set up by the activity of the Bxopodites of the 
maxillipeds. 
The opercular function of tlie third nuixillipeds deserves mention here. The 
operculum is not of importance to the crab on account of any part that it 
plays in feeding. It is freely open dui'iog that process, and is not closed 
again till the mouth-pa-rts have cleaned themselves. It undoubtedly protects 
the more delicate organs within it, as may be seen by the way in which it is 
closed when they are approached by any implement. But the closeness of 
its fitting, and its fringe of hairs, indicate that it is also a part of the 
respiratory apparatus. The current set up by the scaphognathite is a 
wonderfully strong one, partly because it is working in a closed system of 
passages, and there are several places in the neighbourhood of the inner 
mouth-parts where leakage is probably liable to take place, aird throw 
unnecessary work upon the scaphognathite in keeping up a current of the 
swiftness which is needed. The closing of the operculum prevents this. 
4. The hairs which are arranged in so definite a manner upon the limbs 
play no insignificant part in the events which have just been described. 
They are of an immense variety. A few particulars about them have been 
given above, in the course of the descriptions of the limbs, but it is quite 
impossible to do justice to this subject \^ithout devoting to it a special inves- 
tigation. Mcintosh (11) has examined their structure in detail, but their 
fnnctions still remain to be elucidated. The great majority of the hairs are 
in some way feathered or toothed, and the lateral members which they then 
bear may he of every relative dimension and shape, from the finest filaments 
to the coarsest serration, may vary in different parts of the same hair, be 
directed at any angle to the axes, or recurved, as on some of the hairs of the 
epipodites, and be set on one side of it, on two, or on all sides. Most of the 
hairs are placed on edges or surfaces vs'here they meet the water, either 
actively in the movement of the limb or passively by the flow of the gill- 
stream. From their form and position it is clear that they must serve more 
uses than one, though whether any, of them have a double function is less 
obvious. Some of them undoubtedly serve to filter the water entering 
various orifices. This is clear from the particles with which they may often 
be seen to be laden. It is notably the function of those which outline the 
parts of the operculum formed by the third maxilliped, and on its coxa help 
to guard the opening of Milne-Edwards, as all the openings of the gill- 
chamber are guarded by hairs on the carapace and on the coxse of the legs. 
Others are sensory. Most if not all of these are probably tactile, that is 
give information as to the pressure which is exerted against them, but their 
sensibility is used in various waj's. It is clear from its behaviour that the 
crab is able to detect the presence of particles in any part of its gill-stream 
