MOUTH-PAETS OF THE SHOEE CRAB. 139 
and accordingly to stop, reverse, or alter its direction. This can only be due 
to tactile organs at various points in the system. Another use of the tactile 
sense is involved in the appreciation of the strength of the current, which 
the animal must possess. Conceivably this might be due to a muscular sense 
of the power used in producing the current, but, as Doflein (6) has shown, it 
is verj' probable that in Crustacea the muscular sejpse is supplemented, if not 
largel}' replaced, by the information giyen by tactile hairs brought against 
the water by the movement of the limbs. In any case, it is likely that this 
information is possessed by the crab and guides it in the use of its limbs. 
Probably the more delicately feathered hairs are those which are exposed to 
and detect finer changes in the pi essure of the water. From these there is a 
gradation to the coarser ones which are used in filtering. Whelher any or 
all of the latter are sensitive docs not appear. 
Whether, again, any of the pairs subserve a chemical sense is doubtful. 
Certainly the behaviour of the crab shows that it possesses such a sense, ai.d 
by it is informed of the quality of the water and of the nature of the food. 
But it is quite possible that this is due to the aiitennules. Over these the water 
[lasses on leaving the gill-chamber, and the juices and debris of the food are 
rejected towards them, and probably to some extent reach them in spite of or 
by permission of the exopodites of the nuixillipeds. The crab appears, 
though less clearly than a prawn, to taste as well as to smell its food, but 
this may be due to the sensibility of the antennule ; much as in Man, the 
aromas of food are appreciated by the olfactory epithelium. 
Others of the hairs have a cleaning function. This is exercised in very 
different ways by the long threads on the epipodites of the maxillipeds (and 
possibly by the coxopoditic setae of the maxillule, for which I can suggest no 
other function), and by the serrated bristles of the palps of the third maxilli- 
peds. Those organs are continually busy cleaning the various structures in 
front of them. The}' brush the antennules, sometimes acting singly, some- 
times combing an antennule between ihem. Each of them brushes the eye 
and antenna of its side, and reaches across to clean I he mouth-parts of the 
opposite side, attending, for instance, to the delicate and probably sensory 
hairs of the expanded end of the endopodite of the first maxilliped. In 
these activities, different parts of the palp are brought each against a different 
organ, and very possibly the difference in the serration of the bristles in 
various parts is in correspondence with the structures to be cleaned in the 
organs to which they are applied. 
The function of current-makiiuj must perhaps be attributed to the hairs 
on the flagella of the exopodites. 
Lastly, the bristles which are used in manipulating the food are special 
members of the series of hairs. 
