MOUTH-PARTS OF THE SHORE CRAB. 135 
they are often external. From some observations o£ Bohn (4) on the reversal 
of the stroke it would appear tliat the crab is sensitive to changes both in the 
oxygen-content and in the carbon-dioxide-content of the water. Whether 
these circumstances act by altering the composition of the blood, or through 
the sense-organSj or in both ways is not clear. I would hazard a surmite 
that the third of these alternatives is correct, and from some casual 
observations I suspect that the antennules are sensitive to oxygen. Changes 
in the activities of the scaphognathite are accompanied, and ^ts working- 
facilitated, by actions of the subsidiary parts of the apparatus. Probably in 
the normal, quiet breathing of a resting crab, when the water is clear and 
perfectly oxygenated, the third maxillipeds are opposed and the flagella of 
the exopodites at rest. Most commonly, however, at least in experimental 
conditioris, the crab sits with its maxillipeds lowered to some extent. This 
must have the advantage of lessening the work of the scaphognathite, wlien 
■ a considerable volume of water is being dealt with, by allowing it to get 
away with less friction ; and it also enables the fiagella to be brought into 
play, both to help the scaphognathite, and to drive the current and its 
contained particles outward, away from the sense-organs. Like the scapho- 
gnathitej the flagella may work on one side only or on both at once. One, 
two, or three of them may work at the same time on each side, and they may 
■give a single stroke or work continuously for long periods. The flagella of 
one side are complemental to the scaphognathite of the other, driving the 
current in the same direction, and I have not seen the flagella and scapho- 
gnathite of one side both at work while on the other side both are at rest. 
Further, the maxillipeds may be divaricated. This admits more water to the 
gill-chamber, admits it where it has a relatively short distance to travel and 
hence causes relatively little increase in work, and also supplies a set of gills 
which are out of the main stream when the maxillipeds are apposed. 
Changes in the posture of the crab have also, as Bohn has pointed out, an 
effect upon the work of the scaphognathite. When the water is clean and 
well oxygenated, a horizontal position is possible, though there is always some 
upcast, at least when the maxillipeds are opposed. But when the water is 
muddy or foul, the vertical position is necessary to enable reversal of the 
current to obtain more oxygen, as will presently be shown. The exhalent 
opening is then often above the surface, and work is heavier while the 
flow is forward. From time to time the third and second maxillipeds make 
violent excursions outwards. This happens more frequently when the water 
is not clear, and it has the effect of sweeping the gills with the epipodites. 
From time to time also, the scaphognathite reverses its action, and for a 
shorter or longer period draws water in at the exhalent opening and drives it 
backward through the gills and out at the iuhalent opening. This is done 
when foreign particles or distasteful substances are being drawn in, and it is 
noticeable in carmine experiiiients when the particles are too coarse. Its 
