THE RESPIRATORY CURRENT. 81 
Again, if, when a crayfish is resting quietly in the water, 
a little coloured fluid is allowed to run down towards 
the posterior opening of the branchial chamber, it will 
very soon be driven out from the anterior aperture, 
with considerable force, in a long stream. In fact, as 
the scaphognathite vibrates not less than three or four 
times in a second, the water in the funnel-shaped front 
passage of the branchial cavity is incessantly baled out; 
and, as fresh water flows in from behind to make up the 
loss, a current is kept constantly flowing over the gills. 
The rapidity of this current naturally depends on the 
more or less quick repetition of the strokes of the 
scaphognathite ; and hence, the activity of the respira- 
tory function can be accurately adjusted to the wants of 
the economy. Slow working of the scaphognathite 
answers to ordinary breathing in ourselves, quick working 
to panting. 
A further self-adjustment of the respiratory apparatus 
is gained by the attachment of the six gills to the basal 
joints of the legs. For, when the animal exerts its 
muscles in walking, these gills are agitated, and thus not 
only bring their own surfaces more largely in contact with 
the water, but produce the same effect upon the other 
gills. 
The constant oxidation which goes on in all parts of 
the body not only gives rise to carbonic acid; but, so far 
as it affects the proteinaceous constituents, it produces 
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