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 farther 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 



7 



