CH. Il] THE CRAYFISH 57 



this appendage hooks into a small recess upon the mandi- 

 ble near the base of the palp. When the scaphognathite 

 is at work it pulls upon this hook at each stroke. The 

 number of strokes per minute is about sixty. The first 

 and second maxillipeds may be seen to be occasionally 

 thrown into very rapid flickering vibrations. The purpose 

 of these movements is not at all clear. When the animal 

 is at rest the third maxillipeds, chelae, and some of the 

 ambulatory appendages are often kept in gentle move- 

 ment, swaying to and fro from right to left. These 

 actions doubtless assist in changing the water in the 

 vicinity of the respiratory organs. 



The branchial plumes contain blood which flows up the 

 outer and down the inner side of the stem of each plume. 

 The inner wall of the branchiostegite itself has been de- 

 monstrated by Glaus 1 to be highly vascular. No doubt it 

 plays an important part in respiration. The blood is almost 

 colourless 2 , and contains, dissolved in the liquid and not 

 contained within any corpuscles, an organic compound of 

 copper (haemocyanin), which forms an unstable blue com- 

 pound with oxygen as the blood courses through the 

 branchiae and readily parts with the loosely associated gas 

 to the other tissues of the body when it reaches them. 

 When shed the blood soon turns indigo blue in consequence 

 of the oxidation of the haemocyanin. It clots quickly in 

 virtue of the action of a ferment, yielded by its amoeboid 

 corpuscles, upon a globulin proteid present in the plasma. 

 A reddish tinge that may be observed in the blood and 



1 Bouvier, C. R. Ac.ad. Sci. Paris, torn. 110. 



2 Halliburton, Journ. of Physiol. vi. No. 6, 1885. 



