THE BLOOD AND ITS COKPUSCLES. 69 



have thus become integral parts, is a clear fluid, either 

 colourless, or of a pale neutral tint or reddish hue, which, 

 to the naked eye, appears like so much water. But if 

 subjected to microscopic examination, it is found to con- 

 tain innumerable pale, solid particles, or corpuscles, 

 which, when examined fresh, undergo constant changes 

 of form (fig. 14). In fact, they correspond very closely 

 with the colourless corpuscles which exist in our own 

 blood; and, in its general characters, the crayfish's 

 blood is such as ours would be if it were somewhat 

 diluted and deprived of its red corpuscles. In other 

 words, it resembles our lymph more than it does our 

 blood. Left to itself it soon coagulates, giving rise to a 

 pretty firm clot. 



The sinuses, or cavities in which the greater part of 

 the blood is contained, are disposed very irregularly in 

 the intervals between the internal organs. But there is 

 one of especially large size on the ventral or sternal side 

 of the thorax (fig. 15, sc), into which all the blood in the 

 body sooner or later makes its way. From this sternal 

 sinus passages (av) lead to the gills, and from these again 

 six canals (bcv), pass up on the inner side of the inner wall 

 of each branchial chamber to a cavity situated in the 

 dorsal region of the thorax, termed the pericardium (p), 

 into which they open. 



The blood of the crayfish is kept in a state of con- 

 stant circulating motion by a pumping and distributing 

 machinery, composed of the' heart and of the arteries, with 



