THE BLOOD AND ITS CORPUSCLES. 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, se), 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 (bev), 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 
