EPITHELIUM. 177 
ously for two or three minutes, its shape will be seen to 
undergo the constant but slow changes to which passing 
reference has already been made (p. 69). One or other of 
the irregular prolongations will be drawn in, and another 
thrown out elsewhere. The corpuscle, in fact, has an 
inherent contractility, like one of those low organisms, 
known as an Amoeba, whence its motions are frequently 
called ameabiform. In its interior, an ill-marked oval 
contour may be seen, indicating the presence of a sphe- 
roidal body, about 1-2000th of an inch in diameter, which 
is the nucleus of the corpuscle (n). The addition of some 
re-agents, such as dilute acetic acid, causes the corpuscles 
at once to assume a spherical shape, and renders the nuc- 
leus very conspicuous (fig. 49, 9 and 10). The blood 
corpuscle is, in fact, a simple nucleated cell, composed 
of a contractile protoplasmic mass, investing a nucleus ; 
it is suspended freely in the blood; and, though as 
much a part of the crayfish organism as any other of 
its histological elements, leads a quasi-independent ex- 
istence in that fluid. 
2. Under the general name of epithelium, may be in- 
cluded a form of tissue, which everywhere underlies the 
exoskeleton (where it corresponds with the epidermis of the 
higher animals), and the cuticular lining of the alimen- 
tary canal, extending thence into the hepatic ceca. It is 
further met with in the generative organs, and in the green 
gland. Where it forms the subcuticular layer of the 
integument and of the alimentary canal, it is found to 
N 
