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 corj)uscle, in fact, has an 

 inherent contractility, like one of those low organisms, 

 known as an Amoeba, whence its motions are frequently 

 called amoehiform. In its interior, an ill-marked oval 

 contour may be seen, indicating the presence of a sphe- 

 roidal body, about l-2000th of an inch in diameter, which 

 is the nucleus of the corpuscle (ii). 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 protoj^lasmic 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 caeca. 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 

 9 



