ITISTOLOGY. TISSUES. 175 
out the more intimate structure of these parts. The 
tough, outer coat, which has been termed the cuticula, 
except so far as it presents different degrees of hardness, 
from the presence or absence of calcareous salts, is 
obviously everywhere of the same nature; and, by 
macerating a crayfish in caustic alkali, which destroys all 
its other components of the body, it will be readily 
enough seen that a continuation of the cuticular layer 
passes in at the mouth and the vent, and lines the 
alimentary canal; furthermore, that processes of the 
cuticle covering various parts of the trunk and limbs 
extend inwards, and afford surfaces of attachment to the 
muscles, as the apodemata and tendons. In technical 
language, the cuticular substance which thus enters so 
largely into the composition of the bodily fabric of the 
crayfish is called a tissue. 
The flesh, or muscle, is another kind of tissue, which 
is readily enough distinguished from cuticular tissue by 
the naked eye; but, for a complete discrimination of 
all the different tissues, recourse must be had to the 
microscope, the application of which to the study of 
the ultimate optical characters of the morphological 
constituents of the body has given rise to that branch 
of morphology which is known as Histology. 
If we count every formed element of the body, which 
is separable from the rest by definite characters, as a 
tissue, there are no more than eight kinds of such tissues 
in the crayfish; that is to say, every solid constituent 
