HISTOLOGY. TISSUES. 175 



out the more intimate structure of tliese parts. The 

 tough, outer coat, which has been termed the cutlcula, 

 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 apj^lication 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 hody, 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 



