178 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
consist of a protoplasmic substance (fig. 50), in which close 
set nuclei (n) areimbedded. If anumber of blood corpus- 
cles could be supposed to be closely aggregated together 
into a continuous sheet, they would give rise to such a 
structure as this; and there can be no doubt that it 
really is an aggregate of nucleated cells, though the 
limits between the individual cells are rarely visible in the 
fresh state. In the liver, however, the cells grow, and 
become detached from one another in the wider and lower 
Fig. 50.—Astacus flviatilis.—_Upithelium, from the epidermic layer 
subjacent to the cuticle, highly magnified. A, in vertical section ; 
B, from the surface. , nuclei. 
parts of the ceca, and their essential nature is thus 
obvious. 
3. Immediately beneath the epithelial Jayer follows a 
tissue, disposed in bands or sheets, which extend to the 
subjacent parts, invest them, and connect one with 
another. Hence this is called connective tissue. 
The connective tissue presents itself under three forms. 
In the first there is a transparent homogeneous-looking 
matrix, or ground substance, through which are scattered 
many nuclei. In fact, this form of connective tissue 
