178 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



consist of a protoplasmic substance (fig. 50), in which close 

 set nuclei (n) are imbedded. If a number of blood corpus- 

 cles could be supposed to be closelj- 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 ceUs are rarely Tisible in the 

 fresh state. In the liver, however, the cells grow, and 

 become detached from one another in the wider and lower 



Fig. .50. — Astaeus flnviatilis. — Epithelium, from the epidennio layer 

 subjacent to the cuticle, highly magnified. A, in vertical section ; 

 B, from the surface, n, nuclei. 



parts of the csBca, and their essential nature is thus 

 obvious. 



3. Immediately beneath the epithelial layer 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 



