712 THE AMERICAN NATURALIST [Vol. XLIII 



In many digenetic trematodes the cuticula is not of 

 equal thickness on all parts of the body and these varia- 

 tions in thickness are not correlated with corresponding 

 differences in the subcuticular cells beneath them. For 

 instance, the cuticula of the appendiculate distomes has 

 numerous transverse rings which give a longitudinal sec- 

 tion a serrated appearance. These rings are due solely to 



variations in the thickness of the cuticula, the inner sur- 

 face of it being quite smooth, and we might expect the 

 subcuticular cells, if they secreted the cuticular, to be 

 larger or more numerous beneath the rings. This is, how- 

 ever, not the case, these cells showing no variations what- 

 ever beneath these cuticular rings. 



The relations of the spines, hooks, scales and other 

 special cuticular structures to the subcuticular cells also 

 furnish an argument against the epithelial or glandular 

 nature of the latter. These spines and hooks are of very 

 common occurrence and are often very prominent objects 

 in trematodes and cestodes, their function usually being 

 to aid in anchoring the parasite to its host. They are 

 seen in their simplest form in the digenetic trematodes, 

 (Fig. 2) in which they usually appear as specialized parts 

 of the cuticula. In the monogenetic trematodes and the 

 cestodes, on the other hand, they are often of huge size 

 and of more or less complex structure and may extend be- 

 neath the cuticula among the muscles and the parenchyma 

 cells. 



That these organs are similar in essential structure and 



