ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 35 



well-marked thickening. The protoplasm of the free surface of such 

 cells becomes hardened so as to give rise to a thick superficial border, 

 perforated at right angles to its surface by a number of fine canals 

 which give it a striated ap- 

 pearance (intestinal epithe- 

 lium, fig. 21, f, epidermis cells 

 of Petromyzon). If these 

 thickened borders fuse to- 

 gether to as to form a con- 

 tinuous layer which obtains 

 a certain independence, we 

 obtain cuticular membranes, 

 which, according to their ori- 

 gin, may be homogeneous or 

 stratified (fig. 22, a, b, c), and 



a 



FIG. 22. a, Cuticle and hypodermis of the larva of 

 Corethra. b, cuticle and hypodermis of a Gastro- 

 pacha caterpillar, with two poison glands beneath 

 corresponding bristles. 



may exhibit various patterns of different 

 kinds. Very frequently the surfaces 

 of the individual cells are indicated on 

 the cuticle as polygonal figures; and, 

 in addition to the very fine pores, 

 there are also found larger passages pro- 

 duced by out-growth from the cells. 

 These latter lead to the appearance f 

 various cuticular appendages, such as 

 hairs, bristles, scales, etc., which are 

 placed on the cuticular pores, and con- 

 tain as a matrix their special cell or a process of it. Cuticular 

 membranes may obtain a very considerable thickness, and, by the 

 deposition of calcareous salts, a high degree of firmness (cai^apace of 

 Crustacea) so that they acquire the value of skeletal tissue.s, which, 



FIG. 22c. CM, cuticle with bristles in 

 the condition of ecdysis. Cu', 

 newly-formed cuticle (Branchipus). 



