394 THE EXOSKELETON. 



the epidermis either as (i) a cuticular deposit, or as (2) a 

 chitinization, a cornification, or calcification of its constituent 

 cells. These two processes run into each other, and are in many 

 cases not easily distinguished. The protective structures of the 

 epidermis may be divided into two groups according as they are 

 formed on the outer or the inner side of the epidermis. Dermal 

 skeletal structures are in many cases added to them. Amongst 

 the Invertebrata the most widely distributed type of exoskeleton 

 is a cuticle formed on the outer surface of the epidermis, which 

 reaches its highest development in the Arthropoda. In the same 

 class with this cuticle must be placed the molluscan and brachi- 

 opod shells, which are developed as cuticular plates on special 

 regions of the epidermis. They differ, however, from the more 

 usual form of cuticle in their slighter adhesion to the subjacent 

 epidermis, and in their more complicated structure. The test of 

 Ascidians is an abnormal form of exoskeleton belonging to this 

 type. It is originally formed (Hertwig and Semper) as a 

 cuticle on the surface of the epidermis ; but subsequently 

 epidermic cells migrate into it, and it then constitutes a tissue 

 similar to connective tissue, but differing from ordinary epidermic 

 cuticles in that the cells which deposit it do so over their whole 

 surface, instead of one surface, as is usually the case with 

 epithelial cells. 



In the Vertebrata the two types of exoskeleton mentioned 

 above are both found, but that developed on the inner surface of 

 the epidermis is always associated with a dermal skeleton, and 

 that on the outer side frequently so. The type of exoskeleton 

 developed on the inner side of the general epidermis is confined 

 to the Pisces, where it appears as the scales; but a primitive 

 form of these structures persists as the teeth in the Amphibia 

 and Amniota. The type developed on the outer side of the 

 epidermis is almost entirely 1 confined to the Amphibia and Am- 

 niota, where it appears as scales, feathers, hairs, claws, nails, &c. 

 For the histological details as to the formation of these various 

 organs I must refer the reader to treatises on histology, confining 

 my attention here to the general embryological processes which 

 take place in their development. 



1 The horny teeth of the Cyclostomata are structures belonging to this group. 



