CHAPTER X 



INTEGUMENTARY STRUCTURES 



The integument, or covering, of the vertebrate consists of the 

 skin and its derivatives. Amphioxus has the simplest possible 

 skin structure, consisting of a thin layer of mesodermal con- 

 nective tissue, with an ectodermal layer of epidermis only one 

 cell thick. The latter cells secrete a thin cuticle of non-cellular 

 material which covers the animal externally. The vertebrate 

 skin is based on the same principle although it is thicker, lacks 

 the cuticle, and is complicated by other structures which grow 

 from either layer. 



The skin is essentially protective in function. The cyclostomes 

 secrete a protective layer of mucus, the fish have protective 

 scales which arise from the skin, and the reptiles, birds and 

 mammals have a dry integument w^iich is impervious to patho- 

 genic bacteria. 



The ectodermal part of the skin is the epidermis and lies upon 

 the dermis (corium) which develops from the epimere of the 

 mesodermal somite. The dermis is a fibrous structure resembling 

 the typical connective tissues, and is usually a close-packed 

 structure well supplied with nerves and blood vessels. It is con- 

 nected with the underlying muscles by connective tissue fibers 

 which freely interlace with the reticular fibers. There is a layer 

 of subcutaneous connective tissue lying between the well de- 

 fined skin and the muscles. This permits some movement of the 

 skin without affecting the muscles, and explains the ease with 

 which most vertebrates are skinned. In addition, the subcutane- 

 ous tissues are normally loaded with droplets of fat, which 

 assists in heat regulation and supplies a store of food for the 

 animal. This becomes of great importance in animals which 

 hibernate or aestivate. 



The epidermis is several layers thick. Lying next the dermis 

 is a growth layer, the stratum germinatiinim (stratum Malpi- 



107 



