The Ititegumentary System S3 



These ridges which permit the "fingerprints" are present at birth 

 and do not change through the hfetime of the individual. For this 

 reason, they are useful for identification purposes. 



Friction ridges are also found along the inner surface of the pre- 

 hensile tails of the new world monkeys. Located on the feet of certain 

 rodents such as the mouse are distinct elevations or pads known as 

 tori. These are present in the human embryo, but disappear during 

 growth to become flattened and form the friction areas of the hands 

 and feet. 



SKIN DERIVATIVES 



Among the various vertebrate groups, the skin has many deriva- 

 tives which are specialized for the performance of different functions. 

 Like the outer cells of the epidermis, they are often formed of the in- 

 soluble protein, keratin. These derivatives include such widely varying 

 structures as horns, nails, feathers, hairs, scales, claws, and teeth. Often 

 they are of aid to the animal as weapons ; chiefly, however, they 

 function to protect against drying, cold, heat, and other factors of 

 the environment. 



Scales. — While scales of some sort may be found in members 

 of nearly every class of vertebrates, they are particularly character- 

 istic of the fishes and reptiles. In both these classes they are the chief 

 integumentary derivatives, and in both they afford excellent protection 

 against injury. Li fish they have the additional task of preventing 

 excessive absorption of water from the surrounding medium, while in 

 reptiles they prevent excessive drying of the body tissues. 



The placoid scale (Fig. 22,C) found among the sharks consists, 

 in part, of a flat plate having its origin in the dermis. It is embedded 

 in the skin and has a projection of dentine covered with a shiny sur- 

 face composed of harder dentine. These scales are small and placed 

 together, giving a roughened surface to the shark's skin. Before sand- 

 paper was very common, "shagreen" skin from sharks and dogfishes 

 was commonly used as a fine abrasive. 



Ganoid scales (Fig. 22, D) are usually slightly imbricated, platelike 

 structures which form a continuous armor over the surface of the 

 fish. These scales are found among the gars, sturgeons, and the bowfin. 

 These fish are modern representatives of the ganoid fish which in former 

 geologic times were a much more varied and abundant group. 



