50 GENERAL ZOOLOGY 



clown in the corium, the overlying epidermis secretes a hard 

 enamel covering upon the dentinal base and spine. The rough 

 "sandpaper" surface of the shark's skin is thus due to numerous 

 sharp spines, each projecting outward from a basal plate. The 

 exposed, enamel-covered spines are like so many short sharp- 

 pointed teeth on the surface of the body, the basal plates remain- 

 ing embedded in the corium below and the epidermal layer above. 

 This type of scale is thought to be the forerunner of vertebrate 

 teeth and dermal bone (Fig. 25). 



The scales of teleost fishes, such as the perch or bass, consist of 

 bony plates developed in the dermis and overlaid by the epi- 

 dermis. Scales of this type may be hard and bony or soft and 

 flexible. The epidermis takes no part in their development. 



A B 



Fig. 25. — Placoid scale. A, view from above; B, side view, s, spine; p, basal 



plate. 



Horns, Hoofs, Antlers. — The horns and hoofs of sheep, goats, 

 and cattle represent cornifications of the epidermis and resemble 

 nails and claws in their general composition. Usually horns are 

 not shed, a notable exception being the horns of the prong-horn 

 antelope, Antilocapra americana, of Western U. S. Antlers are 

 outgrowths of the frontal bones of the skull and are at first 

 covered with skin which may persist throughout life, as in the 

 giraffe, but which in forms like deer, elk, etc. becomes worn off, 

 exposing the bone. The antlers are shed each year, the succeed- 

 ing ones displaying a greater number of tines, which thus serve 

 as an index of age. 



Invertebrate Integument. — The outermost layer of the integu- 

 ment of some invertebrate animals is cellular, as in the flatworm, 

 Planocera, in which it consists of short columnar epithelial cells, 

 one cell layer in thickness, and provided with cilia on the outer 

 surface (Fig. 26). In many other invertebrates a typical condi- 

 tion consists in the presence of a noncellular structure, the cuticle 

 or cuticula, at the surface of the body. The cuticle is a substance 



