THE INTEGUMENTARY SYSTEM 



117 



further evolution. Each of these scales has a flat basal plate of dentine 

 embedded in the skin, and each has commonly also a projecting spine 

 coated, like a tooth, with hard enamel. From these minute placoid scales 

 of ancient sharks have evolved all the multiform teeth of all the higher 

 vertebrates. 



From these and other types of scale have evolved also, by simple 

 enlargement, the heavy continuous dermal armor of ganoids and other 

 fishes. These same bony plates survive also in man and the higher 



Fig. 110.— 

 The scales are 

 scales of the li 



-ENAMEL SPINE 



-The imbricated pattern of placoid scale arrangement in elasmobranchs. 

 arranged in rows and usually each scale is in line with the interval between 

 nes in front and behind. (Redrawn after Klaatsch.) 



vertebrates as " membrane bones " which, unlike most parts of the skeleton, 

 are not pre-formed in cartilage but develop directly in connective tissue. 



HORNY SCALES 



Vertebrates, besides bony scales, have also horny; but these have 

 played a much less important part in evolution, and are confined to 

 amniotes, more especially reptiles. 



In reptiles, the stratum corneum forms a continuous scaly layer over 

 the entire body, the separate scales being local thickenings which con- 

 tinue to grow by the addition of new keratin from underneath. Serpents 

 commonly shed this scaly coat twice a year. But the rattlesnake retains 

 bits of the old skin at the tip of the tail. These become the rattle, which 

 therefore grows two rings a year. 



