260 COMPARATIVE ZOOLOGY OF CHORDATES 



the epidermis since the layer of ganoin (bone) is a mesodermal 

 structure. The epidermis overlying these scales may possess 

 true denticles. It is also worth noticing that the structure 

 of the dermal bones and of the dermal fin-rays (lepidotrichia) 

 in a given animal tends to be identical with that of the scales. 



In the higher bony fish or Teleosts, the scales lose the layer 

 of ganoin. The scales form in the dermis, but the bone 

 cells become lost and the scales are very thin. It is obvious 

 that these dermal scales together with the dermal scales of 

 Gymnophiona and lizards (osteoscutes) must not be regarded 

 as having anything in common with the epidermal scales 

 (corneoscutes) of higher forms. Dermal scales are retained 

 throughout life ; epidermal scales and denticles are shed. 



Other examples of dermal ossifications are to be found in 

 the bone (os corneum) which forms the core of the " horn " 

 of cattle, and which becomes attached to the frontal bone of 

 the skull. Similar little bones form the knobs on the head of 

 the giraffe, while large bony structures in this position give 

 rise to the antlers of deer. Antlers are restricted to the males, 

 they may be forked, and they are shed every year. The size 

 of the antler often bears an interesting relation to the size 

 of the body (see p. 482). Horns, on the other hand, may be 

 present in both sexes, and, except in Antilocapra (the 

 American prong-buck), they are neither forked nor shed. 



Lastly, when dealing with the skin, mention must be made 

 of colour. Pigment-cells may occur in the epidermal and the 

 dermal layers of the skin. In some cases, the pigment-cells 

 are capable of altering the distribution of their pigment, with 

 the result that the animal may change colour (as, for example, 

 the frog, or the chamaeleon). Pigment may also be present 

 in feathers and in hair, but in these structures the texture of 

 the surface may also produce effects of colour without any 

 pigment being there. 



Literature 



Goodrich, E. S. Vertebrate Craniata, Cyclostomes and Fishes. Black, 

 London, 1909. 



