THE SKIN AND SKELETON. 133 
skeleton: these are plates of cartilage, one of which sur- 
rounds the brain, and hence may be called a skull. To 
this cartilage, not to the “cuttle-bone,” the muscles are 
attached. 
In Vertebrates, the exoskeleton is subordinate to the 
endoskeleton, and is feebly developed in comparison. It 
is represented by a 
great variety of append- 
ages to the skin, which 
are mainly organs for 
protection, not for sup- 
port. Some are horny 
developments of the ep- 
idermis, such as hairs, 
feathers, nails, claws, 
hoofs, horns, and the Fra. 101.—Skeletal Architecture in the Armadil- 
eins of Reptiles ; othe Coto ao raetiog of the carapax to the 
ers arise from the hardening of the dermis by calcareous 
matter, as the scales of Fishes, the plates of Crocodiles 
and Turtles, and the shield of the Armadillo. 
The scales of Fishes (and likewise the spines of their 
vertical fins) lie imbedded in the overlapping folds of the 
skin, and are covered with a thin epidermis. The scales 
of the bony Fishes (Perch, Salmon, etc.) consist of two 
Fie. 102.—Diagrammatic Section of the Skin of a Fish (Carp): a, derm, showing lam- 
inated structure with vertical fibres, b; c, gristly layer; e, laminated layer, with 
calcareous granules; d, superficial portion developing into scales; J, scale-pit. 
layers, slightly calcareous, and marked by concentric and 
radiating lines. Those of the Shark have the structure of 
teeth, while the scutes, or plates, of the Crocodiles, Turtles, 
and Armadillos are of true bone. 
