The Frog: A Representative Vertebrate 55 



The color of the skin of the frog is due to several different struc- 

 tures. Located in the epidermis are scattered pigment granules. Within 

 the dermis are special pigment cells, the chromatophores. These chrom- 

 atophores are of two different types: those having black or brown 

 pigment and those having red or yellow. Other cells contain whitish 

 crystals. The over-all greenish color of the animal is due to the 

 fact that the pigments absorb some colors and reflect others and to 

 the interference of the white crystals. The general coloration of the 

 animal may be either light or dark, depending upon the color of the 

 background. When the pigment within the chromatophores contracts, 

 the color becomes light ; when they expand, the color becomes dark. 

 This color change apparently is mediated through the eyes although the 

 skin has been shown to respond directly to light. 



DcnMis 



.STRATUM CORNEUM 



STRATUM 



GCRMINATIVUM 



CHROUATOPHORE 

 STRATUM SPONGIOSUM 



POISON GLAND 



STRATUM COMPACTUM 



Fig. 10. — Section of skin of frog. 



THE SKELETAL SYSTEM 



Like that of all the other vertebrates, the skeleton (Fig. 11) of 

 the frog forms a supporting structure for the entire body. Not only 

 does it protect the softer organs, but it also provides surfaces for 

 muscular attachment. Principally the skeleton consists of two main 

 portions : the axial skeleton consisting of the skull, vertebral column, 

 and sternum ; and the appendicular composed of the girdles and their 

 appendages. 



The Axial Skeleton.— The anterior portion of the axial skeleton 

 is the skull, which protects the brain and sense organs. The skull of 

 the frog is quite flattened with a small, narrow cranium that contains 

 the brain. Lateral to this cranium are the large orbits that surround 



