40 



MORPHOLOGY OF THE VERTEBRATE BODY 



the adaptation to environment that is so characteristic of both ani- 

 mals and plants; and also as evidence for organic evolution, since 

 anatomical resemblance is regarded as evidence of evolutionary 

 relationship. 



The Skin and its Derivatives. — The skin of vertebrates is 

 variously modified in relation to the conditions of life. Coloration 

 of the skin or its derivatives occurs in all save exceptional cases. 

 The skin glands are often important. Peculiar external modifica- 

 tions of the skin are characteristic of the several classes (c/. 



sp.cd. spxi. ^rn 



1 



- - 1.8. 



^ ab.u. 



Fig. 19. — Transverse section of the frog in region of kidneys and reproductive 



organs. 



ah.v., abdominal vein; cce, coelome; da., dorsal aorta; d.c, digestive cavity; kd, kidney; 

 i.s., subcutaneous lymph sinuses, separated by septa; m, muscles; ms, mesentery; ov, 

 ovary; p.p., parietal peritoneum; p.c, post-caval vein; sk, skin; sp.cd., spinal cord; 

 sp.c/!, spinal column; su.i.s, subvertebra! lymph sinus; <, testis; cs.p., visceral peritoneum. 

 The specimen is shown as a male on one side and as a female on the other. 



table of classification, p. 37, and Fig. 52, p. 95). The elasmo- 

 branchs have one type of scale, the teleosts another; the cyclo- 

 stomes and most Amphibia possess a slimy surface; reptiles are 

 covered with scales peculiar to the class, birds with feathers, 

 mammals with hair. These characteristics are so distinctive that 

 they serve, with few exceptions, as features by which one may deter- 

 mine at a glance the class to which a given vertebrate belongs. 



In most instances the skin is tightly fastened to the muscles so 

 that the " skinning " of the animal necessitates a cutting or tearing 

 of the resistant connective tissue that unites skin and flesh. In 

 contrast to this condition, the skin of the frog is like a glove, fit- 



