26 



MANUAL OF ELEMENTARY ZOOLOGY 



of toads and newts. The pigment in the cells expands and 

 contracts in varying conditions of light and temperature 

 and thus alters the colour of the frog. Cold, dark, or 

 k wet surroundings cause expansion of the pigment and 

 darkening of the skin. Warmth, light, or dryness cause 

 contraction (PI. III.). From time to time the horny outer 

 layer of the skin is shed and eaten by the frog. 



Immediately below the skin is a series of large spaces, 

 General the subcutaneous lymph sacs, containing a fluid 



of Interna? Bt known as lymph (p. 60). Between the lymph sacs 

 organs. the skin is bound down to the underlying flesh 



by tough, white connective tissue, but in consequence of the 



presence of the sacs it 

 is much looser than 

 that of most animals. 

 Below the sacs the 

 body possesses a 

 continuous layer of 

 .-. flesh, which con- 

 sists, as the sub- 

 stance so-called 

 always does, of 

 muscles. There is 



FIG. 9.— A diagram of a transverse section thuS a , h ody-wall 

 through the abdomen of a male frog. composed Ot skin and 



muscles, with bones 



coe., Ccelom ; d. ao., dorsal aorta ; d.s., dorsal lymph j ... r 



sac ; d.s.s., dorsal subcutaneous lymph sac ; g-., and a lining Ot peri- 

 gut; z>.c, inferior vena cava ;£., kidney; !.s.s., toneum (tO be men- 

 lateral subcutaneous lymph sac ; msnt., mesen- . j u l \ j 

 tery; mso., mesorchium ; muse, muscular body- tlOned Shortly), and 

 wall; »., spinal nerves; J>er., peritoneum; si., ..l: 11 Pnr .ln=pc ; n 

 skin; sp.c, spinal cord; <., testis ; v., vertebra; tms waU encloses in 

 v.s. s., ventral subcutaneous lymph sac. the trunk a large 



space, the body cavity 

 or ccelom, in which lie most of the principal viscera. 

 The latter name is applied to the soft internal organs 

 of the body, such as the stomach, bowels, liver, lungs, 

 and heart. The body-wall of the back is much thicker 

 than that of the belly, and in it is embedded a 

 structure known as the backbone, spine, or vertebral 

 column. This consists of a row of ring-like bones, the 

 vertebra, placed end to end to form a tube, the 

 vertebral canal, in which lies a part of the nervous 



