254 TEXT-BOOK OF ZOOLOGY 



cases the change takes place involuntarily, and even contrary to our 

 wish. A blind man standing by our side, but incapable of perceiving 

 any of these sights, neither blushes nor turns pale. Exactly the same 

 happens in animals which are capable of changing their colour. They 

 can only do this while they see, but not if they are deprived of sight. 

 Hence the change of colour is effected through the stimulation of the eye 

 (and that in such a manner that the stimulus acting on the optic nerve 

 is transmitted or reflected to the nerve fibres which are connected with 

 the pigment cells). 



(e) The outermost layers of the skin are periodically cast either as a 

 whole or in patches. 



4. Respiration. In the larval stages (see Section 6) respiration is 

 always effected by gills or branchiae (see fish), in the fully-developed 

 animal usually by lungs. In a few only (see Proteus) branchiae co-exist 

 with lungs throughout life. Eibs being entirely absent (Batrachia), or 

 short rudiments of them only present (Urodela), dilatation of the chest 

 becomes impossible. The respiratory air is consequently swallowed. 

 (Observe the under side of the head in the frog. See also tortoise, 

 p. 249.) Nevertheless, many amphibians breathing exclusively by lungs 

 can remain a considerable time below the water, and even hibernate for 

 months in the mud of ponds, etc. In these animals, in fact, contrary to 

 what obtains in most other animals breathing by lungs, an exchange 

 between carbonic acid gas and oxygen is effected through the agency of 

 the moist and naked skin (cutaneous respiration ; compare also with 

 the gills of fishes). 



5. The Heart consists of two auricles and one ventricle (see Keptiles, 

 p. 229). 



6. Reproduction. Most amphibians are oviparous. The eggs being 

 usually laid in the water (larvae breathe by gills), they are destitute of 

 firm envelopes like those of the eggs of land animals. A few only are 

 viviparous. As a rule, the young as they escape from the egg have a 

 form widely different from that of the parents, and are consequently 

 described as larvce (see Insects). It is only after a series of manifold 

 changes (metamorphosis) that the larva develops into the perfect am- 

 phibian (see edible frog, newt, land salamander). As development usually 

 takes place in the water, the life of the young amphibian resembles that 

 of a fish. These animals, therefore, may truly be called amphibious, 

 since at one stage of life they live actually in the water, in the other on 

 land in air. 



