37^ ZOOLOGY 



animal, resulting from impressions on the retina of the eye. 

 Besides light, — heat and cold, moisture, and probably internal 

 states of the animal, may produce color changes. In many 

 instances these colors are protective by making the animal like 

 its surroundings. In the extinct Stegocephali external protec- 

 tive plates were developed in the dermis. Minute dermal 

 scales are found in some of the lowest present forms ("blind- 

 worms"). 



399. The Skeleton. — The points of contrast with the skele- 

 ton of fishes are, chiefly : the presence of a sternum (formed in- 

 dependently of the ribs) ; the imperfect development of the ribs; 

 the typical limb skeleton; the union of the pelvic girdle with the 

 Spinal column; the closer fusion of the upper jaw with the 

 cranium. 



The vertebras of the lower forms are biconcave as in fishes, 

 in the higher forms (Salientia, and higher Caudata), concavo- 

 convex. The vertebral column consists of one cervical verte- 

 bra; a variable number of thoracic and abdominal vertebrse; 

 usually one sacral, to which the posterior girdle is attached; and 

 a variable number of caudal vertebrse {one, in Salientia). 



400. Respiration. — In early larval stages the respiration is 

 effected wholly by means of the skin, and even after the develop- 

 ment of special organs of respiration the skin continues to serve 

 this function in a greater or less degree. Most amphibians have, 

 when hatched, external gills which may be retained through 

 life (as in Siren, the "mud-eel"), or may give place to internal 

 gills covered by a fold of skin (as in the development of the frog). 

 Typically, lungs replace both kinds of gills in the adidt. The 

 gill slits do not exceed three or four pairs in number. Some of 

 the aquatic forms retain their gills when the lungs are developed, 

 each method of respiration supplementing the other. Those 

 which possess lungs alone in the adult must of necessity undergo 

 profound changes in passing from the water-breathing to the 

 air-breathing habit. The lungs arise as a ventral outgrowth 

 from the esophagus or pharynx. From the short trachea the 

 two sac-like lungs spring. The walls are in folds but the sacs 

 are simple. In some salamanders there are neither gills nor 



