374 ZOOLOGY. 



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

 independently 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 vertebrae of the lower forms are biconcave as in fishes, 

 in the higher forms (Anura, and higher Urodela), concavo- 

 convex. The vertebral column usually consists of one cervical 

 vertebra ; a variable number of thoracic or abdominal vertebrae ; 

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

 variable number of caudal (one, in Anura). 



393. Respiration. In early larval stages the respiration is 

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

 velopment of special organs of respiration the skin continues to 

 serve this function in a greater or less degree. Most amphi- 

 bians 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 develop- 

 ment of the frog). Typically, lungs replace both kinds of gills 

 in the adult. 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 supple- 

 menting 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 oesophagus 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 lungs in the adult, 

 respiration taking place wholly through the body surfaces. 

 The frog breathes through its nostrils. The mouth cavity can 

 be increased by muscular action, thus allowing the entrance of 

 air. The nasal openings are then closed by flaps and the air is 

 forced by muscular action into the lungs. 



394. Supplementary Exercises for the Library. Find as many dif- 

 ferent types of respiration as possible among the amphibians, and cite ex- 



