296 PRACTICAL LESSONS IN SCIENCE. 



and the structure of the heart, make them appear as in some 

 sense a transitional group between the true fishes and the am- 

 phibians. They are not found in this country, and are of no 

 economic value, and are not numerous, although, during recent 

 geological periods, they were prominent forms of life. 



The Amphibia are cold-blooded, usually naked-skinned animals 

 that are oviparous. The fertilized egg develops into a larva, 

 somewhat fish-like in form, that breathes by means of gills. The 

 larva eventually develops lungs and anterior and posteriorlimbs, 

 sometimes retaining the gills during life but usually losing them. 

 The lungs are not well organized and much of the respiration is 

 carried on through the skin. The medium of respiration whether 

 by gills, lungs or skin is supposed to be derived from the cutane- 

 ous surface. 



The skeleton varies considerably, but the bones are usually 

 more compact and calcareous than those of fish, and, with the 

 development of the external limbs, the pectoral and pelvic arches 

 are more fully developed and the amphibian is fitted for crawl- 

 ing, swimming, climbing and walking. 



The nervous system seems, on the whole, to be more highly de- 

 veloped than among fishes, although the sense organs do not 

 seem much superior to those of fish unless, perhaps, the senses 

 of touch and taste may be exceptions. Some of the amphibians 

 have a well-developed vocal apparatus in connection with the res- 

 piratory organs, which is a distinguishing mark of some impor- 

 tance. 



The young, while breathing by means of gills, have the single 

 auricle and ventricle and the same arrangement of arterial trunks 

 as among fishes, but with the development of the lungs the auri- 

 cle is divided into two chambers, one receiving blood from the 

 body, the other from the lungs; but the ventricle remains single, 

 containing mixed blood. The lymphatic system is well developed , 

 in some cases the lymph receptacles are contractile, having the 

 value of lymph hearts. 



Some amphibia are terrestrial during the adult state, but usu- 

 ally live in damp localities since cutaneous respiration necessi- 



