AMPHIBIA 383 



external gills. This is of course a successful adaptation (by 

 eliminating the metamorphosis) to a completely aerial habit. 



From this group we have beautiful illustrations of unequal 

 cleavage of the ovum, of which the student should have the op- 

 portunity of seeing figures in more extended works. If possible 

 the cleaving eggs should be studied. 



408. Place in Nature. — The amphibians are among the oldest 

 vertebrates. They are not, however, a very successful' group as 

 measured by numbers of species or of individuals. The class 

 itself and such species as the frog suggest the way in which the 

 zoologist thinks the land animals arose from the aquatic. 

 Among amphibians we have types that are like fish, gill-breathers 

 all through life; others begin as gUl-breathers, retain their gills 

 and develop lungs; still others lose their gills when the lungs 

 are perfected; and some are hatched out with lungs, never 

 having had functional gills. This makes a very suggestive 

 series of connecting links between water and air-breathing forms. 



None of the species has large individuals. The "giant 

 salamander" of Japan is the largest, reaching a length of 3-5 

 feet. 



Amphibians are practically uniformly helpful to human in- 

 terests, or at least without serious hurt to them. The toads 

 destroy many injurious insects, and are thus particularly valu- 

 able about gardens. Toads are easily reared artificially and 

 might be made to give us even more aid in controlling this diffi- 

 cult group. The legs of some of the larger species of frogs are 

 eaten. The flesh is white and delicate. Frogs are also used in 

 great numbers in the zoological laboratories, for dissection and 

 experimentation. They have thus contributed no little to our 

 knowledge of animal functions, particularly the functions of the 

 nervous system!. 



Profitable frog "farms" are being conducted in various 

 parts of the country to meet the demands referred to above. 

 This suggests possible uses for certain swamp lands not now 

 suitable for other purposes. Edible frogs and turtles might be 

 encouraged in such regions instead of inedible types. 



409. Special Exercises. — Describe the life history and the 

 stages in the metamorphosis of the frog. (Fig. 190.) What 



