44 



General Biology 



tically always laid in water and hatched there. Little tadpoles develop 

 from these eggs and breathe by gills in the larval ( ) 1 condition. 



Some species of frogs retain these gills all through life, even though 

 lungs may be present in the adult forms. The tadpole gradually develops 

 into the mature frog, losing its tail and developing the long hind legs 

 and the short fore legs so familiar in the adult animal. 



EXTERNAL FEATURES 



It is essential that one examine quite carefully the external struc- 

 ture of any plant or animal one may wish to study; for, unless this 

 knowledge is borne in mind, internal structure cannot be interpreted 

 correctly. It is well also to keep in mind our own bodies, and to observe 

 similarities and differences wherever they may occur in animals and 

 plants. 



C, egg con- 

 4, eggs before B, eggs after they taining young 

 they are laid are laid tadpole 



D, young tadpoles attaching 

 themselves to a plant 



B. young tadpole with ex- F, young tadpole with 

 ternal gills internal gill3 



G, young tadpole with hind lega 



H, tadpole with webbed 

 feet 



I, tadpole with legs and arms 



J, young frog 



Fig. 3. 



Eggs, Tadpole, and Adult Stages of Frog. 

 (After Brehm and other authors.) 



It will be noted immediately, that the frog has no neck. The head 

 is broadly united to the trunk. The eyes protrude somewhat, but can 

 be withdrawn readily into the orbits. A pressure put upon the eyes, 

 when the mouth of a frog is open, will extend the inner membrane lining 

 of the roof of the mouth quite prominently, showing that the orbit, or 

 eye socket, is not separated from the mouth by any bones of the skull 

 as in man. The dark oval opening of the eye, the pupil, is surrounded 

 by the iris, a more brightly colored ring. There are upper and lower 

 lids, the upper one moving but slightly, the lower one thin and trans- 

 parent, and capable of covering the entire eye. This lower eyelid is 

 different from that of most animals, and this type of lid will be met with 

 again in other animal-forms to be studied. The nictitating membrane 

 ( ) is separated from the lower lid (Fig. 4), but 



1 The empty parentheses are to be filled by the student, with the derivation of the word as found 

 in the Index-Glossary. 



