44 



GENERAL BIOLOGY 



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, 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 ?, eggs after they taining young D, young tadpoles attaching 

 they are laid are laid tadpole themselves to a plant 



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



ternal gills internal gills G, young tadpole with hind legs 



H, tadpole with webbed 

 feet 



7, tadpole with legs and arma 



J, young frog 



Fig. 



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 

 readily be withdrawn into the orbits.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

 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 of the bones of the skull 

 as in ourselves. The dark oval opening of the eye, the pupil, is sur- 

 rounded 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 

 transparent, 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 mem- 

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

 appears to form a continuation. In birds, for example, this membrane is 

 also very thin and can be thrown over the eye from the inner angle of the 

 orbit. Behind the eye is a more or less circular area called the tympanic 



