XXIII. AMPHIBIANS 



The Frog;^ Body. — In the body of man we find two distinct regions, the 

 head and trunk. Are such regions to be found in the frog ? How is the 

 shape of the body fitted for Hfe in the water? 



Habitat. — There is considerable difference in the habitat of the green 

 frog and the leopard frog. The former may usually be found in ponds or 

 brooks in which considerable vegetation is found. The latter live in pools 

 or woody swamps in which the bottoms are dark from a background of 

 dead leaves or mud. Remembering this, how might the color of the frog 

 harmonize with its surroundings? Is this of advantage to the frog? In 

 what respects? 



Protective Resemblance. — Notice the position of the frog at rest in the 

 water. In its natural habitat a frog in this position would scarcely be 

 noticed, so perfect is the resemblance to the surroundings. Notice that 

 the only parts of the frog that show above the surface of the water are the 

 eyes and that part of the head bearing the nostril holes. 



Appendages. — Compare the anterior limb with j^our own arm. Identify 

 in each, upper arm, forearm, and hand. Note the number of fingers. In 

 the same manner find the thigh, shank, and foot in the posterior limb. In 

 what respects do ankle and foot differ in the frog and in man? What 

 adaptations for locomotion (swimming) do you find in the frog ? 



Skin. — Notice the slimy skin of the frog. This is due to the presence 

 in the skin of cells which secrete and pour out mucus. Might this slime 

 be of any use to the animal ? The skin of the frog is supplied with numer- 

 ous tiny blood vessels. The blood in these thin-walled tubes gains oxygen 

 from the surrounding atmosphere and from the water, while carbon dioxide 

 is given off. Thus the skin is used in the process of respiration. 



The Eye. — The eye of the frog differs somewhat in shape from our own. 

 Note the positions on the side of the head. Touch the eye. How is it 

 protected ? Look for a delicate fold, the nictitating membrane (sometimes 

 called the third eyelid), which may be drawn over the eye. 



Have you any experience as to the keenness of vision in the frog ? Do 

 they jump into the water because they see or hear you ? Any experiment 

 which will throw light on this point will make an interesting piece of original 

 work for extra credit. Can you perform any experiment which will show 

 whether the frog prefers light to darkness ? 



Ear. — The tympanic membrane or eardrum of the frog may be found on the 

 side of the head. It is a circular area of tightly stretched skin. In man the 

 ear drum is beneath the surface of the body near the inner end of the canal 

 or tube which we see in the external ear. In the frog and in man a connec- 

 tion exists between the mouth and the inner surface of the ear drum. This 

 tube is known as the Eustachian tube. 



Home Experiment. — Give an account of any experiment that you may 

 perform that will prove that frogs can hear, or the nature of sounds that 

 attract their attention. 



1 For laboratory exercise see Hunter and Valentine, Manual, page 170. (Either 

 the green frog or the leopard frog may beiised for the laboratory suggestions given 

 in this book.) 



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