422 VERTEBRATE LIFE AND ORGANIZATION 



the gill slits. Water containing oxygen enters the mouth and pharynx, 

 and crosses the internal gills on its way out of the gill slits into the 

 opercular chamber. It leaves this chamber through a small opening on 

 the left side, the spiracle. Late tadpoles also have lungs, and may be 

 seen surfacing to gulp air. Hind limbs appear at the base of the tail, 

 and forelimbs develop within the opercular chamber. 



After two and one half to three months, leopard frogs undergo a 

 metamorphosis, a period of rapid differentiation during which larval 

 features are lost and those of the adult are acquired. The front legs 

 burst through the operculum, the left one first, gills and gill slits are 

 lost and the tail is resorbed. The mouth widens, the horny teeth are 

 lost, a tongue develops and the digestive tract shortens. A tympanic 

 membrane and eyelids appear, and even the shape of the lens changes 

 to provide for good vision in air, which has a different refractive index. 

 Finally gonads develop and differentiate into testes or ovaries. 



Questions 



1. How can the sexes of frogs be distinguished externally? 



2. What makes a frog's skin appear green? What is the advantage to the frog of its green- 

 ish color and dark spots? 



3. What parts of the frog's skeleton are classified as visceral skeleton, axial skeleton, 

 appendicular skeleton? 



4. Describe the parts of a vertebra. 



5. In what ways is the frog's skeleton well adapted for jumping? 



6. Distinguish between the following: the origin and insertion of a muscle, flexion and 

 extension, protraction and retraction, adduction and abduction. 



7. Make a diagram of a cross section of a frog showing the relationship of the internal 

 organs to the coelom, peritoneum and mesenteries. 



8. How do frogs catch their food? 



9. List in correct sequence the parts of the digestive tract of a frog. 



10. Describe the route that air takes in going to the lungs of a frog. How is the air 

 moved in and out of the lungs? 



11. How do frogs produce sound? What is the purpose of their croaking? 



12. List in correct sequence the vessels through which a drop of blood would pass in 



traveling from the intestine of a frog forward through the heart and back to the 

 intestine. 



13. Where does the blood of a frog become aerated? Does oxygen-rich blood mix with 

 oxygen-poor blood in the heart? 



14. Trace the route of sperm from the testis of the frog to the outside. Do eggs have a 

 comparable route? 



15. List the five regions of the frog's brain and the major structures that are present in 

 each. 



16. Are the cerebral hemispheres important integration centers in the frog? What hap- 

 pens if they are destroyed? 



17. Briefly describe the reproduction of the frog. 



18. What is the value of the jelly layers that surround frogs' eggs? Where are these layers 

 added to the egg? 



19. Briefly describe the main features of frog development. 



