138 AMPHIBIA (the frog) 



(c) Do they open into the mouth? Probe with a bristle 



and see. 



(d) Does the frog breathe through the nostrils, or does 



it swallow air, or does it do both? 



(e) Are the nostrils organs of smell only, or does the frog 



both smell with and breathe through its nostrils? 



4. The Limbs. 



(1) The Anterior Limbs. 



(a) Number of digits? 



(b) Webbed or free? 



(c) Use? 



(2) The Posterior Limbs. Same questions as for anterior 



hmbs. 



(3) Compare the anterior with the posterior limbs. In 



what respects do they resemble each other? In 

 what do they differ? Why? 



(4) Name the three modes of locomotion of the frog. 



How is each performed? When? Why? 



5. The Skin. 



(1) Smooth or rough? 



(2) Why so loose on the body? 



(3) Remove the skin from the body. 



(a) Is it thick or thin? 



(b) Hold it up to the light. What do you observe con- 

 cerning its color or color spots? Its blood-vessels? 



6. The Muscles. 



(1) Determine the origin, insertion, and specific use of the 



muscles. 



(2) What do you observe concerning the muscles of the 



posterior limbs? What is the reason for this? 



B. Internal Morphology 



1. Digestive Organs. Pin the frog, on its back with the 

 limbs fully extended, in a wax-bottomed pan or on a soft pine 

 board. Slit open the body-cavity from the posterior end of 

 the abdomen to the breast bone, using great care not to injure 

 the internal organs. On each side of the posterior end of this 

 slit make another short one at right angles to it. Now pin back 



