92 Laboratory Guide in Zoology 



THE FROG 



EXTERNAL ANATOMY 



I. — General. Feel of the frog's skin, noting its smooth, slimy 



nature. The moisture is a secretion from the glands 

 in the skin. Note the general temperature of the body. 

 It is cold even in life. Study the color markings of the 

 dorsal and ventral surfaces, and give reasons for such 

 coloring. The frog has the power to change its color, 

 bringing it into closer harmony with its surroundings. 

 The frog is placed in the group of animals called Am- 

 phibia. All Amphibians belong to the larger group 

 called Vertebrata. Fishes, reptiles, birds, and mam- 

 mals, besides Amphibians, are vertebrates. Fishes and 

 reptiles, like Amphibians, are cold blooded. Notice 

 that the frog has no outer skeleton. Note the absence 

 of an external division of the body axis. 



A segmented condition will be found to exist, how- 

 ever, when we study the skeleton. Examine the limbs 

 with special reference to the animal's habit of locomo- 

 tion. Is the animal well adapted to its environment 

 in this respect .' Remember that the number of Hmbs 

 possessed by the frog is the maximum for verte- 

 brates. 



II. — External Openings. Examine the nature and position 

 of the mouth and cloacal opening. Study the position 

 of the eyes with reference to their field of vision and 

 protection. In the anterior parts of the upper jaw find 

 two small openings into the nasal cavities. Push the for- 

 ceps or some blunt instruments through them into the 

 mouth. Behind each eye find a circular disk of tightly 

 stretched membrane, the external apparatus of the ear. 



