EXTERNAL ANATOMY OF THE VERTEBRATES 311 



(rf) the opening of the cloaca ; (e) the character of the skin (the micro- 

 scopic structure of the skin is studied in a later exercise on page 319) ; 

 and (/) the difference in color between the dorsal and ventral surfaces. 



2. Examine the head and note the following structures : (a) the large 

 mouth ; (6) the pair of small, anterior openings, or nostrils (external 

 nares) ; (c) the pair of circular ear drums (tympanic memrranes) ; 

 (d) the protruding eyes ; and (e) the tiny nodule (brow spot) in the skin 

 between the eyes. Note how the lower eyelids cover the eyes and how 

 each eye may be drawn into a cavity (orbit) in the skull and thus pro- 

 tected from injury. 



3. Examine the fore limb and identify the upper arm, forearm, and 

 hand ; the latter with wrist and digits. Examine the hind limr and 

 identify the thigh, shank, and foot ; the latter with ankle, digits, and 

 interdigital web. Note the difference in length between the two pairs 

 of limbs. 



4. Make a drawing of the Frog, showing as many of the above charac- 

 ters as possible. 



