EXTERNAL ASPECT OP THE RAT 5 



squirrel. Note the scales on the tail. How are they 

 arranged? Are scales characteristic of mammals? In 

 what groups of vertebrates are scales usually found? 



The anus, or opening through which the residual mate- 

 rials (feces) from the digestive tube pass, is immediately 

 ventral to the base of the tail. The conspicuous swelling 

 ventral to the anus of the male rat is the scrotum, which 

 contains the two testes. Sometimes the testes lie up in the 

 body cavity instead of in the scrotum, in which case the 

 size of the latter is very much reduced. The external 

 opening of the male's urogenital system is located mid- 

 ventrally near the anterior end of the scrotum. Carefully 

 insert one blade of a pair of scissors into this orifice and 

 slit the skin, lengthwise of the animal, for three or four 

 millimeters. Note that the foreskin (prepuce), which is 

 hairy externally, surrounds the distal end (glans) of the 

 penis. Examine now the corresponding region of the 

 female. There are three openings in the female : the anus, 

 ventral to the base of the tail; ventral to the anus is 

 the vaginal orifice, or exit from the female reproductive 

 organs; below this orifice the opening from the excretory 

 system at the distal end of a prominent elevation. How do 

 the relations between these openings in the female rat 

 compare with the corresponding openings in other female 

 mammals ? 



Carefully observe the hair color of your animal. If you 

 are working with a wild rat scrutinize the distribution of 

 the pigment within individual hairs. Is each hair of the 

 same color throughout ? Compare the hair color of the rat 

 with that of other wild animals, if they are available either 

 alive or mounted. Compare the color of the albino's eyes 

 with the eye color of the wild rat. 



If time permits, sketch the lateral view of the dead 

 animal. 



