3- This close-up of the head of a Tawny Owl — taken in sunlight — shows the third eyelid in 

 use over the eye nearest the sun while the other eye gives an idea of the large size of the 

 eyeball necessary for nocturnal vision. (Page 60) 



4. The tympanum, or external ear-drum, can be plainly seen in this study of a Marsh 

 Frog. (Page 77) 



