Describe the _mouth_. Observe the extent of the mouth opening (Fig. 251). Are _teeth_ present in the upper jaw? The lower jaw? Are the teeth sharp, or dull? Does the frog chew its food? Is the _tongue_ slender or thick? (Fig. 251.) Is it attached to the front or the back of the mouth? In what direction does the free end extend when the tongue lies flat? Is the end pointed or lobed? How far out will the tongue stretch? For what is it used? Why is it better for the teeth to be in the upper jaw rather than in the lower jaw? That the teeth are of little service is shown by the fact that the toad with similar habits of eating has no teeth. Will a toad catch and swallow a bullet or pebble rolled before it? The toad is accustomed to living food, hence prefers a moving insect to a still one. [Illustration: FIG. 251.--HEAD OF FROG.] =The Senses.=--Compare the _eyes_ with the eyes of a fish in respect to position and parts. Are the eyes protruding or deep-set? Touch the eye of a live frog. Can it be retracted? What is the shape of the pupil? The color of the iris? Is the eye bright or dull? What probably gave rise to the superstition that a toad had a jewel in its head? Is there a third eyelid? Are the upper and lower eyelids of the same thickness? With which lid does it wink? Close its eye? Observe the large oval _ear_ drum or tympanum. What is its direction from the eye? (Fig. 251.) The mouth? Is there a projecting ear? Does the frog hear well? What reason for your answer? As in the human ear, a tube (the Eustachian tube) leads from the mouth to the inner side of the tympanum. How many _nostrils_? (Fig. 251.) Are they near together or separated? Large or small? A bristle passed into the nostril comes into the mouth not far back in the roof. Why must it differ from a fish in this? How do the _fore and hind legs_ differ? How many toes on the fore foot or hand? On the hind foot? On which foot is one of the toes rudimentary? Why is the fore limb of no assistance in propelling the body in jumping? Do the toes turn in or out? (Fig. 250.) How does the frog give direction to the jump? What would be the disadvantage of always jumping straight forward when fleeing? Which legs are more useful in alighting? [Illustration: FIG. 252.--SKELETON OF FROG.]