Describe the =position= of the frog when still (Fig. 250). Of what advantage in this position? Does the frog use its fore legs in swimming or jumping? Its hind legs? How is the frog fitted for jumping? Compare it in this respect with a jumping insect; a jumping mammal. How is it fitted for swimming? Is the general build of its body better fitted for swimming or for jumping? How far can a frog jump? =External Features.=—The frog may be said to have two _regions in its body_, the head and the trunk. A neck hardly exists, as there is only one vertebra in front of the shoulders (Fig. 252), while mammals have seven neck (cervical) vertebræ. There are no tail (caudal) vertebræ, even in the tadpole state of frogs and toads. The _head_ appears triangular in shape when viewed from what direction? The head of a frog is more pointed than the head of a toad. Is the skull a closed case of broad bones or an open structure of narrow bones? (Fig. 252.) [Illustration: FIG. 251.—HEAD OF FROG. ] 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 a pebble rolled before it? The toad is accustomed to living food, hence it prefers a moving insect to a still one. =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 colour 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? [Illustration: FIG. 252.—SKELETON OF FROG. ]