256 TEXT-BOOK OF ZOOLOGY 



C Body Heat. 



In regard to the body heat, the naked integument, and the mucus 

 secreted by the latter, compare p. 252, Section 3, a and b. 



D. Food. 



The frog feeds principally on insects, spiders, and slugs, but also 

 consumes small fish and smaller-sized amphibians, including those of 

 its own species. In places it may do considerable damage to fish. 



1. Organs of Sense.— The frog recognises its prey only by its move- 

 ments ; immovable objects do not arouse its attention. With its large 

 protruding and retractile eyes, which are furnished with an upper lid 

 and nictitating membrane, it constantly surveys its surroundings. The 

 heanng appears to be very acute, since at the slightest suspicious noise 

 the animal leaps into the water, where few of its enemies can pursue it. 

 The tympanic membrane is visible in the form of a circular disc behind 

 the eyes. The nostrils are closed while the animal is in the water by 

 cutaneous flaps. (Why ?) 



2. Movements. — (a) On land the frog pursues its prey by leaping. 

 By a sudden stretching of the bent hind-legs, which are much longer 

 than the broad, disc-shaped body, it can propel itself to a considerable 

 distance. (Notice the remarkable length of the upper tarsal bones — 

 calcaneo-astragalus.) The fore-limbs, which are much shorter and 

 weaker, serve to support the body in falling. 



(b) In the water the frog progresses by swimming by the help of its 

 hind-legs, the five toes of which are long and united by webs. (Man 

 has copied the frog in the motion of his legs in swimming.) While 

 swimming, the fore-legs of the frog are brought up close against the 

 body, and, as they are not called into action, their toes are free and 

 much shorter than the hind-toes. (Compare with crocodile.) 



(c) As progressive movement is carried out almost entirely by the 

 hind-legs, these limbs are extremely muscular. The posterior portions 

 of the skeleton, which are strongly developed, afford these legs a firm 

 support, and also furnish the requisite surfaces of attachment for the 

 muscles. The terminal portion of the vertebral column has the form of 

 a long, rod-shaped bone (formed by the coalescence of the coccygeal 

 vertebrae), parallel to which are two, likewise rod-shaped, pelvic bones 

 (the ilia). The hind-legs of the frog are in many countries considered a 

 delicacy. Unfortunately, they are usually obtained in an abominably 

 cruel manner, being cut off from the live animal, which is thrown back 

 into the water, in the erroneous belief that the legs grow again. (In the 



