THE FROG, OR THE TOAD. 109 



are the same an upper arm, a forearm, and a hand in the 

 former, and a thigh, a shank, and a foot in the latter case. 

 Locate knee, heel, elbow, and wrist. 



c) Count the digits fingers and toes. Do any possess nails? 



What is the use of the web between the toes ? Look for a hard 

 pad on the palm of the toad's hand, and for a spur just inside the 

 inner toe. 



d) The skin is so loose that it can be gathered up in folds here and 



there. It is kept moist by a secretion of mucus, and plays an 

 important part in respiration. This explains how a frog is able to 

 remain under water so long and to hibernate in the mud for months. 

 The fact that the toad's skin is rough and warty has given rise 

 to the superstition that its excretions will, by contact, produce 

 warts on the hands of human beings. 



e) Study the color pattern of the skin with reference to its utility. 



Are there any evidences of protective coloration ? 



f) The gape of the mouth is unusually wide, extending considerably 



behind the eyes. The eyes are large and quite prominent, 

 though capable of some retraction as can be seen by looking at the 

 roof of the mouth while pushing on the eyeball with the finger. 

 Observe a heavy upper eyelid and a thin fold of skin, the nicti- 

 tating membrane, arising from the lower margin of the eye. 

 Move both about with the forceps. 



g) A short distance behind and below the eye is a circular area of 



tightly stretched skin, the tympanum or ear-drum. 

 h) The nostrils are located on the dorsal surface of the snout near the 



tip. Open the mouth and locate the inner nostrils by thrusting 



a blunt probe through the external openings, 

 i) Do you find any teeth in the mouth ? If so, what is their nature 



and where are they located ? 

 j) Push a probe through the ear-drum and look for its appearance in 



the mouth cavity. This internal opening is the entrance to a 



short Eustachian tube designed to equalize the pressure of the air 



on the tympanum, 

 k) Examine the tongue. What is its shape ? How is it attached to the 



