AN INTRODUCTION TO ANIMAL BIOLOGY 29 



Examine the frog's tongue, and note that it is attached at the 

 forward end to the tip of the lower jaw, while the free end extends 

 backward towards the throat. How does this compare with the 

 human tongue? In seizing an insect, the frog's tongue is turned 

 quickly forward out of the mouth, and then quickly withdrawn. 

 Figure 3 will make clear the positions of the tongue at various 

 steps of this peculiar movement. 



36. Internal Structure of the Frog. (L when not marked D). Look 

 at the mounted skeleton of a frog, compare with Fig. 4, and note 

 the position of the following bones : backbone (vertebral column) ; 

 skull ; shoulder-girdle, which is a set of bones attaching the fore 

 legs to the body ; pelvis, a set of bones which attach the hind legs 

 to the body. Notice that the "ribs" are very short. Now turn 

 to the frog you have been studying, and feel the position of the 

 above-named bones through the skin. 



Lay the frog on its back, head pointing away from you. With 

 forceps lift the skin and with scissors carefully cut through it along 

 the median ventral line the whole length of the body. Carefully 

 separate the skin from the underlying parts, cutting the thread-like 

 connections, turn the flaps of skin outward to right and left, and 

 pin to the board or wax in the bottom of a dissecting-pan. Cover the 

 frog with water. 



Notice the muscles of the body-wall of the abdomen, and the 

 bones connecting the fore limbs (shoulder-girdle). 



Again using scissors and forceps, carefully cut through the body- 

 wall in the median ventral line from the pelvis to the shoulder-girdle, 

 and then cut across the body (transversely) just posterior to the 

 girdle. Separate and spread out the two flaps of the body-wall, 

 and pin down to the dissecting-board. The cavity containing the 

 internal organs thus opened is the body-cavity (ccelome). Now iden- 

 tify the organs exposed liver, stomach, intestine, egg-organs or 

 ovaries (if the specimen is a female frog), comparing your specimen 

 with Fig. 5 in order to identify the organs. 



Now cut out, with strong scissors, the ventral bones of the 

 shoulder-girdle (the teacher will demonstrate how this is best done). 

 As you lift up the bones, notice the heart lying beneath. The poste- 

 rior, conical, whitish part of the frog's heart is called ventricle (Fig. 

 5) ; it lies in a depression between two parts of the liver. Anterior 

 to the ventricle are the thin-walled auricles (right and left), usually 

 found filled with dark blood in a dead frog. The ventricle is the part 

 of the heart which forces the blood from the heart into the blood- 

 tubes (blood-vessels) ; while the auricles are reservoirs for holding 



