THE ANIMALS OF THE FRESHWATER AQUARIUM 99 



noticed that after the appearance of the limbs the anterior part 

 of the body is much thinner than before. This is because they 

 develop within the shelter of the gill chamber, and burst through 

 when full formed. After their appearance the tadpole becomes 

 sluggish, often resting on a stone, half in and half out of the 

 water. It no longer feeds, and diminishes in size, the diminution 

 especially affecting the tail, which shrinks rapidly. It is this 

 shrinking tail which is supplying the little creature with food, 

 just as it was the internal store of yolk which supplied it with 

 food during the first quiescent period. At the same time it 

 loses its tadpole-like appearance, and becomes more and more 

 frog-like, with a wide mouth, prominent instead of fish-like eyes, 

 the beginnings of the adult coloration, and in general the attri- 

 butes of the frog. As these changes approach completion and 

 the food supply in the tail becomes exhausted, the animal recovers 

 its activity, leaps on shore, and begins the free insect-eating life 

 of the frog. The whole process is so remarkable that it is only 

 its familiarity which prevents us from perceiving its extraordinary 

 nature. 



Let us consider next what all these changes mean. Con- 

 trasting the frog with the water - tortoise we may perceive 

 that while the latter is a land animal which has gone back to 

 the water, the former is a water animal which is beginning to 

 fit itself for life on land. You will notice that while the tortoise 

 leaves the water to breed, like the seal, the frog goes back to 

 the pond for this process ; that while the frog (tadpole) has gills 

 when it is young, the little tortoise develops within an egg-shell, 

 where it has a special membrane (called the allantois) enabling 

 it to breathe through the walls of its prison ; that while the 

 tortoise has claws at the end of its toes like land animals in 

 general, the frog has not. These are only a few of the characters 

 which enable us to conclude that while the immediate ancestors 

 of the pond-tortoise were certainly terrestrial, the immediate 

 ancestors of the frog were certainly aquatic. Without stopping to 

 consider here the question why land animals should want to adopt 

 the aquatic life, we may consider the other point as to the mean- 

 ing of the frog's voluntary abandonment of the element in which 

 it was hatched, and for which it is in many respects best fitted. 



