THE ANIMALS OF THE FRESHWATER AQUARIUM 05 



and divided into an upper carapace and a lower plastron, the 

 two being movably connected by ligament. From the arched 

 carapace the head, tail, and limbs protrude. Though aquatic 

 in its habits, the animal is a true air-breather, having a pair of 

 lungs, and taking in air at the surface through its nostrils. It 

 is, however, a cold-blooded creature, which only breathes slowly, 

 and is capable if necessary of remaining for a prolonged period 

 beneath the surface. Its structure, including the presence of 

 lungs, shows that the aquatic habitat is secondary, and that its 

 ancestors must have been terrestrial forms like the more familiar 

 land tortoises. This is one of the striking distinctions which 

 we shall find to exist between the denizens of the fresh and salt 

 water aquaria. It is generally true that the inhabitants of the 

 sea have been descended from aquatic ancestors. There are of 

 course exceptions, such as the porpoises and dolphins among 

 animals, and Zostera among plants, but still the statement is 

 generally true. On the other hand, when we examine a fresh- 

 water pond we find that quite a number of the plants and animals 

 give unmistakable signs of having been descended from land 

 ancestors ; the water-crowfoot, the water-spider, the water- 

 beetles, the pond-tortoise are all examples of this. The proofs 

 of this statement will appear in the course of our studies, but it 

 may be noticed in the particular case of the pond-tortoise that 

 lungs are not the best breathing organs for an aquatic animal, 

 that though the pond - tortoise has webbed feet yet in their 

 general structure the feet are the feet of a land animal, and that 

 the fact that the animal comes on shore to breed shows us that 

 its ancestors must have lived on land. In regard to the plants, 

 note how the majority of aquatic flowering plants flower above 

 the water. 



One other point is important as we have already -mentioned, 

 the pond-tortoise hibernates during the cold season. Now we 

 shall find that the power of sleeping either through the cold 

 season, or even through the hot season in freshwater animals of 

 hot countries, is very common among freshwater animals. Not 

 only does the mother tortoise sleep through the winter, but the 

 eggs, which are furnished with hard shells, though laid in the 

 late spring, may not hatch till the next spring. This means that 



