250 The Life Story of the Fish 



ods had to be found to provide the necessaries of life — 

 which, it may be observed, are much the same for humans 

 as for fish : oxygen, food, water, mates, and shelter from ene- 

 mies. Competition has driven fish into all kinds of waters 

 where these necessities could not be had without new adapta- 

 tions. And so it is in connection with the obtaining of one or 

 the other of these essentials that practically all of the adap- 

 tations which we find in fish show themselves. Whole books 

 have been written on themj we shall mention only a few. 



Betta, our old friend the Siamese fighting fish, is a good 

 example of adaptation, and of an adaptation of an adapta- 

 tion. He, as well as such aquarium favorites as the paradise 

 fish and the gourami, belong to a group which are called 

 labyrinth-fishes because they have modified some of the tis- 

 sues on each side of their heads into labyrinth-like breathing 

 chambers. They live in Asia and Africa, where the water in 

 their small pools gets foul in the dry season, and because of 

 the resulting lack of oxygen they have formed the habit of 

 coming to the surface to take in mouthfuls of air which they 

 store in these auxiliary chambers. This they use not only for 

 breathing, but also to make their bubble-nests described in 

 an earlier chapter, for it is from this store of air that the 

 indispensable bubbles are formed. An adaptation originally 

 connected with the need for oxygen has been readapted to 

 suit the need for shelter of the young. And if you want to 

 carry the matter one step further you might consider the 

 way in which the upward-slanting mouth of this fish is so 

 placed as to make it the perfect instrument for releasing 

 bubbles and for inserting eggs into a nest overhead. 



Betta is a case of one adaptive structure serving two pur- 

 poses within the same species. In the ray and the flounder, 

 on the other hand, we have one adaptive form occurring in 

 two dissimilar species for two different purposes and in two 

 fundamentally different ways. Both have the body so flat- 



