SWIMMING ANIMALS 77 



true bone is to be found in their skeletons, but that they 

 consist of that curious transparent substance known as 

 cartilage, which, for all its delicate appearance, is in reality- 

 very tough. In the bony fishes on the other hand, the 

 skeletons are all made of true bone, which is hard and 

 brittle. This is only a further stage in evolution, bone 

 being actually cartilage within which strengthening deposits 

 of lime have been added. The skeletons of very young 

 animals consist of cartilage, and it is only as they grow 

 that the lime is deposited to build up bone. This being the 

 case it is not surprising to find that the cartilaginous fishes 

 are more primitive than the bony fishes, and appeared first 

 in the course of evolution. In the struggle for existence 

 during the history of the world, however, the fishes with 

 true bony skeletons have been by far the most adaptable 

 and are now far more numerous both in kinds and in 

 numbers than are the shark family. 



But this is not the place to enter into a scientific dis- 

 cussion on the evolution of fishes or to describe in detail all 

 the many different species of fishes that exist at the present 

 day. It is rather the intention of these pages to show how 

 some of the various fishes behave and live in nature and the 

 part they play in the world under the sea. 



Spawning Habits 



In describing the life of any fish, it is reasonable to begin 

 from the day of its birth. Most marine fishes, but not 

 quite all, lay eggs ; only a few are viviparous, that is, are 

 bom alive and do not hatch from an egg previously shed 

 by the mother. Amongst these few are the viviparous 

 blenny {Zoarces), one or two kinds of dog-fish and the saw- 

 fish {PrisHs). 



But let lis consider the majority, that is the egg-laying 

 fishes. Some fish lay their eggs on the sea bottom attached 



