SHARKS, DOG-FISHES, SKATES AND RAYS 43 



These rings assist in concealing the sharp-cut free edge of 

 the fin among the round stones on the bottom. The ray- 

 under consideration is the same fish as that shown on 

 the plate facing p. 44. Here several rings are again 

 clearly seen on the edge of the fins, but if the ground 

 is examined other rings of a similar appearance will be 

 detected. These are due to the stones on the bottom, 

 for when looked down upon from above the stone forms 

 the centre of the ring, and the shadow itself the dark 

 ring round it. Similar rings can easily be seen if round 

 stones are dropped into a deep tub so that the light 

 falls on them only from above. 



Sharks and rays are classed together, and it seems 

 strange that fish so very different in their shape and 

 habits should be so nearly related ; but this dissimilarity 

 in external appearances in nearly-related species is con- 

 stantly met with. Sharks and rays are among the most 

 ancient of fishes, and probably had a common ancestor. 

 Some of these took to active life, chasing and devouring 

 swimming fishes, and hence they became elongated in 

 shape, and developed powerful tails. Others became 

 bottom feeders, and by a gradual process of evolution, 

 extending over countless ages, they became flat, and 

 as their habits of feeding do not necessitate rapid move- 

 ment, the tail as a swimming organ became functionless. 



The manner in which rays are flattened is of interest, 

 since it differs from the method of flattening of the 

 so-called " flat fishes," such as the plaice and the turbot. 



The ray is flattened from above downwards, and 



