SHARKS, DOG-FISHES, SKATES AND RAYS 41 
feed on the bottom, and when intent upon their food 
they do not notice the flat, colour-protected ray. Sud- 
denly the ray raises itself up and flops on to the top 
of the herring, and before the victim can extricate him- 
self he has been seized in the horny-lipped mouth of 
his aggressor. 
Sharks and rays have many characteristics in com- 
mon, and they differ from bony fishes in having a 
gristly or cartilaginous skeleton. They also have five 
separate gill slits on each side of the head, instead of 
the single gill opening behind the protecting gill cover. 
Many sharks and rays are born alive, whereas the young 
of bony fishes, with very few exceptions, hatch from 
liberated eggs. 
Lastly, the embryos of sharks and rays possess 
external gills, similar in appearance to those of a tad- 
pole, but these disappear in the thornback about four 
months before hatching. 
As we shall be considering the early life histories 
of several fishes, let us look at what happens when an 
egg has been fertilised. On the yolk of every egg is 
a spot known as the germinal area, and from this 
germinal area the little fish develops. The rest of the 
yolk is used as food to nourish it, both while it is in the 
egg and also after it has hatched, until such time as it 
is able to find its own food. The yolk food enclosed in 
a yolk sac is nature’s feeding bottle, and is attached to 
the body of the little fish. We have seen how the egg 
case of the thornback ray is fixed to the weeds on the 
