These teeth may all be alike, not some “double” and some “single” as you have. But sometimes fish have teeth of different patterns. The most common form of teeth in fish that live on fish or other animal food, is that of a slim cone, bent a little inward to hold firmly the fish caught, so that it cannot slip away. The fish that feed on weeds have short, roundish teeth with a flat top, to make a good mill with which to grind or crush their food. Fish that eat insects do not need such large teeth. They have a great number of little teeth almost as fine as hairs. Rows of these teeth look like a little brush. Some fish are said to have “mill” teeth, as the carp, because their large flat teeth roll upon each other like the stones of a mill grinding grain. The ray fish, that feed on crabs, shell-fish, and flat fish, need very strong teeth for crushing such food. If you should look into their mouths, you would see that they are made like a mill, and the upper and under teeth roll against each other and crush fine all that is between them. By looking at the mouth of fish and examining its teeth, you can find out what kind of food it used, just as by looking at the beak and claws of a bird, you can tell what were its habits and food. Some fish have the inside of the stomach thick and furrowed, very like the gizzard of a fowl. This is to aid in cutting or grinding up the food. The teeth of a carp have much the appearance of the teeth of sheep and cows, only they are much smaller. They work against a plate of gristle in the roof of the mouth and reduce the food to pulp. The strong, sharp teeth of fish are used not only for eating their food, but for biting and fighting other fish. Two great salmon have been known to fight until one was killed. The dead one was found to be badly wounded and torn by the teeth of its enemy. Pike have very sharp, strong teeth, and a big pike has been known to seize hold of the foot or nose of a dog, fox, or other animal, that came to the water’s edge to drink. The mouth of a dog-fish is set back on the under side of the head. This mouth has a row of strong, sharp teeth all round it. When the fish seizes its prey or its enemy with its teeth, it whirls itself over, and this action enables it to tear out a large piece of the flesh held by its teeth. LESSON XLVI. BIG AND LITTLE BROTHERS. Yesterday, as I was going along the street, I saw a large globe full of tiny fish. They were from one to two inches long. I stopped to examine them. They were sticklebacks. You must know that little Mr. Stickleback is a very pretty and curious fellow, and well worth buying. [Illustration: THE LITTLE HOUSE BUILDER.] Looking into this globe made me think of some of the queer things I have heard and seen of this little fish. Shall I tell you about him?