I have seen these black snails gather on a dead fish from a distance of half a mile; in less than a day's time nothing was left of the fish but his bones and scales, and these were picked so clean that they had a polished look. These snails are provided with ribbon-like tongues, from which project a great number of minute and beautifully constructed teeth. By passing these tongues backward and forward rapidly they cut their food down much as a mowing-machine cuts grass. These snails are the scavengers of all dead fish and vegetable substances found in our bays and rivers, and to them we owe a great deal of the purity of our waters. The little hermit-crab lives alone in an empty shell, which he carries about with him wherever he goes. His reason for living in a shell is because the hind part of his body is soft, and not protected with a hard shell, like the fore part of his body. The end of the soft body of the hermit-crab is provided with hooks, or claspers, with which he holds on to the inner chamber of his shell so tightly that it is almost impossible to get him out except by breaking the shell. Very often these crabs are to be found with a colony of living polyps growing on their shells. These polyps are very interesting from the fact of their being the parents of one of our most beautiful jelly-fishes. [Illustration: HERMIT-CRAB OUT OF SHELL.] When a hermit-crab grows too large, or so fat that his shell pinches him, he hunts up a new one. First he pushes his long claws far into it, just to see that no one is inside, and that it is nice and clean; then he rolls it over and over, often lifting it so as to judge of its weight. If it suits, he drags it close to the entrance of his old home, and in an instant he has whisked into his new house. Hermit-crabs are great house-hunters, often moving just for the fun of it. They are always skylarking with one another like monkeys, and, in truth, they are the monkeys of an aquarium. When the water in an aquarium becomes bad, they are sure to indicate it by leaving their shells, and trying to crawl out of the tank. In all respects they are the most valuable and interesting inhabitants of the aquarium. [Illustration: PIPE-FISH.] Pipe-fish are apt to be delicate; still, if your aquarium is in perfect health, and the water is teeming with minute animal life, they will get along nicely. Their favorite food consists of the eggs of all small crustaceans, such as shrimps, sand-hoppers, and lady-crabs. Mrs. Pipe-fish does not take care of the children, but Mr. Pipe-fish places them in a long folding pocket that runs along the under side of his body (which I have tried to show in the engraving). When he lets them out of this pocket into the vast ocean world to shift for themselves, they are only a quarter of an inch long, no thicker than a bristle, and almost transparent. [Illustration: DANDY-CRAB.]