Hundreds of young people are now spending their vacation on or near the sea-shore, and have a good opportunity to study the wonderful habits of animal and vegetable marine life. Therefore I have undertaken to throw out a few plain hints as to the management of a salt-water aquarium, in which these interesting forms of nature can be observed to greater advantage. We will start off with one of the small tin frame tanks sold in New York so cheap, or a candy jar, or a small-sized wash-tub--any vessel that will hold water, and is not of iron, tin, or copper, either of which will poison the water. [Illustration: ULVA.] After washing out the tank carefully, and filling it with clear sea-water, we will place in it twelve silver-shrimps (bait shrimps). At the end of two days they are dead, and you ask why did they die when they had so much water to live in. They died of suffocation, after they had breathed all the air contained in the water. We will take out the dead shrimps, and in the same water place a good handful of ulva (sea-lettuce, sea-salad), one of the most common of all marine plants, and place the aquarium in a strong and direct sunlight, by this means exciting the ulva to work, or, as it is termed, aerify the water. In less than an hour's time a froth will be seen forming on the surface of the water, and adhering to the sides of the aquarium. Now observe the ulva closely, and from its edges and surface very fine threads of silvery bubbles are pouring out and ascending to the surface. In an hour's time the water will be thoroughly charged with air. We will again place twelve more shrimps in the aquarium. This time they will live, and we will have established a _true_ aquarium--an aquarium based on the self-sustaining principles of nature, wherein _it will not be necessary to change the water_. [Illustration: SHRIMP.] Fish as well as human beings breathe air. Air is contained in all water. After the shrimps had breathed or used the air contained in the water several times over, it became unfit to sustain animal life any longer, and so they smothered: just the same as if a number of people were placed in a room, and all the doors and windows and ventilators were sealed up tight, so that no new air could enter. They, too, would suffocate in a short time and die. All plants living in water are constantly manufacturing new and pure air for their friends and companions the fishes, particularly when under the action of sunlight.