Respiration under water depends, not on the water itself, but on the small percentage of free oxygen which it contains, and this is utilised for the aëration of the blood of animals, by that wonderful and often extremely beautiful apparatus of delicate fibres or laminæ penetrated with blood-vessels, which we call a gill. Except those lowest creatures which aërate their blood merely at the general surface of the body, all animals capable of respiration in water are provided with gills in some form, though in many of the humbler types, like that of the familiar Oyster, the gills are used for the double purpose of aërating the blood and, by their minute vibrating threads or cilia, drifting food to the mouth. In the great group of radiated animals, the _Protozoa_, _Cœlenterata_, and _Echinodermata_, no air-breathing creature exists, or, in so far as is known, has existed, so that this vast group of animals is limited altogether to the waters; and this is undoubtedly one mark of its inferiority. In the sub-kingdom of the Mollusks the highest class, that of the Cuttle-fishes and Nautili, has been, singularly enough, rejected as unfit for this promotion, though it was early introduced, and attains to a high development of muscular energy and nervous power. The group next in order, that of the Snails and their allies, alone ventures in some of its families to assume the _rôle_ of air-breathing. As might be expected, in creatures of this stamp the simplest means are employed to effect the result. In the sub-aquatic species the gills are contained in a chamber, where they are protected and kept supplied with water. In the air-breathing species, this gill-chamber is merely emptied of its contents and converted into an air-sac or functional lung. Thus a rude and imperfect method of air-breathing is contrived, which scarcely separates the animals that possess it from their aquatic relatives, but which nevertheless gives to us the beautiful and varied groups of the Land-snails and of the air-breathing fresh-water Snails. In the worms and Crustaceans the gills are placed at the sides of the body, and connected with its several segments. But the Crustaceans, like the Cuttle-fishes, though the highest aquatic type, never become air-breathers. It is true some of them, like the Land-crabs, live in the air; but they retain their gills, and have to carry with them a supply of water to keep these moist. But in order to elevate the Annulose type to the true dignity of air breathing, three new classes had to be introduced, differing altogether in their details of structure; and all three seem to have been placed on the earth about the same time. They are: First, the Myriapods, or Gallyworms and Centipedes; secondly, the Insects; and thirdly, the Arachnidans, or Spiders and Scorpions.