have these organs placed on the outside of the body, sometimes forming prominent warts or papillœ, disposed in rows, or in tufts, sometimes resembling little branching trees, and at others, arranged as a number of elegant plumes, set, like the petals of an exquisite flower, around a circle.
In the Tunicata, examples of which may be found on our rocky beaches, closely adhering to the under surface of stones at low water, and looking like shapeless masses of a substance something between gristle and jelly,—the breathing organ is developed to a very great extent. It occupies a capacious chamber in the interior of the animal, the two sides of which are studded on their inner surfaces with little oval cells, arranged in a regular pattern of rows. Each of these cells is formed by an oval ring of cilia, which, when in full play, present a most beautiful and interesting spectacle. The accompanying figure, taken from the life, is a magnified representation of a tiny creature, not larger than a pin's head, but as transparent as the purest crystal. The oblong rings conspicuously seen are the ciliary cells of the breathing organ; but no figure can convey an adequate impression of the beauty of the sight, when the observer gazes upon forty or more of these ovals, all set round their interior with what look like the cogs on a watch-wheel, dark and distinct, running round and round with an even, moderately rapid, ceaseless motion.
One large tribe of the Gasteropoda comprises animals, however, which breathe air, and are terrestrial in their habits. Of these the Slugs and Snails of our fields and gardens afford familiar examples. The delicately-formed, and often bril-
