Mr. Darwin's description of a kind of Sea-pen, _Virgularia Patagonia_, throws some curious light on the habits of these creatures. "This zoophyte consists of a thin, straight, fleshy stem, with alternate rows of polypi on each side, and surrounding an elastic stony axis, varying in length from eight inches to two feet. The stem at one extremity is truncate, but at the other is terminated by a vermiform fleshy appendage. The stony axis, which gives strength to the stem, may be traced at the extremity into a mere vessel filled with granular matter. At low water, hundreds of these zoophytes might be seen projecting like stubble, with the truncate end upwards, a few inches above the surface of the muddy sand. When touched or pulled, they suddenly drew themselves in with force, so as nearly, or quite, to disappear. By this action, the highly elastic axis must be bent at the lower extremity, where it is naturally slightly curved; and I imagine it is by this elasticity alone that the zoophyte is enabled to rise again through the mud. Each polyp, though closely united to its brethren, has a distinct mouth, body, and tentacula. Of these polyps, in a large specimen there must be many thousands, yet we see that they act by one movement. They have also one central axis connected with a system of obscure circulation, and the ova are produced in an organ distinct from the separate individuals. For," adds Mr. Darwin, in a note, "the cavities leading from the fleshy compartments of the extremity were filled with a yellow pulpy matter which, under a microscope, consisted of rounded semi-transparent grains aggregated together into particles of various sizes. All such particles, as well as separate grains, possessed the power of rapid motion, generally revolving round different axes, but sometimes progressive." The description of the Island of Cocos or Keeling is as follows:--"The ring-formed reef of the lagoon island is surmounted, in the greater part of its length, by linear islets. On the northern, or leeward side, there is an opening through which vessels can pass to the anchorage within. On entering, the scene was very curious, and rather pretty; its beauty, however, entirely depended on the brilliancy of the surrounding colours. The shallow, clear, and still water of the lagoon, resting in its greater part on white sand, is, when illumined by a vertical sun, of the most vivid green. This brilliant expanse, several miles in width, is on all sides divided, either by a line of snow-white breakers from the dark heaving waters of the ocean, or from the blue vault of heaven by the strips of land crowned by the level tops of the cocoa-nut tree. As a white cloud here and there affords a pleasing contrast to the azure sky, so in the lagoon bands of living coral darken the emerald-green water.