There is a colourless little fresh-water species of the genus Naïs, remarkable for the beauty of its bristled feet. There are two pairs on each ring of the worm, consisting of wart-like perforated protuberances, through which a number of microscopic bristles protrude, arranged in a radiating pencil like a fan. They are very slender, bent at the tip, and so transparent that they look like threads of spun glass; the worm thrusts them out and draws them in with extreme rapidity. A blood-red Naïs lives in burrows in the mud at the bottom of springs and pools in immense multitudes; large tracts of the mud of the Thames are red with a species of them; half of their bodies stretched out of their burrows maintain a constant oscillating motion on its surface, but, like the earth-worm, they instantly shrink into their burrows on the least alarm. They have no respiratory organs; but their blood is aërated through their skin, which is so transparent that, with a microscope, the whole of the internal structure, the motions of the liquids, and the particles they contain are distinctly visible. The blood acts the part of internal gills, by aërating the colourless liquid contained in a set of vascular coils surrounding the organs of digestion. The Tubicola are marine worms, forming the third order of Annelida, according to the system of M. Milne-Edwards. They live in tubes, either of a shelly calcareous substance, which forms naturally on the tenacious mucus of their skins, or in tubes artificially constructed by themselves of sand and particles of shell glued together. All the Tubicola can protrude their gills and the anterior part of their bodies, and some can leave their tube and return to it again. These worms, which form beautiful objects for the microscope, have ringed bodies with tubular bristled feet, and respiratory organs or gills fixed either on the head or near it. They have an alimentary canal loosely attached to the ventral wall of the body, and two systems of circulating liquids, one red, the other colourless. In the Tubular Annelids the principal organs of respiration are the contractile plumes on the head.