The _Reticulosa_, or _Foraminifera_, form an order of this group. They are small calcareous shells, as a rule, nearly invisible to the naked eye, and enclosing, or once having enclosed, a living organism. The sand of the sea‐shore is often one‐half composed of them. M. d’Orbigny found in three grammes (forty‐six grains troy) of sand from the Antilles no less than 440,000 of these minute shells. Ehrenberg, the German microscopist, was once invited by the Prussian Government to assist in tracing the robbery of a special case of wine. It had been packed in sand only found in an ancient sea‐board of Germany, and from this fact and knowledge of locality the thief was detected. The _Foraminifera_, small as they are, have helped to form enormous deposits, obstruct navigation in gulfs and straits, and fill up ports, as may be seen at Alexandria. In various geological strata they are found; they exist in immense quantities in the chalk cliffs of this country. In the Paris chalk their remains are so abundant that a block of little more than a cubic yard has been computed to contain _three thousand millions_! “As,” says Figuier, “the chalk from these quarries has served to build Paris, as well as the towns and villages of the surrounding departments, it may be said that Paris, and other great centres of population which adjoin it, are built with the shells of these microscopic animals.” [Illustration: FORAMINIFERA IN A PIECE OF ROCK.] The _Infusoria_ almost baffle the attempts of naturalists to classify them, while their very existence would have escaped us but for the discovery of the microscope, “the sixth sense of man,” as Michelet happily termed it. In the tropics, water collected at a great depth was found to contain 116 species; in the Antarctic regions the very ice was found to contain nearly fifty different species. The very largest kinds can hardly be seen by the naked eye. They are generally nearly colourless, but some of them are nevertheless green, blue, red, brown, and even blackish. Some of those most commonly noted, on account of their superior size, are furnished with hairy _cilia_, which act as paddles, while certain of them appear to be employed in conveying food to the mouth. [Illustration: INHABITANTS OF THE BRITISH SEAS. 1. Pilot fish, 2. Piper, 3. Eagle Ray, 4. Oysters, 5. Spotted Ray, 6. Star fish, 7. Hermit Crab, 8. Common shore Crab, 9. Common Lobster, 10. Sea Anemones (various) 11. Corals, 12. Conger Eel, 13. Octopus, 14. Sea weeds.]