Two hundred years ago the Dutch naturalist Leuwenhoek, who made many discoveries with the highly magnifying lenses which he himself ground and mounted, wrote to the Royal Society of London that he had "discovered several animalcula that protrude two wheels out of the forepart of their body as they swim, or go on the sides of the glass jar in which they are living." He says that "the two wheels are thick set with teeth as the wheel of a watch," and he sent to the society for publication drawings of these wonderful little creatures. This was the first account of the Wheel Animalcules. Since then they have been studied by many microscopists, especially by Ehrenberg, who figured many in his great book on animalcules in 1838. Fourteen years later the delightful English naturalist, P. H. Gosse, who studied and illustrated the "sea-anemones" so ably–and, by his example and charming descriptions, made the keeping of these beautiful things in marine aquaria a favourite occupation among people of leisure, blessed with a "curiosity concerning the things of nature"–published some microscopical studies on Wheel Animalcules, and continued throughout his life to make them a special subject of his investigation. The microscope was greatly improved–in fact, reached its present state of perfection–during Mr. Gosse's lifetime, and a wonderful amount was added to our knowledge not only as to the various kinds of wheel animalcules (which now number not less than 900 species), but also with regard to the minutest details of their structure, their growth from the egg, and their habits. Another English lover of these minute creatures, Dr. C. T. Hudson, of Clifton (Bristol), began his observations a few years later, and also discovered many wonderful kinds. It was my good fortune to bring these two devotees of the Rotifera, or Wheel Animalcules, together, and to induce them to write a conjoint work on their favourites–after, as they say in their preface, they had each continued their studies almost daily for thirty years, and had made innumerable drawings from living specimens, which are reproduced in the many hundred (mostly coloured) figures engraved in the thirty-four quarto plates of their monumental book. This was published in 1889, a year after Mr. Gosse's death at the age of 78. My friend, Mr. Edmund Gosse, the distinguished man of letters, is the son of the naturalist; the microscope, the aquarium, and the rock-pools of the seashore were the familiar delights of his boyhood, as of mine.