THE animalcules described in the following pages were found in great numbers in the bottom of a small vessel or "aquarium," in which colonies of Plumatella, Melicerta, and Limnias had been kept. Of all the forms which can with certainty be referred to the animal kingdom, there are few which at first sight are so little likely to be recognized as animals as those about to be described. If the reader will imagine a bag made of some soft extensible material so thin as to be transparent like glass, so soft as to yield readily by extension when subjected to internal pressure, and so small as to be microscopic; this bag, filled with particles of sand, shells of Diatomaceæ, portions of Algæ or Desmidieæ, and with fragments of variously colored cotton, woolen, and linen fibres, will give a picture of the animal; to complete which it is only necessary to add a few loose strings to the bag, (figs. 1 and 2,) to represent the variable radiant processes which it possesses around the mouth.