Although the Rotifera are microscopic objects, their organization is higher than that of the Annelida in some respects. They are minute animalcules, which appear in vegetable infusions and in sea-water, but by far the greater number are found in fresh-water pools long exposed to the air: occasionally they appear in enormous numbers in cisterns which have neither shelter nor cover; a few can live in moist earth, and sometimes individuals are seen in the large cells of the Sphagnum or Bog-Moss.
The bodies of the Rotifers have no cilia; they are perfectly transparent, elongated, or vermiform, but not segmented; they have two coats, both of which in some genera are so soft and flexible that the animal can assume a variety of forms; while in others the external coat is a gelatinous horny cylindrical shell or tunic enclosing the whole body except the two extremities, which the animal can protrude or draw in. The soft kind can crawl over solid surfaces by the alternate contraction and extension of their bodies like a worm, and the stiff Rotifers are capable of doing the same by the contractility of their head and tail. All can swim by means of cilia or lobes at their head. The greater number possess means of attaching themselves to objects by the posterior end of their bodies and of removing to another place.
The wheel-like organs from which the class has its name, are most characteristic in the common Rotifer (fig. 137), where they consist of two disk-like lobes projecting from the body whose margins are fringed with long cilia. It is the uninterrupted succession of strokes given by these cilia, passing consecutively like waves along the lobes, and apparently returning into themselves, which gives the impression of two wheels in rapid rotation round their axes.
The Brachionus pala (fig. 136) affords another instance of the two-wheeled Rotifers. Though of unusually large dimensions in its class, it is just visible to the naked eye as a brilliant particle of diamond when moving in a glass of water. Its transparent horny tunic, when viewed in front with a microscope, is a cup of elegant form, bulging at the sides. One side of the rim is furnished with four spines, of which the middle pair are slender and sharp as needles, with a deep cleft between them; the other side of the rim is undulated but not toothed, and the bottom of the cup ends in two broad blunt points.