The Tardigrades are slow creeping animalcules, which seem to form a link between the Worms and the Rotifers, though they are more nearly allied to the former in having a vermiform body divided transversely into five segments, the first of which is the head, and each of the others has a pair of little fleshy protuberances furnished with four curled hooks. They resemble the Rotifers in their jaws, in their general grade of organization, and in the extreme length of time they can remain dried up without loss of life. When in the dried state they can be heated to a temperature of 250° Fahr. without the destruction of life, although when in full activity they cannot endure a temperature of more than from 112° to 115° Fahr. When alive the transparency of their skin is such as to show a complicated muscular system, the fibre of which is smooth; and as no respiratory organs have yet been found, their respiration must be cutaneous. These animalcules have no nerve-centre in the head, but they have one in each segment of the body; and they are furnished with a suctorial mouth at the end of a retractile proboscis, on each side of which are two tooth-like styles, the rudiments of lateral jaws. The structure of these creatures is microscopic.
Rotifera.
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.
Between the terminal blunt points there is a round opening for the protrusion of the foot of the animal. The tunic is of glassy transparency, so that every organ and function of the animal can be traced with perfect distinctness.
Fig. 136. Brachionus pala, with three eggs attached to its foot.
The foot of the animal is long, rough and wrinkled, not unlike the flexible trunk of an elephant. It can be lengthened, shortened, drawn within, or pushed out of the tunic in an instant. It terminates in two short conical fingers or toes, which can be widely separated or brought into contact. By means of these, the Brachion has the power of mooring itself even to the smooth surface of glass so firmly, that it can stretch itself in all directions, shaking itself to and fro with sudden violence without letting go its hold. The Rotifers usually fix themselves before they set their wheels in motion in search of food.