Familiar examples of roundworms are the so-called vinegar-eels (Anguillula) (fig. [32]) to be found in weak vinegar, and other species of this same genus which live in water or moist ground or in the tissues of plants, doing much injury. The hair-worms (Gordius) or horse-hair snakes, which are believed by some people, to be horse-hairs dropped into water and turned into these animals, are also familiar examples of roundworms. They are often found abundantly in little pools after a rain, and it is sometimes said that these worms come down with the rain. They have in reality come from the bodies of insects in which they pass their young or larval stages as parasites. The hair-worms all live as parasites during their larval stage, and as free independent animals in their adult stage. Some of them require two distinct hosts for the completion of their larval life, living for a while in the body of one, and later in the body of another. The first host is usually a kind of insect which is eaten by the second host. The eggs are deposited by the free adult female in slender strings twisted around the stems of water-plants. The young hair-worm on hatching sinks to the bottom of the pond, where it moves about hunting for a host in which to take up its abode.

Fig. 33.—Trichina spiralis, encysted
in muscle of a pig.
(From specimen.)

The terrible Trichina spiralis (fig. [33]), which produces the disease called trichinosis, is another roundworm of which much is heard. This is a very small worm which in its adult condition lives in the intestine of man as well as in the pig and other mammals. The young, which are borne alive, burrow through the walls of the intestine, and are either carried by the blood, or force their way, all over the body, lodging usually in muscles. Here they form for themselves little cells or cysts in which they lie. The forming of these thousands of tiny cysts injures the muscles and causes great pain, sometimes death, to the host. Such infested muscle or flesh is said to be "trichinosed," and the flesh of a trichinosed human subject has been estimated to contain 100,000,000 encysted worms. To complete the development of the encysted and sexless Trichinæ the infested flesh of the host must be eaten by another animal in which the worm can live, e.g. the flesh of man by a pig or rat, and that of a pig by man. In such a case the cysts are dissolved by the digestive juices, the worms escape, develop reproductive organs and produce young, which then migrate into the muscles and induce trichinosis as before. But however badly trichinosed a piece of pork may be, thorough cooking of it will kill the encysted Trichinæ, so that it may then be eaten with impunity. Some people, however, are accustomed to eat ham, which is simply smoked pork, without cooking it, and in such cases there is always great danger of trichinosis.

Wheel animalcules (Rotifera).—Technical Note.—Live specimens of Rotifers can be found in almost any stagnant water. Examine a drop of such water with the compound microscope, and find in it a few small, active, transparent creatures, larger than the Paramœcium and other Protozoa in the water and which have the appearance shown in fig. [34]. They may be known by the constant whirling, or rather vibrating, circlet or wheel of cilia at the larger or head end of the body. These wheel animalcules may be studied alive by the class. Although usually darting about, the animalcules occasionally cease to move, when, because of their transparency, almost the whole of their anatomy can be made out. Their feeding habits can also be readily observed, and the food itself watched as it moves through the body. Make drawings showing as much of the anatomy as can be worked out. Note especially the "mastax" or gizzard-like masticating apparatus in the alimentary canal.

Fig. 34.—A wheel animalcule,
Rotifer sp. (From living specimen,
Stanford University.)

The wheel animalcules (fig. [34]) or Rotifers look little like the other worms we have studied. But they are nevertheless more nearly related to the worms than to any other branch of animals. They are all small, about 1/3 mm. long, and have a compact body. They are aquatic and feed on smaller animals and plants or on bits of organic matter which they capture by means of the currents produced by the vibrating cilia of the "wheel." Small as they are they have a complex body-structure, with well-organized systems of organs. For a long time, however, they were classed by naturalists with the Protozoa on account of their size. They are found all over the world, mostly in fresh water; a few are marine. More than 700 species of them are known.