SUB-KINGDOM V.
Vermes (worms). Animals having head and tail composed of segments. The digestive organ is tubular, and the nervous system a double chain of ganglia[4] on the ventral[5] surface. There are six classes of vermes. The animals differ greatly in appearance.
Class I.—Flat worms are best known as the parasites that infest animals, such as the liver fluke of the sheep, and the tapeworm. The flat worms pass through a very peculiar metamorphosis, some varieties taking as many as seven different forms.
Class II.—Round or Thread Worms are represented by the pin worm and Trichina. The latter is the dangerous worm which finds its way into the human system from pork flesh, in which it is imbedded.
Class III.—Wheel Animalculæ, or Rotifera. A most interesting microscopic worm, abounding in fresh water and in the ocean. They will remain dried up for years, and then recover life. Their shapes are very peculiar.
Class IV.—Moss Animals, or Polyzoa, are the animals which form a coral-like shell. They are abundant on the seashore, and are called sea mosses.
Class V.—Lamp Shells (Brachiopoda). These worms are marine, and form a bivalve shell, the valves being on either side of the body. The body has long arms on one side of the mouth, which bear fringes; the motion of the fringes draws food into the mouth. They are also used in respiration. But few species of the Brachiopods are now living, though they were once very plenty.
Class VI.—Annelidæ. This last class includes the leeches, a flat worm, whose body is divided into segments; the earth or angleworm, a familiar worm of many segments, and the marine worms. Each segment of the latter bears clusters of bristles, used in swimming.