All the worms deposit eggs, and nearly all are remarkable for the wonderful changes through which they pass before they attain maturity. This is well illustrated in a planarian worm (Fig. 60), which seems to require the presence of another animal to enable it to complete its development. The little creature which breaks from the egg (A) is a free-swimming creature surrounded by cilia or hairlike swimming organs. By these it moves through the water, and with strange instinct searches for some animal, generally a snail, which it enters. There it becomes surrounded by a sack and produces a little creature called the nurse (_b_), which soon grows to resemble the tadpolelike creature (C), which is filled with small egglike or germlike objects (_a_). It now changes into a wormlike creature (D), in which the germs have assumed the shape of worms (_a_), and soon breaks forth as a little form with a tadpolelike tail (E)--a remarkable performance. But the end is not yet; another animal is necessary to complete the change. Swimming about, the little creature is swallowed by some animal in drinking, and finds its way to the liver, where it lives, the tail being lost. The animal now changes into a perfect flukeworm (F), which finally leaves the animal or host and lays eggs in the water; these pass through the same wonderful transformation. The flukeworms (Fig. 61) are disagreeable flat creatures, not often seen, the marine forms attaining large size. [Illustration: FIG. 61.--A flukeworm.] [Illustration: FIG. 62.--Thorn-headed worm.] [Illustration: FIG. 63.--Supposed horsehair worm (_Gordius_): _A_, adult; _B_, young (larva).] Many of the worms are parasites living upon other animals. The thorn-headed worm (Fig. 62) is an example. Who has not heard the story of the living horsehair? Almost every country newspaper has told the story, that some farmer after washing his horses had found several hairs taken from the horse's tail which "were alive," and to prove the story the farmer produces the "living horsehair" which is a remarkable imitation of the long hair of a horse's tail. But the hair is a well-known worm (Fig. 63) called Gordius aquaticus. It is almost exactly like a horse's hair, two or three feet in length, and found coiled up in ponds or snugly tucked away in the interior of a beetle or grasshopper which it has seized upon as a host. The deadly Trichina spiralis belongs to this group (Fig. 64). If the vinegar bottle is examined, in what is popularly called the "mother" at the bottom, still another member of the family will be found. This is a minute round worm almost invisible to the naked eye. It is very active and disagreeable to contemplate, living in the sour, fiery liquid. [Illustration: FIG. 64.--_Trichina spiralis_: a deadly worm from pork.] [Illustration: FIG. 65.--The rotifer.]