Sheep and the hog are more tormented with cysted worms than any other domestic animals used for food. If introduced into the human intestines by eating raw ham or sausages, the larvæ soon acquire the perfect form. The eggs of the Tænia may be introduced into the human or animal stomach; for dogs and other carnivora which eat raw unwholesome meat are infested by full grown tænia, which fix themselves to their entrails by their hooks and suckers, while at the same time egg-bearing segments separate successively from their posterior extremity, and being voided scatter the eggs far and wide on land and in water.
The young of some Entozoa undergo various transformations, as those of the Distoma of the Lymnæa. When full grown that entozoön is like a sole, flat, broad, and long, with a kind of head at the broad end, and two suckers on its under-surface, in one of which there is a pore serving as a mouth, whence an alimentary canal extends, which spreads in branches almost throughout the whole body. This animal has a filamentary nerve round its gullet, from which minute fibres pass to the mouth, and two filaments extend backward on each side as far as the second sucker. The eggs which occupy the whole margin of the body are developed into worms, each of which seems to be merely a mass of structureless cells enclosed in a contractile case. By a second change each of these cells is transformed into a freely swimming ciliated zooid endowed with eyes. Having escaped from their contractile case, they remain for a time in that state, and then imbed themselves in the mucus on the foot of the fresh-water mollusk Lymnæa, or pond snail, where they are transformed into true Distomata, and ultimately enter into the body of the Lymnæa itself, where they lose their eyes and cilia, which are no longer of use in their dark and permanent abode. The Fluke found in the livers of sheep that have the rot is a Distoma.
The Nematoid order, or thread-worms, that live in the muscles of men and animals, are long, smooth, and cylindrical, with a structureless skin covering layers of longitudinal and circular fibres, by means of which they can stretch and contract themselves. They are generally pointed at both ends with a mouth at one extremity and an orifice at the other. The Filariæ are slender, sometimes of great length, as the Guinea worm, which varies in length from six inches to two, eight, or even twelve feet. In Persia they are believed to be introduced into the system by drinking water in which their eggs have been deposited. This worm may grow in the muscles of a man to the size of five or six feet without giving much annoyance, but when its head bores through the skin it produces a painful sore unless extracted. In Persia, where the worm is common, the natives seize it by the head, draw it carefully out, and wind it round a bit of wood, an operation which may require several days to accomplish. It has a numerous viviparous progeny, which come out through the mouth. There are certain very small slender species of Filaria which attack the eyes both of men and horses; some bury themselves close to the eye, and a very minute kind enters the ball itself.
The Ascaris lumbricoïdes, a common intestinal thread-worm of the hog, ox, and the human race, is sometimes of great length. The sexes are distinct, and their fertility enormous. The ovaries are two tubes sometimes several feet long, in each of which the eggs are arranged in whorls round a central stem, like the flowers of a plantago. By counting the number of microscopic eggs in a whorl, and the number of whorls, Dr. Eschricht ascertained that in a full grown female the average number of eggs amounted to sixty-four millions. In this species of worm the embryo is not developed from the egg while within the victim, so that most of the eggs perish.
Different species of Anguillulæ, which are minute eel-like worms slender as a hair, inhabit the alimentary canal of fresh-water snails, frogs, and fishes, but many species are not parasitic. These are often united in swarming masses that nestle in mud, wet moss, wet earth, and aquatic plants. One species causes the cockle in wheat, appearing like a living tuft of white wool in the blackened grains. They appear in sour paste and in other decomposing substances, and are so tenacious of life that, after being completely dried for months, and apparently dead, they revive on being moistened.
Turbellariæ.
The Turbellariæ are fresh- and salt-water animals, distinguished by having the whole surface of their bodies covered by cilia, under which in some species there are thread-cells containing six, eight, or a greater number of darts. Most of the members of this tribe have elongated flattened bodies, and move by a sort of crawling or gliding motion over the surface of aquatic plants and animals. Some of the smaller kinds are sufficiently transparent to allow their internal structure to be seen by transmitted light. The mouth, which is situated at a considerable distance from the rounded end of the body, opens into a sort of gullet leading into the stomach, which has no other orifice, but a great number of branching canals are prolonged from it, which carry its contents into every part of the body. A pair of oval nerve-centres are placed near the rounded end of the animal, whence nerves extend to various parts of the body; and near to these there are from two to forty rudimentary eyes according to the species, each of which has its crystalline lens, its pigment layer, nerve bulb, and its cornea. The power of the Planaria to reproduce portions which have been removed is but little inferior to that of the Hydra.
Annelids.
The Annelids are the most highly organized of all the worm tribe. They are exceedingly numerous and varied; some are inhabitants of fresh water, others are terrestrial, but by far the greater number and most highly endowed are marine. They generally have a long, soft, and smooth body, divided or marked by transverse rings into a succession of similar segments. In many the first and last segments are alike; in others the first segment can scarcely be called a head, though it exercises several functions, while in the highest two orders the head is the seat of several senses. On each side of the bodies of the Annelida there are one or two long rows of tufted bristles or feet, which may be regarded as the earliest form of symmetrical locomotive organs. Most of the Annelids have ocelli or eye-specks, and in many of them the head supports soft cylindrical tentacles, which are obviously organs of touch. These worms are divided into four orders, the Suctorial, Terrestrial, Tubercular, and Errantia, or Wandering Worms.[[32]]
The first order consists of Leeches of different kinds: their body is long, slightly segmented, with a suctorial disc at each end. Their skin is smooth, whitish, and translucent; beneath it are cells filled with brown or greenish matter, and three layers of muscular fibres follow; the first are transverse, the second cross one another diagonally so as to form a network, and the third are longitudinal. The mouth, which occupies the centre of the principal sucking disk, varies in form with the genera. In the common leech it has an enlarged lip, and opens into a short gullet leading into a capacious and singularly complicated stomach, divided by deep constrictions into eleven compartments, the last of which is connected with an intestinal canal, which ends in a vent in the middle of the terminal sucker.