coiled "castings" at the entrance of the burrows. In thus mixing the soil and rendering it porous they are of great service to the agriculturist. Although earthworms are omnivorous they also manifest a preference for certain kinds of food, notably cabbage, celery, and meat, which leads us to think that they have a sense of taste. All these substances are carried into their retreats and devoured, or are used to block the entrance during the day. The food thus carried into the body is digested by a system (Fig. 40) composed of several portions, FIG. 40.-Earthworm (Lumbricus) dissected from left side. b, brain; c, crop; d, outer opening of male reproductive system; dv, dorsal blood-vessel; g, gizzard; h, pulsating vessels or "hearts"; i, intestine; k, kidney; m, mouth; n. c., nervecord; oe, esophagus; o, ovary; od, oviduct; ph, pharynx; r, testes; s.r., seminal receptacles; v.v., ventral vessel. each of which is modified for a particular part in the process. The mouth (m) leads into a muscular pharynx (ph) whose action enables the worm to retain its hold on various objects until swallowed, and this in turn is continuous with the esophagus. From here the food is passed into the thinwalled crop (c), and from this storehouse is gradually borne into the gizzard (g), whose muscular walls reduce it to a fine pulp now readily acted upon by the digestive fluids. These, resembling in their action the pancreatic juice of higher animals, are poured out from the walls of the intestine into which the food now makes its way; and as it courses down this relatively simple tube the nutritive substances are absorbed while the indigestible matters are cast away. 67. Circulatory system. In all the groups of animals up to this point the digested food is carried through the body by a simple process of absorption, or in the threadworms by means of the fluid in the body cavity; but in the earthworm Some of the duties of this vascular system are also shared by the fluid of the body cavity, which is made to circulate through openings in the partitions by the contractions of the body wall of the animal in the act of crawling. In this rough fashion a considerable amount of nutritive material and oxygen are distributed to various organs, and wastes are carried to the kidneys to be removed. 68. Excretion. In nearly all of the segmented worms there is a pair of kidneys to every segment (Figs. 40, 41). W FIG. 41.-Diagram of earth- Each consists of a coiled tube wrapped in a mass of small blood-vessels, and at its inner end communicating with the body cavity by means of a funnel-shaped opening. In some unknown way the walls of the kidney extract the waste materials from the blood-vessels coursing over it and pass them into its tubular cavity. At the same time the cilia about the mouth of the funnel-shaped extremity are driving a current from the body-cavity fluids, which wash the wastes to the exterior. 69. Nervous system. The nervous system of the earthworm consists first of a brain composed of two pear-shaped masses united together above the pharynx (one shown in Fig. 34), from which nerves pass out to the upper lip and the head, which are thus rendered highly sensitive. Two other nerves also pass out from the brain, and, coursing down on each side of the pharynx like a collar, unite below it and extend side by side along the under surface of the digestive system throughout its entire extent. In each segment the two halves of this ventral nerve-cord are united by a nerve, and others are distributed to various organs, which are thus made to act and in proper amount for the good of the body as a whole. In its relation to the outside world the chief source of information comes to the earthworm through the sense of touch, for definite organs of sight, taste, and smell are but feebly developed, while ears appear to be entirely absent. Nevertheless these are sufficient to enable it to lead a successful life, as is evidenced by the great number of such worms found on every hand. 70. Egg-laying.-In digging up the soil where earthworms abound one frequently finds small yellowish or brownish bodies looking something like a grain of wheat. These are the cocoons in which the earthworms lay their eggs, and the method by which this is performed is unique. We have already noted the presence of a swollen girdle (the clitellum) about the body of the worm. At the breeding season this throws out a fluid which soon hardens into an encircling band. By vigorous contractions of the body this horn-like collar is now slipped forward, and as it passes the openings of the reproductive organs the eggs and sperms are pushed within it. They thus occupy the space between the worm and the collar, and when the latter is shoved off over the head its ends close as though drawn to gether by elastic bands. A sac, the cocoon, is thus produced, containing the eggs and a milky, nutritive substance. In a few weeks the worm develops and, bursting the wall of its prison, makes its escape. 71. Distribution. The earthworms and their allies are found widely distributed throughout the world, and all exhibit many of the characters just described. The greatest differences arise in their mode of life: some are truly earthworms, but others are fitted for a purely aquatic existence in fresh water or along the seacoast; a few have taken up abodes in various animals and plants, and in some of these situations they extend far up the sides of the higher mountains. In all, the head is relatively indistinct, the number of A B bristles on each segment FIG. 42.-A marine worm (Nereis). A, apfew, and for this and other reasons all are included in pearance at breeding season, and B, at other times. the subclass Oligochata, or "few-bristle worms. 72. Nereis and its allies.-In many of the above-mentioned situations members of a more extensive group of worms are found, with highly developed heads and many bristles arranged along the sides of the body. These are the Polychates or "many-bristle" worms, and as a representative we may take Nereis (Fig. 42), a very common form along almost any seashore. The body presents the same segmented appearance as the earthworm, but the head (Fig. 43, A) is provided with numerous sense organs, chief among which are four eyes and several tentacles or "feelers." The segments behind the head FIG. 43.-A, head and one of the lateral appendages (B) of a marine worm (Nereis brandtii); al, intestine; f, "gill "; k, kidney; n, nerve cord; s, bristles for loco motion. differ very little from one another, and, unlike those of the earthworm, each bears a pair of lateral plates (Figs. 41, 42, B) or paddles with many lobes, some of which bear numerous bristles. By a to-and-fro movement these organs aid in pushing the animal about, or may enable certain species to swim with considerable rapidity. As in all other worms, respiration takes place through the surface of the body, the area of which is increased by the development, on certain portions of the paddles (parapodia), of plates penetrated with numerous blood-vessels, which thus become special respiratory organs or gills (Fig. 42, B). In their internal organization the Polychætes are constructed practically on the same plan as the earthworms, the principal difference being in the reproductive system. In the earthworm this is restricted to some of the forward segments, while in the present group the eggs and sperms |