CHAPTER V.

_WORMS._

The WORMS and their allies embrace a numerous assemblage of animals which exhibit a remarkable amount of variation both in structure and habits. A fundamental distinction which serves to separate readily even the most highly organised members of the group from the other articulate or jointed-bodied animals, such as Crabs, Insects, or Centipedes, is furnished by the character of the locomotive appendages. These in the Worm Tribe never assume a jointed character, but take the form of unjointed membranous processes which may or may not be supplemented by bristles. Frequently bristles alone constitute the essential locomotive organs. In certain groups, such as the Leeches, Flat-worms, Thread-worms, and others, even these are unrepresented.

The appropriate title of BRISTLE-WORMS has been conferred upon the section in which the locomotive organs take the form of bristles. Among these the COMMON EARTH-WORM is included. At first sight the worm's body appears to be perfectly smooth and naked; it is found, however, on closer investigation to be furnished, according to the species, with either two or four longitudinal rows of fine, hook-like bristles. Although these bristles project but slightly above the surface of the skin, they constitute very effective aids to locomotion, enabling the animal to obtain a secure grip upon the surface of the ground over which it may be travelling. Progression under such conditions is effected, in fact, on the same principle as that of the snake, the ends of the stiff bristles with which the segments are armed fulfilling the same role as the projecting edges of the reptile's scales. Earth-worms are chiefly vegetable-feeders, dragging into their holes fallen leaves, straws, and every other description of vegetable débris. They also swallow and pass through their systems large quantities of earth, absorbing from it its organic constituents, and depositing the indigestible residuum therefrom in the form of "earth-casts." The useful function thus performed by worms in bringing up earth from considerable depths and redepositing it upon the surface of the ground has been fully demonstrated in one of Mr. Darwin's works. There are some twenty species of British earth-worms, none of which, however, attain to the proportions of certain kinds indigenous to Australia and South Africa. Some of these giant species are as much as 3 or 4 feet long when unextended, and will on the stretch measure twice such lengths. Their thickness, which is proportionate, may vary from that of a man's finger to that of an ordinary sausage.

The most numerically abundant and structurally varied representatives of the bristle-bearing worms are inhabitants of the sea, and are divisible into two easily recognised natural groups. In one of these the animals resemble earth-worms in their ability to move about freely from place to place. In the other group they secrete and permanently reside within a tubular edifice, which may be calcareous and of shell-like hardness, or composed of particles of sand, mud, or other substances. The free-roving group, which embraces by far the larger number, includes such forms as the LUG-WORM, or LOB-WORM (held in high repute for fishing-baits), and a host of other allied species. In all of these the development of bristles and other appendages is more pronounced than in the Earth-worms. In another group, known as the NEREIDS, the elongate worm-like body is more or less flattened in shape; unjointed leg-like appendages, supplemented by bristles, are developed from the majority of the segments, and the animal presents a somewhat centipede-like aspect. This likeness is further enhanced by the presence of antennæ-like organs at the anterior extremity, while the mouth is armed internally with a pair of sharp-pointed, horny jaws. In many of the nereids the lateral organs are flattened out and paddle-like, constituting effective swimming-structures. Some of the larger species attain a length of several feet, and are especially noteworthy for the brilliantly iridescent tinting of their skins. The palm of beauty with respect to its brilliant colouring must undoubtedly, however, be awarded to the so-called SEA-MOUSE, frequently cast up by storms on the British coast. In this creature the body is comparatively short and thick, 3 or 4 inches long by 1½ to 2 inches wide. The centre of the back is covered in by a felt-like mass of fine interlacing hairs of a brownish hue, underneath which are broad, flat scales which protect the breathing-apparatus. The sides are, however, thickly clothed with long, slender hairs and bristles, each of which reflects the most brilliant prismatic tints.

The TUBE-DWELLING WORMS are noteworthy for the elegant and often beautifully coloured flower-like gill-tuft with which the head is crowned. Its separate filaments are clothed with vibrating hairs, which create currents bringing food-particles to the mouth. In those forms which build up a hard calcareous dwelling-tube, one of the gill-filaments is usually so modified as to constitute a stopper-like organ, wherewith the animal, on retreating into its domicile, can effectually bar out the ingress of intruders. In some members of the group the gill-tufts are elegantly branched and supplemented by long, simple, thread-like filaments, that are thrust out to long distances in every direction both for food and the materials required for the further lengthening and enlargement of the tube.

The LEECHES differ essentially from the Bristle-worms in the absence of bristles or supplementary appendages, in the presence of an adhesive sucking-disk at the posterior and sometimes also the anterior extremity, and on their well-known blood-sucking propensities. While the MEDICINAL and so-called HORSE-LEECHES inhabit fresh-water, some, more especially in tropical countries, infest the moist jungles and scrubs in vast numbers, and are among the most actively aggressive pests with which the traveller has to contend. A few leeches also inhabit the sea, preying upon the skate and other fishes. The bodies of these marine species are cylindrical, with a sucker at each extremity, and roughly corrugated or warted.

The FLAT-WORMS embrace a large number of intestinal and other parasitic species, including TAPE-WORMS, THREAD-WORMS, LIVER-FLUKES, and others. Among the free-living non-parasitic members of this group, the so-called INDIA-RUBBER-WORM is remarkable for the extraordinary elasticity of its tissues. Black in hue, it lives among rocks and seaweeds, and preys upon small fishes and other organisms. These, being seized by the suctorial mouth, are unable to effect their escape, the worm's body being capable of stretching out to a length of 20 feet or more, and "playing" the captured victim like a living elastic fishing-line until its struggles are exhausted.

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