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Nearly all species, like the sea-anemones, are brilliantly colored during life, and several are highly phosphorescent. All are marine, and while they are found everywhere, from the shore-line to great depths, the more abundant and larger species inhabit the clear, warm waters of the tropics down to a depth of one hundred and sixty feet. In such regions the stag-horn corals especially grow in the wildest profusion, and become tall and greatly branched. Except in quiet water they are continually being broken by the waves, beaten into fragments, and the resulting sand is deposited about their bases. As a result of this continuous growth and erosion, there have been formed from coral sand mixed with the shells of mollusks and the skeletons of various Protozoa several of the islands along the Florida coast and many of those of the Pacific, some of them hundreds of miles in extent.
52. General Characteristics.—The bodies of the animals comprising the two preceding groups are exposed on all sides equally to the water in which they live and are radially symmetrical; but in the worms, one side of the body is fitted for creeping, and for the first time we note a wellmarked dorsal (back) and ventral (under) surface. In the former, the body, like a cylinder, may be divided into similar halves by any number of planes passing lengthwise through the middle; but in the worms, the right and left halves only are exposed equally to their surroundings, and there is, accordingly, only one plane which divides the body into corresponding halves, so that these animals, like all higher forms, are bilaterally symmetrical. In creeping, also, one end of the body is directed forward and it thus becomes correspondingly modified. It usually bears the mouth, and may be provided with eyes, feelers, or organs of touch, and various other structures which enable the worm to recognize the nature of its surroundings. The nervous and muscular systems are better developed than in the foregoing groups, and we note a greater vigor and definiteness in the animal's movements, and in various ways the worms appear better able to avoid or ward off their enemies, recognize and select their food, and in general adapt themselves to the conditions of life.
The division of the worms is a very large one, and in some respects difficult to define, owing to the close resemblance which many of them show to animals in other groups. All the invertebrates, therefore, except the crabs and insects, were placed in one group until subsequent study made it possible to classify them more exactly. According to the general shape of the body, and the arrangement of internal organs, worms are divided into a number of groups, chief among which are the flatworms, the thread or roundworms, and the ringed worms or annelids.
53. Form and habitat.—The flatworms, as their name indicates, are much flattened, leaf-like forms, some species living in damp places on land, in fresh - water streams or ponds, or along the seacoast, while a variety of other species are parasitic. The free forms (Fig. 32) are usually small, barely reaching a length greater than five or seven centimeters (2 to 3 inches), but some of the parasitic species (Fig. 36) attain the great length of six to thirteen meters (20 to 40 feet).
The free-living forms usually occur on the under side of stones, and frequently are so delicate that a touch is sufficient to destroy them. A few species are almost trans- Flo 32 parent, while many are colored to harmonize completely with their surroundings, so that, even though fragile and defenseless,, they escape the attacks of enemies by being overlooked. The night-time or dark days are their hunting season, and at such periods they may be found moving about with a steady gliding motion (due to cilia covering the entire body), varied occasionally by a looping, caterpillar movement, or by swimming with a flapping of the sides of the body. When watched at such times they may sometimes be seen to snatch up small worms, snails, small crabs and insects, which serve as food.
More closely examining one of these forms, for example, the species usually found on the under side of sticks and stones in our shallow fresh-water streams (Fig. 32, A), we noto that the forward end is not developed into a well-defined
head as in the higher worms, but is readily determined by the presence of very simple eyes and tentacles, while the lower creeping surface is distinguished by a lighter color and the presence of the mouth. Through this small opening a slender proboscis (in reality the pharynx) may be extended some distance, and may be seen to hold the small organisms upon which it lives until they are sufficiently digested to be taken into the body.
5i. Digestive system.—In the smaller flatworms, some of which are scarcely larger than many of the Protozoa, the alimentary canal is a simple unbranched tube; but in the larger forms such an apparatus is replaced by a greatly branched digestive tract which furnishes an extensive surface for the rapid absorp
tion of food, and extending deep into the tissues of the body, carries nutriment to otherwise isolated regions. In the fresh-water forms and their allies there are three main branches of the intestine (Fig. 33), while in many of those from the sea there are several, and their arrangement affords a basis for their general classification.
55. Excretory system.—In the sponges and ccelenterates the wastes are cast out by the various cells into the gastric cavity or at once to the exterior without the aid of any pronounced system of vessels; but in the flatworms several of the organs are deeply buried within the tissues of the body and a drainage system becomes a necessity. This consists of a paired system of vessels extending the length of the animal (Fig. 33) and provided with numerous branches, some of which open at various points on the surface of the body, while the others terminate in Fio.at-Ftame-eeiiof flat
. . worm (after Lang). /,
spaces (Fig. 34, s) among the organs in flageiium; n, nucleus; what are known as flame-cells. The «, spaces among the or
, , . , . . , gans of the body; v,
substances which accumulate in these waste materials, spaces are gathered up by the flamecell, poured into the space it contains, and by means of the vibratory motion of its flageiium (/), a movement bearing a fancied resemblance to the flickering of a flame in the wind, are borne through the tubes to the exterior.
56. Nervous system and sense-organs.—In the sponges no definite nervous system is known to exist, the slight movements which the cells are able to undergo being regulated somewhat as they are in the Protozoa. Among the ccelenterates certain of the cells scattered over the surface of the body are set aside as nerve-cells, and, more or less united by means of fibers extending from them, convey impulses over the body. In the flatworms the larger number of nerve-cells