CHAPTER VI THE WORMS 42. 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 resem blance 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. THE FLATWORMS 43. Form and habitat.-The flatworms, as their name indicates, are much flattened, leaf-like forms, some species living in damp places on land, or A in fresh water streams ponds, or along the seacoast, while a variety of other species are parasitic. The free forms (Fig. 27) are usually small, barely reaching a length greater than five or seven centimeters (2 to 3 inches), but some of the parasitic species (Fig. 31) 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 B naria); B, marine flatworm (Lepto few species are almost trans- FIG. 27.-A, fresh-water flatworm (Plaparent, while many are col- plana). Enlarged, from Nature. ored 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. 27, A), we note 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. 44. 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 ap paratus 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. 28), while in many of those from the sea there are several, and their arrangement affords a basis for their general classification. 45. Excretory system. In the sponges and cœlenterates the wastes are cast out by the various cells into the gastric cavity or at once to the exterior with out 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 vesseis extending the length of the animal (Fig. 28) and provided with numerous branches, some of which open at various points on the surface of the worm (after LANG). f, flagellum; n, nucleus; 8, spaces among the organs of the body; v, waste materials. body, while the others terminate in FIG. 29.-Flame-cell of flatspaces (Fig. 29, s) among the organs in what are known as flame-cells. The substances which accumulate in these spaces are gathered up by the flamecell, poured into the space it contains, and by means of the vibratory motion of its flagellum (f), a movement bearing a fancied resemblance to the flickering of a flame in the wind, are borne through the tubes to the exterior. 46. 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 cœlenterates 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 are collected into two definite masses (Fig. 28, B), which constitute a simple brain on which the eyes are situated and from which bundles of nerve fibers pass to all parts of the body, the two extending backward being especially noticeable. As in the squirrel, these are distributed to the muscles and other organs to regulate their activity, while those distributed to the skin, especially in the forward part of the body, convey stimuli produced by touch. The branches connecting with the eyes enable the animal to distinguish light from darkness, but are probably too simple to allow it to clearly distinguish objects of the outside world. The sense of smell and possibly that of taste are also present, but are relatively feeble. Some other characters of this class will be noted in the consideration of the two following classes. 47. Parasitic flatworms (trematodes)—parasitism.-Mention has already been made of the associations of two animals as "messmates" for mutual benefit, such as the Hydractinia growing on the surface of the shell inhabited by the hermit crab, to which it gives protection by means of its nettle-cells, while in turn being borne continually into regions abounding with food. More frequently, however, one animal derives benefit from another without making any compensation. For example, many species of flatworms live within the shells of certain snails and upon the bodies of sea-urchins and starfishes, where they gather in their food supply safe from the attacks of enemies. Such associations are probably without much if any inconvenience to the animal thus inhabited, and it also appears probable that the tenants are transients, using the mollusk or starfish only as a temporary home. But from this condition of affairs it is only a short step to the parasitic habit, where the association becomes permanent and the occupant is provided with various structures which prevent its separation from its host. This latter kind of union occurs throughout the group of trematodes; all are parasitic, and |