The round, or thread, worms include free forms, as the vinegar eel; parasitic forms, as the pin worm (Ascaris) and trichina; and forms free when adult, and parasitic when young, as the hair worm (Gordius): The body is usually elongated and cylindrical in shape, whence the name. In most forms there are plainly marked digestive and nervous systems. The trichina is usually derived by man from the flesh of the pig. It exists in the muscles, inclosed in micro I. II scopic cases or cysts, FIG. 39.-Trichina spiralis (much enlarged): are produced and bur I, male; a, mouth; c, intestine; II, capsules, with trichinæ in muscle. row their way into the muscles, usually of the back and limbs, where they become encysted in the muscle fibers. In burrowing they cause great pain and fever, and sometimes death. The adult trichina is about 2 of an inch long. The "horse-hair snake," a hair worm (Gordius), passes the early part of its existence in larval or adult insects, e.g., the cricket. When mature the worms leave the body of the insect and lay their eggs in damp places. The eggs or the immature worms are then taken into the bodies of other insects in which the parasites later reach their full development. The wheel animalcules, or rotifers, mostly found in fresh water, are composed of a few ill-defined segments, and have on the anterior end a disk which is ciliated on the edge, the motion of the cilia causing the appearance of a rotating wheel, whence the name. They are from 200 to 36 of an inch long. They have a welldeveloped digestive system, the food consisting of minute organisms, and a rudimentary nervous system. Rotifers have been kept for several years in a dried condition and have afterward been revived (Fig. 40). These ani- FIG. 40. Rotifer, or mals have gen erally a body "Wheel animalcule " (Hydatina), highly magnified. FIG. 41. Diagram of a Polyzoan: 2. lophophore bearing tentacles, t; m, mouth; d, digestive cav ity; i, intestine: a, anus; e, excretory organ; b, "brain." Much magnified. cavity, in which lies the alimentary canal, bent in such a manner that the mouth and the anal opening are close together. Near the mouth is a curved ridge, the lophophore, bearing tentacles. There is a very rudimentary nervous system (Fig. 41). The Polyzoa resemble polyps in appearance, living in clusters, each individual inhabiting a delicate cell, or tube, and having a simple mouth surrounded with ciliated tentacles. The colony often takes a plantlike form; sometimes spreads, like fairy chains or lacework, over other bodies; or covers rocks and seaweeds in patches with a delicate film. The majority secrete carbonate of lime. A polyzoan shows its superiority to the coral, which it resembles, in possessing a distinct alimentary canal and a nervous system. The cells of a group are never con FIG. 42. Polyzoans: 1. Hornera lichenoides, natural size. 2. Branch of the same; magnified. 3. Discopora Skenei, greatly enlarged. nected by a common tube, as in cœlenterates. are both marine and fresh-water species. There The Brachiopoda or "lamp shells" have a bivalve shell, the valves being applied to the dorsal and ventral sides of the body. The valves are unequal, the ventral being usually larger, and more convex; but they are symmetrical, i.e., a vertical line let fall from the hinge divides the shell into two equal parts. The ventral valve has, in the great majority, a prominent beak, perforated by a foramen, or hole, through which a fleshy stalk protrudes to attach the animal to submarine rocks. The valves are opened and shut by means of muscles, and in most cases they are hinged, having teeth and sockets near the beak. The mouth faces the middle of the margin opposite the beak; and on either side of it is a long fringed "arm," generally coiled up, and supported by a calcareous framework. The animal, FIG. 44. having no gills, respires by the arms and the mantle. Brachiopods were once very abundant, over two thousand extinct species having been described; but only about a hundred species are now living.13 These are all marine, and fixed. The animals, in this group are related to the mollusca. Branch VIII. ECHINODERMATA The echinoderms, as starfishes and sea urchins, are characterized by the possession of a distinct nervous system (a ring around the mouth with radiating branches); an alimentary canal, completely shut off from the body cavity, having both oral and anal apertures; a watervascular system of circular and radiating canals, connected with the outside water by means of the madreporic tubercle, and a symmetrical arrangement of all the parts of the body around a central axis in multiples of five,14 this radial arrangement, however, concealing a definite bilateral symmetry. They are, thus, much more highly organized than the coelenterates, with which group they have very little in common except Forms of Echinoderms, from radiate to annulose type: a, Crinoids: 6, Ophiurans; c, Starfish; d, Echini; e, Holothurians. their apparent radial symmetry. In the course of development in echinoderms metamorphosis occurs, the larval forms bearing no resemblance to the adults. There are five principal classes, all exclusively marine and solitary, and all having the power of secreting more or less calcareous matter to form the skeleton. Ordinary starfishes consist of a flat central disk, with five or more arms, or lobes, radiating from it, and containing branches of the viscera. The skeleton is leathery, hardened by small calcareous plates (twelve thousand by calculation), but somewhat flexible. The mouth is below; and the rays are furrowed underneath, and pierced with |