anemones, described in the next section. In the course of time remarkable changes ensue, which first manifest them a m FIG. 25.-Stages in the development of a scyphozoan jelly-fish. a, the attached young, which in b has separated into a number of disks, each of which becomes a jelly-fish, c.-After KORSCHELT and HEIDER. selves in a series of grooves encircling the body. These grow deeper, and the body of the animal finally comes to resemble a pile of saucers with the edge of each developed into a number of lobes (Fig. 25, b). One after another each saucer, to preserve the simile, raises itself from the top of the pile and swims away, and is clearly seen to be a jelly-fish, though considerably unlike the adult. As growth proceeds, however, it un FIG. 26. An attached scyphozoan jelly-fish (Haliclystus). Natural size, from Nature. dergoes a series of transformations which result in the adult form. 50. Sea-anemones.-In its external appearance the seaanemone (Fig. 27) bears some resemblance to the Hydra, but is of a much larger size (1 to 45 c.m., or inch to 14 feet in diameter), and is frequently brilliantly colored. The number of tentacles is also more numerous, and the mouth leads into the body by means of a slender esophagus (Fig. 28). Numerous partitions from the body wall extend inward, and many unite to the esophagus, keeping the latter FIG. 27.-Sea anemones (the two upper figures) and solitary coral polyps. in position. Below the esophagus each partition projects into the great cavity of the body and bears upon its inner free edge several important structures. The first of these, known as the mesenteric filaments (Fig. 28), appearing like delicate frills, plays an active part in the digestion of the food. Associated with these are long, slender threads, closely packed with innumerable lasso-cells, which may be thrown out through openings in the body wall when the animal is attacked. Lasso-cells are also very numerous on the tentacles, which are thus to some extent defensive, but are chiefly active in capturing the crabs and small fish which serve as food. The partitions also carry eggs which may undergo the first stages of their growth within the body, and when finally able to swim are sent out through the mouth opening by hundreds to seek out favorable situations, there to settle down and remain. In some species the young may sometimes arise as buds, as in Hydra (Fig. 27), and in others the animals have been described as splitting longitudinally into two equal-sized young. 51. Corals, The coral polyps also FIG. 28.-Longitudinal section through the body of a sea-anemone. oe, esophagus; m. f., mesenterial filaments; r., reproductive organs. belong to this group, showing a very close resemblance to the sea-anemones. In most cases they develop a firm skeleton of lime, commonly known as "coral," which serves to protect and support the body. In a few species the polyps throughout life are solitary, and with skeleton comparatively simple (Fig. 27); but the larger number of species be-. come more complex by developing buds, which retain their connection with the parent, and in turn produce other outgrowths with the ultimate result that highly branched colonies are produced (Fig. 29). At the same time the outer layer of the body is continually forming a skeleton which encloses the colony as a sheath, except at the termination of each branch, where the mouth and tentacles are located. In certain species-for example, the sea pens (Pennatula) and sea fans (Gorgonia)-a skeleton may be FIG. 29.-Small portions of coral colonies, with some of the polyps expanded. formed of myriads of lime spicules, somewhat like those of the sponge, which are bound together by the fleshy substance of the body; but the skeleton of most of the common forms in the ocean, and the coral found in general collections, is stony. According to their method of branching, such specimens have received various popular names, such as brain, stag-horn, organ-pipe, and fungous corals. |