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39. General remarks.—This division of the many-celled animals includes the jelly-fishes, sea-anemones, and corals. A few species live in fresh water, but the majority are confined to the sea, being found everywhere from the shoreline and ocean surface to the most profound depths. Adapted to different surroundings and modes of life, they constitute a vast assemblage of the most bewildering diversity. In some cases their resemblance to plants is remarkable, and the term zoophyte or " plant animal," occasionally applied to them, is the relic of former times when naturalists confounded them with plants. Even to-day certain species are sometimes collected and preserved as seaweeds by the uninformed.
The general plan on which all ccelenterates are constructed is a simple sac, in some respects resembling that of the lower sponges, yet, since the modes of life of the members of the two groups are usually quite unlike, we shall find many profound differences between them.
40. The fresh-water Hydra,—The bodily plan comes out most clearly in the Hydra (Fig. 19, A, D), which occurs upon the stems and leaves of submerged fresh-water plants in this and other countries. Its body, of a green or grayish color, according to the species, scarcely ever attains a diameter greater than that of an ordinary pin nor a length exceeding half an inch. One end of the cylindrical organism is attached to some foreign object by means of a sticky secretion, but as occasion requires it may free itself, and by means of a " measuring-worm " movement travel to another place.
Examined under a hand lens, the free end of the body will be found to support six to eight prolongations known
as tentacles, which s J? § /~\ serve to convey
food to the mouth, centrally located in their midst. This opening, unlike that of the sponges, is the only one leading directly into the large central gastric cavity which occupies nearly the entire animal (Fig. 19, D). As in the sponge, the cells of the body are arranged in the form of definite layers, but the middle one is represented only by a thin gelatinous sheet.
41. Organs of defense. — These are the so-called lasso or nettle-cells (Fig. 19, C). Some of the cells of the outer layer possess, in addition to the elements of the typical cell, a relatively large ovoid sac filled with a fluid, and also a spirally wound hollow thread provided with barbs near its base. On the outer extremity of the nettle-cell projects a delicate bristle-like process, the trigger hair. These cells are especially abundant on the tentacles (Fig. 19, A, D), forming close, knob like elevations or "batteries," thus rendering it practically impossible for any free-swimming organism to avoid touching them in brushing against the tentacles. In such an event the dis- . turbances conveyed through the trigger hair set up in some unknown way very rapid changes in the cell. This causes the sac to discharge the coiled thread and barbs into the body of the intruder, which is rendered helpless by the paralyzing action of the fluid conveyed through the thread. Thus benumbed it is rapidly borne to the mouth and swallowed. In time new nettle-cells develop to take the place of those discharged and consequently worthless.
42. Digestion of food,—Upon the interior of the body of Hydra and all of the ccelenterates the food, by reason of its large size, is incapable of being taken into the various cells. It is necessary, therefore, to break it up into smaller masses, and this is accomplished through the solvent action of the digestive fluid poured over it from some of the cells of the adjacent inner layer. When subdivided, the granules swept about the gastric cavity by the beating of the flagella (Fig. 19, D) are seized by the processes on the free surfaces of the remaining inner layer cells, where they undergo the final stages of digestion; then in a dissolved state they become absorbed and assimilated by all the cells of the body.
43. Methods of multiplication.—Very frequently, especially if the Hydra has been well fed, two or three processes arising as outpushings of the body wall may be noted upon the sides of the animal (Fig. 19, A, D). If these be watched from time to time they are found to increase in size, and finally, upon their free extremities, to develop a mouth and surrounding tentacles. Up to this point growth has taken place as a result of the assimilation of nutritive substances supplied from the parent; but a constriction soon occurs which separates the young from the parent, and from that time on the two lead independent existences. At other times this asexual method of multiplication is replaced by sexual reproduction, where new individuals arise from fertilized eggs. Both eggs and sperm arise in Hydra and in some other animals in the same individual, but in all such cases the eggs are fertilized by sperm which escape from some other individual. The fertilized egg, surrounded by a firm coat, separates from the parent, drops to the bottom, and after a period of rest develops into a little Hydra which hatches and enters upon a free existence.
Fig. SO.—Different types of Hydrozoan colonies. From Nature, the lower species magnified about 50 diameters.
44. Hydrozoa, or Hydra-like animals.—Attention has already been directed to the fact that the structure of Hydra is the simplest of the ccelenterates; nevertheless, the thousand or more species belonging to this class which present a much more complicated appearance (Fig. 20) possess many fundamental Hydra-like characters. It is owing to this fact that this assemblage of forms has been placed in the class of the Hydrozoa, or Hydra-like animals.
With but very few exceptions the members of this class are marine, usually living near the shore-line, where at times their plant-like bodies occur in the greatest profusion attached to rocks, seaweeds, or the bodies of other animals, particularly snails and crabs. Fig. 20 (upper colony) gives a good idea of one of the more complex forms, whose treelike body attains in some cases the relatively giant height of from 15 to 25 c.m. (six to ten inches). In early life it bears a close resemblance to a Hydra. Buds form in much the same way, but they retain permanently their connection with the parent, and in turn bear other buds, until finally the form shown in the figure is attained. In the meantime root-like processes have been forming which afford firm attachment to the object upon which the body rests. Also during this process the cells of the outer layer form a horny external skeleton ensheathing the entire organism except the terminal portions (the hydranths, Fig. 2i, B) bearing the tentacles. The gastric cavities of all communicate, and the food captured by one ministers in part to its own needs and, swept through the tubular stalks and roots, is also shared by all other members.
\) 45. Jelly-fishes and the part they play.—During the process of growth a number of stubby branches arise which differ from the ordinary type in shape, and also in many cases as regards color. These club-like, fleshy portions develop close-set buds (Fig. 21, c) which early assume a belllike shape, the point of attachment corresponding to the handle, while the clapper is represented by a short, slender