process bearing on its end an opening which becomes the mouth (Fig. 21, A). Around the margin of the bell numerous tentacles develop, and at the same time the gelatinous substance situated between the outer and inner layers of the bell expands to a relatively enormous degree, giving it an increasing globular form and glassy appearance.

FIG. 21.-A jelly-fish (Gonionemus), slightly enlarged. The stalked mouth is shown in dotted outline. B, C, enlarged portions of a hydroid colony bearing the mouth and tentacles; j, à capsule within which the jelly-fish develop; D, diagram of jelly-fish, illustrating its method of locomotion.

Finally, vigorous movements rupture the connection with the parent, and this newly developed outgrowth, usually small, becomes an independent organism popularly termed a jelly-fish. While the external form of the jelly-fish appears to be widely different from the hydranths, a more careful study shows the difference to be superficial. Some zoologists believe that jelly-fishes are simply buds which have become fitted to separate and swim away from the colony. in order to distribute the young, as described hereafter.

When the stalked colonies are very abundant the jelly

fishes may be liberated in such multitudes that the upper surface of the ocean for many miles may be closely packed with them in numbers reaching far into the millions. In these positions they are carried both by oceanic currents and through the alternate expansion and contraction of the bell, a movement resembling the partial closing and opening of an umbrella. In the jelly-fish the contraction is more vigorous and rapid than the opening in the velum or veil (Fig. 21, 6) which is so narrowed that the water in the subumbrella space (a) is driven through it with considerable force, which results in driving the body in the opposite direction.

The life of a jelly-fish is perhaps of short duration, lasting not more than a few hours in some species, up to two or three weeks in others, but during that period they produce multitudes of eggs which develop into minute freeswimming young. These settle down on some rock or seaweed, and soon develop a Hydra-like body which, after the fashion described above, grows into another tree-like colony.

46. Alternation of generations.—It will be noticed that the offspring of the jelly-fishes are not jelly-fishes, but stalked colonies, and these latter forms give rise to jelly-fishes. This is known as the alternation of generations, the jellyfish generation alternating with the colonial form. This characteristic is of the greatest service in preventing the extermination of the race. Were the stalked forms to give rise directly to other stationary colonies, it is obvious that before long all the available space in the immediate locality would be filled. The food supply, always limited, would not suffice, and starvation of some or imperfect development of all would result; but by means of the free-swimming jelly-fish new colonies are established over very extensive areas, and favorable situations are held by all.

47. More complex types.-As mentioned above, there are perhaps upward of a thousand species of Hydrozoa, all with

essentially the same structure but with various modes of branching (for some of the commoner modes, see Fig. 20). In some of the higher forms a division of labor has arisen among various members of the association which has led to most interesting results. For example, Fig. 22 represents a species of hydroid found investing the shells of sea-snails occupied by hermit crabs (Fig. 66). To the unaided eye its appearance is that of a delicate vegetable growth, but when placed under the microscope it is found to consist of

a

FIG. 22.-An enlarged portion of a hydroid

colony (Hydractinia), showing (a) the nutritive polyp, (b) the defensive polyp, and (c) the reproductive polyp.

a multitude of Hydra-like animals united by a hollow branching root system connecting the gastric cavities of all of them (Fig. 22). Certain individuals (a) with tentacles and a mouth resemble a Hydra; others, without a mouth and tentacles, are reduced to a club-like form (6) liberally supplied with nettle-cells upon their free extremities; while the third type

(c), likewise devoid of a mouth, possesses rudiments of tentacles below which are borne numerous clumps of reproductive cells. The first type, the only one possessing a mouth, captures the food, and after digesting it distributes the greater portion to the remaining members by means of the connecting root system; those of the second form, defending the others by means of their nettle-cells against the inroads of a foreign enemy, are the soldiers of the colony; while the third type produces the eggs from which new individuals develop.

In some of the higher Hydrozoa, the Portuguese manof-war (Fig. 23), this division of labor has reached a more advanced stage of development, and in addition the entire

colony is fitted for a free-swimming existence. What corresponds ordinarily to the attached stalk in other forms terminates in a bladder-like expansion, distended with gas, that serves as a float. From it are suspended individuals resembling great streamers sometimes many feet in length, without mouths, but loaded with nettle-cells that enable them to capture the food, which is conveyed to the second type, the nutritive polyps. Each of these is a simple tube bearing a mouth, and within them the food is digested and distributed by means of a branching gastric cavity extending throughout the entire colony. Then there are individuals like mouthless jelly-fishes which bear the eggs and care for the perpetuation of the colony; and besides these there may be some whose duty it is to defend the rest, and others whose active swimming movements, together with the wind, drive

From Nature.

the colony about. Thus uni- FIG. 23.-A colonial jelly-fish (Physalia). ted, sharing the food supply

and working for the general welfare of all, the members of this colony live in greater security and with less effort than if, as separate individuals, each was fighting the battles of life alone.

48. Scyphozoa. The greater number of the larger and more conspicuous jelly-fishes are included under this term. In general shape and locomotion they resemble those of the

preceding group (Fig. 24), but while the latter are generally very small, these forms are commonly from four to twelve inches in diameter, and some measure one to two meters (three to six feet) across the bell. They are also distinguished by means of tentacles which extend from the corners of the mouth sometimes to a distance of several feet,

and together with the marginal tentacles are formidable weapons for capturing small crabs, fishes, and other animals which serve as food. In turn these forms serve as the food of many whales, porpoises, and numerous fishes which hunt them down, though the amount of nourishment they contain is probably relatively small owing to the fact that in their composition there is a large percentage of water (99 per

cent in some species). The lobed margin of the bell, the absence of a definite swimming organ or velum, and the character of several of the internal organs, distinguish the larger from the smaller jelly-fish; but the greatest difference, however, is in the method of development.

49. Development. The eggs arise from the inner layer of the jelly-fish and drop into the gastric cavity, where each develops into a ciliated two-layered sac in some respects like that of a young sponge. Swimming away from the parent, they finally settle down, and attaching themselves (Fig. 25, a) assume the external form and habits of the sea