striction 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 fertil

ized 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. 20.-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 cœlenterates; 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. 21, 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 ciapper is represented by a short, slender

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, a 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, b) 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

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.

FIG. 22. An enlarged portion of a hydroid

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

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