The science of life : fully illustrated in tone and line and including many diagrams

BOOK ae

are forms in which sexual meduse develop directly from the fertilized ege—medusze otherwise very similar to those of Obelia. On the other hand there are forms without meduse, and we may pause for a moment to examine one of these.

Hydra is a minute creature found commonly enough in fresh-water ponds ; it may be a third of an inch long. It is like a single, free-living polyp of Obelia ; it has a hollow, cylindrical body, with a mouth surrounded by tentacles. It usually remains fixed, catching such prey as Cyclops with its tentacles, which, when the animal is hungry, trail in the water like a driftnet. Sometimes it moves from place to place, gliding along on its base or looping like a looper caterpillar. Like Obelia, it is a loosely organized cell-community ; if it is cut into halves, or even into small bits, each fragment can reconstitute itself and form a whole Hydra. Its methods of reproduction are two. It may produce buds—a lump of tissue appears somewhere on the side of the creature, grows, develops a mouth and tentacles, and breaks away. Sometimes, when food is very plentiful, the bud may start to grow new buds from its own flanks even before it breaks away from its parent; this is a hint, so to speak, of the colony-formation seen in Obelia. Normally, however, Hydra is not colonial. Sometimes little swellings may appear on the sides of Hydra—not in definite places, but anywhere—which do not form buds. These swellings produce eggs if they are near the base of the body, spermatozoa if they are near the top. The eggs are relatively large, passive cells like our own eggs ; the spermatozoa are tiny, active tadpole-like creatures as our own are. The sperms swim up to and fertilize the eggs, which then develop into little embryo Hydras without any intermediate stage.

Hydra, then, contrasts with Obelia in two respects : first, there is no medusa and the polyp is itself ‘sexual ; second, there is no colony-formation and no division of labour between polyps like that between feeding-polyps and blastostyles in Obelia.

On the other hand, there is another group of Hydrozoans, the Siphonophores, in which colony-formation is carried to an extreme degree. ‘They are found floating in the open sea—chiefly in warm waters. The ** Portuguese man-of-war,’’ Physalia, is a good example ; it has an elongated bladder filled with air, from one to twelve inches long, by means of which it floats, and having above a raised crest or sail; the whole is peacock-blue or orange. From the lower

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CHAPTER 4

side of this float there hang bunches of miscellaneous-looking objects—blue, violet, and carmine, with a silvery sheen. Each object is a unit, either a polyp or a fixed medusa, and they specialize in the performance of various duties; some, not unlike the Obelia polyps, are feeders; others, having no mouths and a single long tentacle very richly provided with stinging capsules, are soldiers; the medusz, clustering like dark blue erapes, bear sexual cells; from the whole there depend particularly formidable tentacles, up to sixty feet in length and with a sting that can raise weals on a human being, like the sting of a giant nettle, and give him fever for several days. Other Siphonophores are differently planned and coloured. We may note that although a Siphonophore is conveniently described as a colony of specialized individuals, those individuals are not nearly so sharply defined as in Obelia—they are continuous with each other, and it is often very hard to say where one begins and the other ends. The problem of assigning to any part of the colony an individuality comparable to our own individuality is much harder than even in Obelia.

The second class of Ccelenterates, the Scyphozoa, includes all the larger and more familiar jelly-fish. The medusa of a Hydrozoan is like a miniature jelly-fish in its general appearance, but the Scyphozoan medusa is more complex in its organization. The Scyphozoa are all marine, and most of them float at the surface of the ocean; they are generally transparent, with patches of purple, blue, or orange, especially on their reproductive organs; the deep-water forms are often red or brown. Many are phosphorescent. The greater part of their bodies consists of a structureless jelly that is in fact mostly sea-water ; only about one per cent. of the weight of a jelly-fish consists of organic matter. One or two species are curious, not swimming but living on the sea-bottom ; Haliclystus, shown in Fig. 102, affords a good example—it has the structure of a jelly-fish, but leads the life of a polyp, attaching itself to weeds and the like.

Many jelly- -fish develop directly from the egg, but in some species there is an intermediate stage reminiscent of the alternation found in Obelia. In Aurelia, for example, the commonest jelly-fish of our shores, the eges hatch into tiny planula larve like the larvee of Obelia, which settle and give rise to a single feeding-poly p- The polyp later divides itself into a series of discs, like a pile