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

LESS INDIVIDUALIZED ANIMALS

individuals but the plastic living tissue, growing and budding and branching, throwing out now a feeding-polyp, now a blastostyle, now a medusa ; not definitely committed to any one of these kinds of body, but sprouting and reorganizing itself as circumstances determine. It is a living continuum ; a mob of cells which may be turned to this or that. So long as any part of the mob lives it may continue the race. Even in ourselves there is an underlying reality of this kind, for we, the individuals, die and yet through us the race continues ; small portions of tissue, single cells detached from our bodies may hand on the life of the species by growing and building themselves into new individuals. But in our case, the capacity for reproduction is retained only by a few specialized cells. Through them human protoplasm lives on and may live for ever; human individuals are the temporary shapes that it assumes—excrescences, so to speak, that the race throws out as the Obelia stuff- throws out polyps. We are so accustomed to thinking in terms of individual experience, to regarding a species as merely a convenient groupname for a number of separate persons, that the idea of the living, material race as a more enduring reality than the individual is at first sight altogether strange; nevertheless, it is clear when we come to examine these lowly-organized creatures that the elaborate, selfcentred individual is not by any means an indispensable part of the scheme. Life can go on without his or her intervention.

§ 3

Polyps, Felly-fish, Sea-Anemones, Corals

Now Obelia belongs to a phylum, the Celenterata, which includes a large number of different forms, all aquatic and nearly all marine. Like Obelia, their bodies are always simply organized, without elaborate organ-systems and with little differentiation of tissue, and their symmetry is nearly always radial like the symmetry of a starfish, and not bilateral like the symmetry of a vertebrate. For the latter reason the Cozlenterates used to be classified with the Echinoderms (Fig. 89) in the phylum “ Radi-

ata ’—hbut it is now recognized that they

are utterly distinct from each other; the Echinoderm has an altogether more highly organized and specialized interior than the Coelenterate. The bodies of Ccelenterates are soft, although they often make rigid houses to live in, or frameworks to live round, and the group is characterized by their peculiar stinging-capsules. Many of them are sessile and flower-like, so that they have been called the Zoophyta, or plant-like animals.

There are three classes of Ccelenterates, of which the Aydrozoa includes Obelia. ‘The members of this class are all variations, so to speak, upon the Obelia theme; the kind of life-history that we have described, withits alternation between the free-swimming

Fig. 101. Two of the characteristic stinging-capsules with which a Celenterate benumbs its prey, very highly magnified.

The one on the left is undischarged ; the barbed thread is still coiled up within it.

The one on the right has shot qut its thread.

sexual medusa and the colony of budding polyps, underlies the group, although it may be more or less profoundly modified. Best known are a great number of forms, growing like Obelia on hard objects round our coasts and often mistaken for small seaweeds ;_ they differ from each other in the structure of the polyps and medusz and of the horny tubes and cups in which the colony lives. The structure of our common representatives of the group can be seen clearly only with a magnifying glass, but in some forms the polyps may be brilliantly coloured flower-like objects two or three inches across, and the medusz, as transparent as glass, may be fifteen inches across the bell. In some species the life-history is simpler. There may be no colony stage at all ; there

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