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

BOOK 2

sea. Like the feeding-polyps, they are predatory; they have stinging-cells and they paralyse and engulf a variety of microscopic creatures. But they are altogether more complicated in structure than the polyps ; they have a definite stomach leading into a system of canals which, like the branched stomach of a flatworm, carries food to all parts of the body; they have special sense-organs; they are male or female. Dangling down under the bell there are tiny bags filled with eggs or spermatozoa.

e

Fig. 100.

two perfect polyps. Koology,”? 1911.) The little jelly-fishes, or meduse, swim in shoals at the surface of the sea and, when they are ripe, they shed their spermatozoa or eggs, as the case may be, into the seawater. The two mix together, and in this primitive manner the eggs are fertilized. They develop into simple elongated larve, called planule, mouthless bags which swim by means of cilia with which they are covered. ‘These larve are the founders of new colonies. They settle on pieces of wood, seaweed, and so on, and develop into feeding-polyps ; the polyps bud out stems which give rise to other polyps ; and thus a new colony is

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THE SCIENCE OF LIFE

The tissues of Pennaria (a creature very like Obelia) have been pressed through fine silk and the cells separated from each other ; nevertheless, they have crept

together again to rebuild polyps (magnified). The smaller cell-mass (inset), three days old, is beginning to organize itself ; the larger one, six days after the operation, has already formed

(From Wilson, ** Journal of Experimental

CHAPTER 4

formed. Evidently there is an alternation of phases in the life cycle of Obelia ; first, the fixed, budding colony of polyps, and then the free-swimming sexual medusa, charged with the duty of founding new colonies in distant places.

We have used the phrase “ alternation of phases * rather than “ alternation of generations ”’ because it is open to question whether the fixed stage should be called a generation at all. It consists, we have seen, of a central tube with an indefinite number of side-

branches; of the side-branches the = majority end in those living snares, the tentacled polyps, whose duty it is to nourish the whole colony, while a few end in the _ blastostyles—polyps which specialize in reproduction. Now, the feeding-polyps can perform independent movements of a considerable degree of complexity—they look like individuals—and the blastostyles have essentially the structure of polyps, so that we may, if we choose, regard the feeding-polyps and blastostyles as individuals, specialized in different ways in order to build up a colony, much as our cells are individuals specialized in different ways in order to build up a body. We may regard the polyp as the homologue of a man. Then we should distinguish three grades of individuality—the cells, the polyps, and a higher order of individual, the colony. But the organization of Obelia is very loose, very plastic. ‘There is no rigid specialization of tissues like that found in ourselves. If a small fragment of the colony be cut off from the rest—a single feeding-polyp, for example—it will sprout out a new central stem from which both feeding- and breedingpolyps will arise. What is more, and most vividly interesting, the tissues of the creature can be forced through fine muslin so that its cells are completely separated from each other ; nevertheless, they will come together again into masses, and organize themselves into new polyps, new colonies. Imagine the tissues of a man doing this !

In ourselves the species consists entirely of discrete and highly complicated cellcommunities that we call individuals. Moreover, in our case, this kind of organization is necessary, for human tissue cannot live unless it is part of a human body.! But in Obelia the reality is not a number of separate

1 Unless it is elaborately nursed in a tissue-culturemade to believe, so to say, that it iy part of a body.