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

BOOK 2

ultimately equally troublesome tail. Meanwhile, the hind end starts on an independent life, to be rebelled against in its turn by its own posterior. Moreover, in this flatworm phylum we find alternation of generations carried to its highest and most elaborate pitch ; we have already seen how in the liver-fluke there is a regular succession of a whole series of generations, externally very unlike each other—the sexually reproducing flukes in the liver of the sheep, and the minute sexless forms in the water-snail.

We are accustomed to think of ourselves as individuals and of our species as consisting entirely of individuals that are all alike, except for the relatively slight distinction between male and female. But manifestly

Fig. 98. The branching growth of the worm Spllis ramosa.

in these invertebrate animals there are different kinds of individuals, and a different sort of species. We find one half of the body rebelling against the other half, and we find them splitting apart and reconstituting themselves ; we find that the liver-fluke, a creature two-thirds of an inch in length, spending its life in a sheep’s liver, and the redia larva, about one sixteenth as long, living in a pond-snail and with every organ different from that of a fluke, are both members of one and the same species, begetting each other with a regular, inevitable rhythm. The rigidity and uniformity of organization with which we are familiar in ourselves is not universal. Our ideas of individuality have to be revised and extended. We shall find them still more profoundly shaken by the animals that we are next to examine.

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THE SCIENCE OF LIFE

CHAPTER 4

§ 2 Obelia

It is common to find a light brown furlike growth just below low-water mark on seaweeds, the wooden piles of piers, and the like. The growth consists of branching filaments of about the thickness of fine sewing-cotton, the branches terminating in tiny knob-like enlargements. The whole thing is suggestive at first sight of a miniature forest of seaweed, but the filaments are animals, not plants; they belong to the genus called Obelia.

On examining the filaments with a microscope, we find that they consist of hollow strands of living substance ensheathed by a transparent, horny tube. At most of the knobs the tube opens out as a delicate goblet ofsimple design, Dy and in each goblet there lives a Ife polyp. The polyps—the individuals of the little colony—are very simple in their structure. They consist of plain cylindrical bags of living tissue ; at one end they are joined to the central filament of the branch on which they rest, and at the other they have tiny funnel-shaped mouths surrounded by single rings of writhing tentacles. Scattered all over their surfaces, and particularly numerous on the tentacles, are minute stinging-capsules. Each of these is a tiny oval vesicle with a spiral filament coiled up inside and a sensitive “‘trigger-hair’’ projecting from the surface of the animal into the outer world. When the trigger-hair is stimulated, the spiral filament, barbed and poisonous, is shot out with surprising vigour. The flower-like polyp is a living snare ; it waits, with its tentacles stretched out, and if a suitable victim —such as one of the microscopic crustaceans —brushes one of the tentacles as it passes by it is suddenly riddled with the stinging, numbing filaments, then grasped in the other tentacles and crammed through the mouth into the hungry bag. The cells lining the bag secrete a digestive juice which disintegrates the prey, and they can also take particles of food into their own bodies very much as our white blood-corpuscles consume bacteria. Finally, after a little time, the indigestible remains of the victim are discharged through the same door that served as an entry, and the net of stinging fingers is spread again. That, in brief, is the life of the polyp ; occasionally, if prey is not too abundant, the