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

BOOK 2

regular rhythm of generations that differ from each other—first the fluke, a creature over an inch long, living in the sheep’s liver, then the barely visible creatures living in the snail, then the fluke again, and so on. It is as if the offspring of men were mice and the offspring of mice were men.

The flukes include a number of different forms with more or less picturesque and adventurous life-cycles. The one most troublesome to ourselves is Bilharzia of tropical countries, which spends its fluke

Fig. 94. Tenia, a common tapeworm.

Inset is a magnified view of the head, showing the armoury of adhesive hooks and suckers with which it ts provided.

stage in the human intestine or urinary bladder, and its microscopic stage, like that of Fasciola, in a fresh-water snail. It invades us through the mouth or through the skin, and reaches the bladder along the veins.

The third class of platyhelminthes is the tapeworms—also parasitic, and also showing an alternation of generations in two different hosts. ‘The animals have long bodies—sometimes up to eighty feet of them—and they have suckers and hooks on their ‘ heads ” by means of which they cling to the wall of the host’s intestine. ‘They have no digestive

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THE SCIENCE OF LIFE

CHAPTER 3

system of their own, but absorb the food at the surfaces of their bodies when it has been digested for them by these hosts. The body consists of a chain of segments—two or three to many hundreds—each with a complete reproductive system in it. As the hinder ones ripen they drop off, and swollen pockets of eggs are loosed into a wider world with the feces of the host ; but they are continually replaced by new segments which originate in a zone of growth close to the head. Of those which infect man some spend their adult stages in our own bowels and their intermediate stages in the muscles of pigs or cattle, entering us with our meat; another, even more troublesome, spends its adult stage in the dog’s bowel and its intermediate stage in our own bodies, where it forms large “hydatid cysts’? in the lungs, liver, and other organs in the hope that our flesh will be devoured by dogs. In the brains of sheep these cysts cause the disease known as staggers.

Strange indeed are Nature’s ways. Here are two special classes, flukes and tapeworms, and they exist, it seems, for no other purpose than the affliction of the innumerable variety of forms which the dominant vertebrate, arthropod, and molluscan phyla have produced.

§ 7 Ei Cetera

So far we have dealt with seven animal phyla. Now comes the ragbag. We have left over a residuum of minor phyla containing few forms and having little economic or theoretical importance. We will not catalogue them here. Briefly we will mention four groups.

The Nemertine worms are thread-like or ribbon-like creatures, ranging in length from under an inch to twenty-five metres. Most of them are marine, creeping under stones and among seaweed ; some are able to swim. One or two species live in fresh water and a few are terrestrial ; one or two are transparent and leaflike and swim like marine butterflies at the surface of the sea. For the most part they are carnivorous. ‘They are in a phylum by themselves.

The Rotifers, or wheel-animalcules, another phylum, are minute but highly organized animals, found chiefly in ponds, gutters, damp moss and the like, and a great