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

BOOK 2

CHAPTER 3

FURTHER PATTERNS OF INDIVIDUALIZED ANIMAL LIFE

§ 1. Other Ammal Phyla.

Pentagonal Experiment. worms.

§ 1 Other Animal Phyla

WwW: have spoken already in our introduction of the dumps used by the early biological classifiers. At first the only real phylum of animals to be picked out was the vertebrates. The remainder was all a miscellaneous dump labelled (wrongly) ** Bloodless Animals.” ‘The next great phylum to be separated away from this confused heap was the arthropods we have just dealt with. Linneus (1707-1778), the father of precise biological classification, made the distinction, using the word “insect” to cover all the arthropods—but he did not regard the arthropod division as equivalent to the vertebrates ; he grouped animals into the six classes—mammals, birds, amphibians (including also the reptiles), fishes, insects, and worms. ‘This last class, the ““ worms,” is his dump ; here all other creatures were left together because as yet there was insufficient anatomical knowledge for them to be sorted out.

Gradually we have come to realize that quite a number of other phyla—of other “general ideas”? and general plans of animals, that is to say—exist in the Animal Kingdom. We have, we hope, made clear that there is no sort of precedence or subordination between these phyla. The lobster is not lower or higher than the frog or fish. He is altogether different. He belongs to a different series of experiments that Nature seems to have made. So, too, we now recognize another very important and various phylum, another plan of living mechanism, in what are called molluscs (such as oysters, mussels, snails, squids, octopuses), and yet another in the echinoderms (the starfish, sea-urchins, sea-cucumbers, and _ sea-lilies). There are three perfectly distinct phyla of worms, the segmented worms, the roundworms, and the flatworms, each a large and important group. And there are the phylum of jelly-fishand polyps, the phylum of sponges, and the phylum of animals too small for the naked eye to see. But besides these there are other phyla, which give a less ample

130

§ 2. The Molluscs. § 4. Segmented Worms. § 5. Roundworms. § 6. Flat-

§ 3. Echinoderms ; Nature’s

7. Et Cetera.

spectacle of range and variations. One might speak of them as Nature’s less successful or less versatile trials. She is like an enterprising manufacturer who has put a number of devices on the market and followed up those that caught on. The chordata, when it got to the vertebrate pattern, was a huge success, and the call for arthropods was steady and abundant. Both these phyla conquered sea, land, and air. The molluscs and flatworms and roundworms went well, the segmented worms and echinoderms went fairly well. But certain other phyla, we shall note, never ran away with matters in that fashion. The zoologist to-day has still to wind up his tale with a collection of odd and apparently unrelated things, minor phyla, and at last something it will be most truthful to label “et cetera.” There are phyla Nature has not gone on with; in

’ the past there may have been endless phyla

she has set aside and ceased altogether to produce.

To each of these less exciting patterns of animal construction we will devote only a section or a part of a section here. That must not hide from the reader that we are dealing with phyla, with series of fundamentally different kinds of animal body, each, from the point of view of physiological mechanism, as important as the chordata and the arthropods.

§ 2

The Molluscs

Molluscs are creatures nearly all of which possess shells—double (bivalve) shells like those of oysters, clams, and cockles, single, hollow shells like those of snails, shells embedded in the body-wall like the “‘ bone” of a cuttle-fish. Except for these shells they are soft-bodied, without any skeleton. ‘They are not segmented as arthropods are, but their blood is coloured pale blue with hemocyanin, like the blood of a crustacean. In the great majority of cases (the shell of the bivalves being a notable exception) their muscles do not form accurately defined bands playing on jointed skeletal levers, like the volun-