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

BOOK 2

CHAPTER 2

THE SECOND GREAT PHYLUM : THE ARTHROPODS

§ 1. A Contrast of Arthropod and Vertebrate. § 3a. Shrimps, Crabs, Water-fleas, and Barnacles. § 3d. Centipedes and Millipedes. § 3¢. Peripatus.

Scorpions. § 3c. Insects.

$1 A Contrast of Arthropod and Vertebrate

Aw now we come to the second great phylum of living things. This includes all the insects which creep and buzz about us, bite and pester us, sting and poison us, the spiders that shroud our rooms and cellars, the lice that infest us, and earwigs that creep and run from every stone we turn. It is a strange, competitive world side by side with the chordate phylum, unsympathetic, alien, and for the most part hostile and malignant to it.

We can best consider its distinctive differences from other phyla by taking first a concrete example, and for that the size and accessibility of the lobster (Homarus vulgaris) make it a convenient type.

The general appearance of a living lobster is sufficiently well known. His prevailing colour is dark blue; the familiar red colour of boiled lobster only appears in the boiling. On contrasting him with a vertebrate, two outstanding differences immediately present themselves ; first, his whole body is covered with sheet armour; second, in the place of our legs and arms he is provided with a great number of hard, jointed limbs.

The front half of the lobster is protected above and at the sides by a single sheet of stiff armour. This part is called the cephalothorax—meaning head and chest in one. The hinder half consists of a chain of seven movable segments ; each segment is strongly protected, but in between the segments the layer of armour is very thin—so thin that it is flexible. This part is called the abdomen —absurdly, as we shall explain. Its segments are jointed together in such a way that the abdomen can either be held straight out as in the figure or curved forward under the cephalothorax ; by suddenly flapping his abdomen forward in this way the lobster can jerk himself rapidly backwards through the water, and it is by means of a series of such flaps that he retires when seriously alarmed.

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§ 2. The Arthropod Plan of Structure. § 3b. Spiders, Mites, and

On the lower side of the abdomen there are six pairs of limbs or appendages, one pair to each segment except the last. Below the cephalothorax, there are thirteen more pairs of appendages. The structure of these appendages varies with their situation and the duties that they are intended to perform. Most conspicuous are the two pairs of whip-like feelers in front, the enormous claws, the four pairs of walking legs, and the last pair of all which, together with the last abdominal segment, form a flat plate used in the abdomen-flapping mode ofretreat. But besides these obvious appendages there are others. There is a battery of six pairs round the mouth, having various shapes and used for smelling, tearing up and sorting food ; there is a series below the abdomen of which in the male the first two pairs are specially modified for transferring spermatozoa to the female. Moreover, some of the appendages of the cephalothorax bear gills on their bases, and are constructed in various ways so that a current of water may be passed over the gills.

It is interesting to compare this series of specialized appendages with our own two pairs of limbs. Our legs are designed for one function only—they support and propel us—but our arms are essentially unspecialized structures. From an evolutionary point of view they are primitive. Our hands have not committed themselves, so to speak, to the exclusive performance of any one act ; they are plastic, in the sense that they can do a variety of different things. A lobster does not possess anything that could be called a hand. Every one of his nineteen pairs of limbs is designed for the efficient and exclusive performance of some particular duty. They are not so much limbs as tools. A lobster is like an armless carpenter who has his mallets and planes, his awls and chisels and reamers, his saws, bits, drills, axes, augers, and sandpaper—even his knives and forks and spoons and toothpickssprouting in a double row out of the front of his chest and belly. He is like a penknife