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

THE SECOND GREAT PHYLUM:

embryonic development and in other respects they show many resemblances to the insects. The best-known members of the group are the flat-bodied centipedes and the rounded millipedes; they may be distinguished by the fact that in the latter the body-segments are welded together in pairs, so that there appear to be four legs to each segment, while in the former there are only two. The centipedes are fierce, active creatures, stalking small invertebrates and killing them by means of their strong, poisonous jaws. The millipedes are inoffensive plant-feeders, with a preference for decaying plants ; their bite is not poisonous, but some of them, if seriously disturbed, can exude a foul-smelling fluid in self-defence.

§ ge Peripatus

Finally we come to a class, the Onychophora, containing a few species only, all belonging to the single genus Peripatus. The animals are humble creatures, found hiding under stones or damp bark in South

America, Africa, Malaya, and Australasia.

To the layman they are harmless unimportant grubs, but to the zoologist their anatomy is full of interest.

Peripatus differs in a number of important respects from other arthropods. Its chitinous integument is comparatively thin and beset with little spiny warts ; being flexible

THE ARTHROPODS

it shows no jointing on the limbs or division into body-segments. The numerous appendages show less specialization of structure than do those of other arthropods. In its general appearance the animal looks half-way between a centipede and a worm.

In its internal anatomy the animal shows many arthropod features. It has a bodycavity that contains blood, simple unbranched trachee, and so on. But in other respects it resembles the worms. It has a series of kidney-tubes, one pair corresponding to each pair of legs like the kidney-tubes of an earth-worm, and its central nervous system, instead of lying along the belly, forms two distinct cords running along the two sides of the body and connected together by regular communications like the rungs of a ladder—a condition that is paralleled by the flat-worms and by some molluscs.

And in its development, it, like seasquirts or Sacculina, drops clues as to its affinities. Early in embryonic life, it has an ordinary blood-system, with veins and arteries, and an ordinary body-cavity with no blood in it, just like a worm. Gradually some of the veins enlarge, compress the original body-spaces into insignificance and become the blood-filled body-cavity of the adult.

Peripatus, then, is an arthropod which shows many resemblances to animals of other phyla, particularly the segmental worms ; it seems to bridge much of the gulf between the arthropods and this latter group.

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