Whether this loss of locomotion is to be regarded as a retrogression on the part of the plant, or animal, which becomes fixed, may be questioned; but there are curious indications of positive retrogression from a higher standard in the metamorphoses of some animals. Thus the beautiful marine worm, _Terebella_, which secretes a tube for itself, and lives in it, fixed to the rock, or oyster-shell, has in early life a distinct head, eyes, and feelers; but in growing to maturity, it loses all trace of head, eyes, and even of feelers, unless the beautiful tuft of streaming threads which it waves in the water be considered as replacing the feelers. There are the Barnacles, too, which in the first stage of their existence have three pairs of legs, a very simple single eye, and a mouth furnished with a proboscis. In the second stage they have six pairs of legs, two compound eyes, complex in structure, two feelers, but _no mouth_. In the third, or final stage, their legs are transformed into prehensile organs, they have recovered a mouth, but have lost their feelers, and their two complex eyes are degraded to a single and very simple eye-spot. [Illustration: Fig. 12. WATER BEETLE and its larva.] But to break up these digressions, let us try a sweep with our net. We skim it along the surface, and draw up a quantity of duckweed, dead leaves, bits of stick, and masses of green thread, of great fineness, called Conferva by botanists. The water runs away, and we turn over the mass. Here is a fine water-beetle, Dytiscus, and a larva of the same beetle, called the “Water-tiger,” from its ferocity (Fig. 12). You would hardly suspect that the slim, big-headed, long-tailed Water-tiger would grow into the squat, small-headed, tailless beetle: nor would you imagine that this Water-tiger would be so “high fantastical” as to breathe by his tail. Yet he does both, as you will find if you watch him in your aquarium. [Illustration: Fig. 13. DRAGON-FLY LARVÆ: A ordinary aspect; B with the huge nipper-like jaw extended.] [Illustration: Fig. 14. A LIMNÆCA STAGNALIS, or water snail. B PLANORBIS.] [Illustration: Fig. 15. PALUDINA VIVIPARA.]