developmental stages, so that at that time it looks like and really is like the mature stage of some tailed crustacean like a crayfish. A barnacle, which looks little like a crayfish or crab in its mature stage, is hardly to be distinguished in its immature life from a young crab or lobster. Sacculina, which is a still more degenerate crustacean, is only a sort of feeding sac with rootletlike processes projecting into the body of the host crab on which it lives as a parasite, but the young free

swimming Sacculina is essentially

like a barnacle, crayfish, or crab in its young stage.

However, it is obvious that this recapitulation or repetition of ancestral stages is never perfect, and it is often so obscured and modified by interpolated adaptive stages and characters that but little of an animal's ancestry can be learned from a scrutiny of its development. The fascinating biogenetic law of Müller and Haeckel summed up in the phrase, "ontogeny is a recapitulation of phylogeny," must not be too heavily leaned on as a support for any speculations as to the phyletic affinities of any species or group of species of organisms. "Embryology is an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand."

FIG. 137.

Metamorphosis of a barnacle, Lepas: a, Larva; b, adult.

While a young robin when it hatches from the egg or a young kitten at birth resembles its parents, a young starfish or a young crab or a young butterfly when hatched does not at all resemble its parents. And while the young robin after hatching becomes a fully grown robin simply by growing larger and undergoing comparatively slight developmental changes, the young starfish or young butterfly not only grows larger, but undergoes some very striking developmental changes; the body changes very much in appearance. Marked changes in the body of an animal during post embryonic or larval development constitute what is called metamorphic development,

or the animal is said to undergo or to show metamorphosis in its development.

This metamorphosis is familiar to all in insects; to zoologists, it is familiar among numerous other kinds of animals. Fig. 138

FIG. 138.-Metamorphosis of the Monarch butterfly, Anosia plexippus: a, Egg; b, larva; c, pupa; d, imago, or adult.

shows the different stages in the metamorphic development of the common large red-brown milkweed butterfly, Anosia plexippus. From the egg hatches a crawling, wormlike larva, wingless, without compound eyes, and with strong jaws and other mouth parts fitted for biting. This creature develops into the winged butterfly with different eyes, different antenna, different mouth parts, different almost everything. And, by the intervention of a curious quiescent stage called the pupal

or chrysalid stage, the changes seem to be made by sudden leaps. Of course, this is not so. It is all done gradually, although there are certain periods in the course of the development when the changing is more rapid and radical than at other times. The changing is masked by the outer covering of larva and pupa, and although it is indeed startlingly radical in its character, it is wholly continuous.

The metamorphosis of frogs and toads also is familiar.

The eggs of the toad are arranged in long strings or ribbons in a transparent jellylike substance. These jelly ribbons with the small, black, beadlike eggs in them are wound around the stems of submerged plants or sticks near the shores of the pond. From each egg hatches a tiny, wriggling tadpole, differing nearly as much from a full-grown toad as a caterpillar differs from a butterfly. The tadpoles feed on the microscopic plants to be found in the water, and swim easily about by means of their long tails. The very young tadpoles remain underneath the surface of the water all the time, breathing the air, which is mixed with the water, by means of gills. But as they become older and larger they come often to the surface of the water. Lungs are developing inside the body, and the tadpole is beginning to breathe as a land animal, although it still breathes partly by means of gills, that is, as

an aquatic animal. Soon it is apparent that although the tadpole is steadily and rapidly growing larger, its tail is growing shorter and smaller instead of longer and larger. At the same time, fore and hind legs bud out and rapidly take form and become functional. By the time that the tail gets very short indeed, the young toad is ready to leave the water and live as a land animal. On land the toad lives, as we know, on insects and snails and worms. The metamorphosis of the toad is not so striking as that of the butterfly, but if the tadpole were inclosed in an unchanging opaque body wall while it was losing its tail and getting its legs, and this wall were to be shed after these changes were made, would not the metamorphosis be nearly as extraordinary as in the case of the

At left, strings of eggs; in water, various tad

pole or larval stages; and on the bank, the adult toads. (Partly after Gage.)

butterfly? But in the metamorphosis of the toad we can see the gradual and continuous character of the change.

Many other animals, besides insects and frogs and toads, undergo metamorphosis. The just-hatched sea urchin does not resemble a fully developed sea urchin at all. It is a minute

wormlike creature, provided with cilia or vibratile hairs, by means of which it swims freely about. It changes next into a curious bootjack-shaped body called the pluteus stage. In the pluteus a skeleton of lime is formed, and the final true seaurchin body begins to appear inside the pluteus, developing and growing by using up the body substance of the pluteus. Starfishes, which are closely related to sea urchins, show a

stage.

b

similar metamorphosis,

except that there is no pluteus stage, the true starfish-shaped body forming within and at the expense of the first larval stage, the ciliated free-swimming stage.

A young crab just issued from the egg (Fig. 141) is a very different appearing creature from the adult or fully developed crab. The body of the crab in its first larval stage is composed of a short, globular portion, furnished with conspicuous long spines and a relatively long, jointed tail. This is called the zoëa

The zoëa changes into a stage called the megalops, which has many characteristics of the adult crab condition, but differs especially from it in the possession of a long, segmented tail, and in having the front half of the body longer than wide. The crab in the megalops stage looks very much. like a tiny lobster or shrimp. The tail soon disappears and the body widens, and the final stage is reached.

In many families of fishes the changes which take place in the course of the life cycle are almost as great as in the case of the insect or the toad. In the ladyfish (Albula vulpes) the very young are ribbonlike in form, with small heads and very loose texture of the tissues, the body substance being jelly