CHAPTER TWELFTH.

METAMORPHOSES OF ANIMALS.

366. UNDER the name of metamorphoses are included those changes which the body of an animal undergoes after its birth, and which are modifications, in various degrees, of .ts organization, form, and its mode of life. Such changes are not peculiar to certain classes, as has been so long supposed, but are common to all animals, without exception.

367. Vegetables also undergo metamorphoses, but with this essential difference, that in vegetables the process consists in an addition of new parts to the old ones. A succession of leaves, differing from those which preceded them, comes on each season; new branches and roots are added to the old stem, and woody layers to the trunk. In animals, the whole body is transformed, in such a manner that all the existing parts contribute to the formation of the modified body. The chrysalis becomes a butterfly; the frog, after having been herbivorous during its tadpole state, becomes carnivorous, and its stomach is adapted to this new mode of life; at the same time, instead of breathing by gills, it becomes an air-breathing animal; its tail and the gills disappear; lungs and legs are being developed, and, finally, it is to live and move on land.

368 The nature, the duration, and importance of metamorphoses, as also the epoch at which they take place, are infinitely varied. The most striking changes which naturally present themselves to the mind when we speak of metamor

phoses, are those occurring in insects. Not merely is there a change of physiognomy and form observable, or an organ more or less formed, but their whole organization is modified. The animal enters into new relations with the external world, while, at the same time, new instincts are imparted to it. It has lived in water, and respired by gills; it is now furnished with air-tubes, and breathes in the atmosphere. It passes by, with indifference, objects which before were attractive, and its new instincts prompt it to seek conditions which would have been most pernicious during its former period of life. All these changes are brought about without destroying the individuality of the animal. The mosquito, which to-day haunts us with its shrill trumpet, and pierces us for our blood, is the same animal that, a few days ago, lived obscure and unregarded in stagnant water, under the guise of a little

worm.

369. Every one is familiar with the metamorphoses of the silk-worm. On escaping from the egg, the little worm or caterpillar grows with great rapidity for twenty days, when it ceases to feed, spins its silken cocoon, casts its skin, and remains enclosed in its chrysalis state. During this period of its existence, most extraordinary changes take place. The jaws with which it masticated mulberry leaves are transformed into a coiled tongue; the spinning organs are reduced; the gullet is lengthened and more slender; the stomach, which was nearly as long as the body, is now contracted into a short bag; the intestine, on the contrary, becomes elongated and narrow. The dorsal vessel is shortened. The ganglions of the thoracic region approach each other, and unite into a single mass. Antennæ and palpi are developed on the head, and instead of simple eyes appear compound ones.

* In the raising of silk-worms this period is not waited for, but (he ani mal is killed as soon as it has spun its cocoon.

The muscles, which before were uniformly distributed, (159,) are now gathered into masses. The limbs are elongated, and wings spring forth from the thorax. More active motions then reappear in the digestive organs, and the animal, bursting the envelop of its chrysalis, issues in the form of a winged rnoth.

370 The different external forms which an insect may assume is well illustrated by one which is unfortunately too well known in this country, namely, the canker-worm. Its eggs are laid on posts and fences, or upon the branches of our apple-trees, elms, and other trees. They are hatched about the time the tender leaves of these trees begin to unfold.

The caterpillar (a) feeds on the leaves, and attains its full growth at the end of about four weeks, being then not quite an inch in length. It then descends to the ground, and enters the earth to the depth of four or five inches, and having excavated a sort of cell, is soon changed into a chrysalis or ymph, (b.) At the usual time in the spring, it bursts the skin, and appears in its perfect state, under the form of a moth, (d.) In this species, however, only the male has wings. The perfect insects soon pair, the female (c) crawls up a tree, and, having deposited her eggs, dies.

371. Transformations no less remarkable are observed among the Crustacea. The metamorphoses in the family of Cirrhipedes are especially striking. It is now known that he barnacles, Balanus,) which have been arranged among the mollusks, are truly crustaceans; and this result of modern researches has been deduced in the clearest manner from the

study of their transformations. The following figures represent the different phases of the duck-barnacle, (Anatifa.)

372. The Anatifa, like all crustacea, is reproduced by eggs, specimens of which, magnified ninety diameters, are represented in figure 148, a. From these eggs little animals issue, which have not the slightest resemblance to the parent. They have an elongated form, (b,) a pair of tenacles, and four legs, with which they swim freely in the

water.

373. Their freedom, however, is of but short duration. The little animal soon attaches itself by means of its tentacles, having previously become covered with a transparent shell, through which the outlines of the body, and also a very distinct eye, are easily distinguished, (Fig. 148, c.) Figure 148, d, shows the animal taken out of its shell. It is plainly seen that the anterior portion has become considerably enlarged. Subsequently, the shell becomes completed, and the animal casts its skin, losing with it both its eyes and its tentacles. On the other hand, a thick membrane lines the interior of the shell, which pushes out and forms a stem, (e,) by means of which the animal fixes itself to immersed bodies, after the loss of its tentacles. This stem gradually enlarges, and the animal soon acquires a definite shape. such

as it is represented in figure 148, f, attached to a piece of floating wood.

374. There is, consequently, not only a change of organization in the course of the metamorphoses, but also a change of faculties and mode of life. The animal, at first free, becomes fixed; and its adhesion is effected by totally different organs at different periods of life, first by means of entacles, which were temporary organs, and afterwards by means of a fleshy stem developed especial.y for that

purpose.

375. The Radiata also furnish us with examples of various metamorphoses, especially among the star-fishes. A small species living on the coast of New England (Echinaster sanguinolentus) undergoes the following phases, (Fig. 149.)

S.

Fig. 149.

*

376. If the eggs are examined by the microscope, each one is found to contain a small, pear-shaped body, which is the embryo, (e,) surrounded by a transparent envelop. On escaping from the egg, the little animal has an oblong form, with a constriction at the base. This constriction becoming deeper and deeper forms a pedicle, (p,) which soon divides into three lobes. The disk also assumes a pentagonal form, with five double series of vesicles. The first rudiments of the rays are seen to form in the interior of the pentagon. At the same time, the peduncle contracts still more, being at last entirely absorbed into the cavity of the body, and the animal soon acquires its final form, (m.)