METAMORPHOSIS IN ANIMALS, the changes which in many animals take place be tween the time of birth and maturity. The causes are most probably changes of habitat, of seasons, of food and the acceleration in growth resulting from the approach of sexual maturity. Familiar examples are the change of the cater pillar into the butterfly, of the tadpole into the toad or frog.
In the frog and toad, metamorphosis is com plete and thoroughgoing. The embryo on hatch ing from the egg has a large head and body, but no tail; in a few days its tail grows om and it becomes a tadpole, when it is fisb-like but without fins or limbs. With the growth of the tail, the external gills appear, and the mouth is formed, while the intestine become very long and closely coiled. The mouth it armed with horny, comb-like plates, on which develop great numbers of microscopic teeth. which are shed continuously. In changing from the tadpole to the toad or frog, the body, in cluding the skull and rest of the skeleton, ar.: the viscera are made over anew ; the extenla gills disappear as the lungs develop, the Inc teeth appear in the jaw, the intestine become short and straight, and the creature instead of nibbling decaying leaves or dead animals feeds on living snails and insects. Soon the tail be comes absorbed, finally the hind legs grow ow. the gills disappear, the front legs bud out and the adult form is attained. While most am phibia pass through such a metamorphosis, in a few forms, owing to the absence of water or other changes in the environment, developmem is direct, the metamorphosis being suppressed. Metamorphosis may be retarded by cold and shortened by hunger, and in the amphibians or in insects the changes are greatest in model: and specialized forms, as frogs and toads.
The metamorphosis of the butterfly is the most complete of those of all insects. The of the insects is divided into four stages, that is. the egg, larva, pupa and imago or adult. DUI• ing the larval and pupal periods the insect is, sc to speak, a different animal from the adult The caterpillar is provided with big jaws and eats voraciously; in shape and structure r differs widely from the winged adult. The
pupa or chrysalis is also different from the larva, and also from the imago; it talces food and rests almost motionless. On the other hand, the butterfly has no jaws, while its maxilla form a long coiled-tip tongue, beauti fully adapted for probing the corollas of flowers. Though these changes appear to be sudden, the internal alterations of cells ani tissue which lead to them are gradual. Free one to three days before assuming the pupa state the caterpillar becomes restless and stop< eating. If a spinner it spins a cocoon, or if n enters the earth to undergo its transforma tion into a chrysalis, or if a larval butterfly attaches itself to some fixed object, as a tete or fence. Profound changes now take place in the mouth-parts as well as the nervous, mus cular and other systems of internal organs.
But the changes are most marked in the flesh flies and their allies. At the end of the got stage, the internal organs are destroyed. breaking up and forming a creamy mass, aid the appendages and wings arise from minvic internal masses of cells called "imaginal buds' which are present in the maggot. The body thus entirely made over anew. But while the process of destruction of the larval organs ar' appendages goes on, there is also a constructil: process, during which the organs of the adnIT state are being built up. It is thus evident dui the sharp division of the life-history of the insect into larval, pupal and imaginal stages oziv applies to the external surface of the both The internal processes of development, on the other hand, form a continuous series of trans formations between which is no sharp line ef demarcation. Yet as a whole the form of the larva, papa and imago are kept distinct in adaptation to their separate environments and habits.