TYPES OF AMPHIBIAN METAMORPHOSIS

For most of us the words "amphibian metamorphosis" call up a mental image of the life history of some common frog like Rana pipiens; a vision of a long-tailed, round-mouthed, fat-bodied, polliwog swimming sluggishly among the vegetation of a pond and then "undergoing extensive alterations without any interruption to business" to become a tailless, pop-eyed, insect-eating participant in long-distance jumping contests. We can describe the many changes, both external and internal, that are involved in this transformation and the sequence and spacing of these alterations which make up the complex pattern of metamorphosis. And we can tell our students that the amphibians are so called because of this double life they lead, beginning as weli-adapted aquatic herbivorous animals but later changing to equally well-adapted terrestrial carnivorous forms. Actually, however, we are aware that the striking metamorphosis which occurs in the leopard frog with its sharp contrast between tadpole and adult in body form, in habits, and in habitat are by no means characteristic of all amphibians. In fact a great many amphibians are not "amphibious" at all. Some live in the water throughout their lives and retain essentially the larval structure even as adults. Others, by way of contrast, do not even lay their eggs in water and the young hatch as completely terrestrial little animals with the usual adult body form. Between these two extremes are various intermediate types of life history involving a great diversity of metamorphic patterns.