前程百利小编为考生带来了与变态动物相关的托福阅读材料。变态动物作为动物大群体中独特的一类，是不是大家也该认识一下它们呢?
metamorphosis is a biological process by which an animal physically develops after birth or hatching involving a conspicuous and relatively abrupt change in the animal’s body structure through cell growth and differentiation. Some insects amphibians molluscs crustaceans cnidarians echinoderms and tunicates undergo metamorphosis which is usually accompanied by a change of habitat or behavior.
Scientific usage of the term is exclusive and is not applied to general aspects of cell growth including rapid growth spurts. References to "metamorphosis" in mammals are imprecise and only colloquial but historically idealist ideas of transformation and monadology as in Goethe’s metamorphosis of Plants influenced the development of ideas of evolution.
A. Frogs and toads
With frogs and toads the external gills of the newly hatched tadpole are covered with a gill sac after a few days and lungs are quickly formed. Front legs are formed under the gill sac and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long spiral‐shaped gut to digest that diet.
Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw and its gills disappear along with its gill sac. Eyes and legs grow quickly a tongue is formed and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision loss of the lateral line system etc.) All this can happen in about a day so it is truly a metamorphosis. It isn’t until a few days later that the tail is reabsorbed due to the higher thyroxin concentrations required for tail resorption.
B. Butterflies
How does a caterpillar become a Moth or a Butterfly?
The answer to that can actually get quite complicated but basically what happens is this. When the caterpillar has eaten enough it turns into a pupa more about this later on because it is different for different groups of Lepidoptera. To do this it stops eating and finds somewhere safe here it becomes very still (pupa never eat and seldomly move at all) it then moults its skin the same as it does when it is growing only instead of another larval skin it secretes a pupal skin (inside its old larval skin) that is much thicker and stronger. Generally this pupa then breaks out of the old larval skin though in many moths the pupa remains inside the old larval skin you can often find the remains of the caterpillar skin around the tail of a Butterfly pupa. All that is fairly straight forward where it gets tricky is how the caterpillar inside its new pupal case changes itself into a Butterfly or Moth. The first thing that happens is that a lot of the caterpillars old body dies. It is attacked by the same sort of juices the caterpillar used in its earlier life to digest its food it would not be far wrong to say the caterpillar digests itself from the inside out this process is called "histolysis". Not all the tissue is destroyed however some of the insects old tissue passes on to its new self the amount that does this varies between different insects and is not very much in the Lepidoptera. There is one particular sort of tissue left in a number of places in the insects body are collections of special formative cells which have played no part in the insects larval life and have stayed hidden or protected during this partial death each of these groups of cells is called an "imaginal bud" or a "histoblast". The job of these histoblasts is to supervise the building of a new body out of the soup that the insects digestive juices have made of the old larval body. This they do using the same biochemical processes that all insects use to turn their food into part of their bodies. This rebuilding process is called "histogenesis". During this time the insect is very vulnerable because it cannot run away and this is why insects try to choose somewhere safe to hide away when they are going through this incredible change still I think you have to be very brave to be a Caterpillar and become a Butterfly or a Moth.
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects amphibians or cnidarians typically have a larval phase of their life cycle.
The larva’s appearance is generally very different from the adult form. A larva often has unique structures and organs that do not occur in the adult form while their diet might be considerably different.
Larvae are frequently adapted to environments separate from adults. For example some larvae such as tadpoles live exclusively in aquatic environments but can live outside water as adult frogs. By living in a distinct environment larvae may be given shelter from predators and reduce competition for resources with the adult population.
Animals in the larval stage will consume food to fuel their transition into the adult form. Some species such as barnacles are immobile as adults and use their mobile larval form to distribute themselves.
It is a misunderstanding that the larval form always reflects the group’s evolutionary history. This could be the case but often the larval stage has evolved secondarily as in insects. In these cases the larval form may differ more than the adult form from the group’s common origin.
以上就是前程百利小编为大家带来的托福阅读材料，平时的阅读练习是托福考试阅读备考的重要一环。希望大家利用好每份材料，尽可能多地从中获取知识。预祝大家托福考试取得好成绩。
