Postanal tail - extensions of the notochord and nerve cord. Class names in this key are linked to other zoology class keys. All vertebrate systems work toward osmoregulation , the removing of excess water in freshwater species and conserving of water in marine and terrestrial species. The vertebrate body is much more complex than the invertebrate. There is a longer period of development until vertebrates reach sexual maturity.
Amphibians are the only vertebrates that go through any type of metamorphosis after being born.
Under certain conditions amphibian reproduction by a larvalike adult occurs, neoteny , but this is a very rare exception. The fertilized egg has a complete set of chromosomes and begins to go through rapid cell division, clevage . This developing mass of cells reaches a critical size and must make a transformation. In order for all the cells to remain in contact, they must form a blastula , a hollow sphere of cells. The blastula continues to increase in size, but the surface remains only a few cell layers thick.