Animal Forest

History Of Animals:

Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms for sustenance.

Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.

Contents [hide] 1 Etymology 2 Characteristics 2.1 Structure 2.2 Reproduction and development 2.3 Food and energy sourcing 3 Origin and fossil record 4 Groups of animals 4.1 Porifera, Ra diata and basal Bilateria 4.2 Deuterostomes 4.3 Ecdysozoa 4.4 Platyzoa 4.5 Lophotrochozoa 5 Model organisms 6 History of classification 7 See also 8 References 8.1 Notes 8.2 Bibliography 9 External links

A newt lung cell stained with fluorescent dyes undergoing mitosis, specifically early anaphase.Nearly all animals undergo some form of sexual reproduction. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals.

Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, or in some cases through fragmentation.

A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs