Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!

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'''Development and evolution'''

Animals are eukaryotes, and diverged off the same stock of monoflagellate protozoa that gave rise to the green plants, fungi and choanoflagellates.  The last are especially close relatives, with collared cells appearing only among them, the sponges, and rarely in certain other animal forms.  Motile cells all have a single posterior flagellum.

Adult animals are typically diploids, producing small motile sperm and large non-motile eggs.  In all forms the fertilized zygote intiallvy divides to form a hollow sphere called a ''blastula''.  This then undergoes rearrangement and differentiation.  Blastulae are probably representative of the sort of colonies animals evolved from; similar forms occur among other flagellates, e.g. ''Volvox''.  However none of these other groups really ever progressed further, and large multicellular forms tend to develop by progressive growth instead.

What makes animals different?  The answer lies in the way the cells are held together.  Instead of being simply stuck together or held in place by thick walls, animal cells are linked by septate junctions, composed mainly of elastic proteins - collagen is characteristic - that make up the extracellular matrix.  Sometimes this is calcified to form shells, bones, or spicules, but otherwise it's fairly flexible and can serve as a framework, upon which cells can move about and be reorganized.

The first forms that might represent animals appear in the fossil record around the end of the [[Precambrian]].  These are called [[Vendian Biota]] and are exceedingly difficult to relate to later forms.  Other than them, virtually every phylum makes a more or less simultaneous appearance during the [[Cambrian]].  This massive adaptive radiation may have come about because of climate change or a simple genetic innovation, and is so sudden that it is usually called the cambrian explosion.


The sponges ([[Porifera]]) were separated from the other animals early on, and are very different.  Sponges are sessile and usually feed by drawing in water through pores all over the body, which is supported by a skeleton typically divided into spicules.  The cells are differentiated, but not organized into distinct groups.

There are three problematic phyla that show an unclear relation to the other animals - the [[Rhombozoa]], [[Orthonectida]], and [[Placozoa]].  After these, all animals belong to a clear group called the Metazoa.  In these forms the blastula invaginates to form a ''gastrula'' with a clear digestive chamber and three separate layers of cells - called the ectoderm, mesoderm, and endoderm - that differentiate into various tissues, including muscles and nerves.

Lower metazoa:

* Phylum [[Cnidaria]] (=coelentrata)
* Phylum [[Ctenophora]]
* Phylum [[Platyhelminthes]] - flatworms, probably paraphyletic to higher groups
* Phylum [[Rotifera]]
* Phylum [[Acanthocephala]]


* Phylum [[Nemertea]]
* Phylum [[Mollusca]]
* Phylum [[Sipuncula]]
* Phylum [[Echiura]]
* Phylum [[Pogonophora]]
* Phylum [[Vestimentifera]]
* Phylum [[Annelida]]


* Phylum [[Cephalorhyncha]] (=priapulida, kinorhyncha, loricifera)
* Phylum [[Nematoda]]
* Phylum [[Nematomorpha]]
* Phylum [[Onychophora]]
* Phylum [[Tardigrada]]
* Phylum [[Arthropoda]] (e.g. insecta, arachnida, crustacea)


* Phylum [[Brachiopoda]]
* Phylum [[Ectoprocta]] (=bryozoa)
* Phylum [[Phoronida]]


* Phylum [[Chaetognatha]]
* Phylum [[Echinodermata]]
* Phylum [[Hemichordata]]
* Phylum [[Chordata]] (e.g. vertebrates)