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Blah, blah, blah.

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 animals other than the sponges, and a few problematic phyla, form a clear subgroup called the Metazoa. In these forms the blastula invaginates and cells migrate or pinch into the interior. The resulting form is called a gastrula and has a clear digestive tract, with three separate layers of cells - called the ectoderm, mesoderm, and endoderm - that differentiate into various tissues, including muscles and nerves.

The first forms that might represent animals appear in the fossil record around the end of the PrecambrianEra?. These are called VendianFauna? and are exceedingly difficult to relate to later forms. Other than them, virtually every phylum makes a more or less simultaneous appearance during the CambrianPeriod?. 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 CambrianExplosion?.

Systematics and ecology

Blah, blah, blah. Arthropods and annelids turn out not to be related, as suggested by the very different developments of the two groups. Instead arthropods seem to be related to the roundworms (NemaToda?), linked by a chitinous cuticle which is molted. What this does to proposals of their polyphyly I don't know.

Phyla of animals

Parazoan and mesozoan animals:

Lower metazoa:





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Last edited February 13, 2001 11:51 am by JoshuaGrosse (diff)