Wikipedia 10K Redux

Reconstructed by Reagle from Starling archive; see blog post for context.

Animalia

Introduction

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 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.

Systematics

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 - the Rhombozoa, Orthonectida, and Placozoa - that have an unclear position with respect to other animals. After these, all animals belong to a monophyletic group called the Metazoa, characterized by a digestive chamber and separate cell layers that differentiate into various tissues, including nerves and muscles.

The simplest Metazoa are radially symmetric and diploblastic, that is, they have two germ layers. The outer layer (ectoderm) corresponds to the surface of the blastula and the inner layer (endoderm) is formed by cells that migrate into the interior. It then invaginates to form a digestive cavity with a single opening. This form is called a gastrula or planula when it is free-swimming. The Cnidaria (jellyfish, anenomes, corals, etc) are the main diploblastic phylum; the Ctenophora (comb jellies) may also belong here.

The remaining forms comprise a group called the Bilateria, since they are usually bilaterally symmetric, and are triploblastic. The blastula invaginates without filling in first, so the endoderm is simply its inner lining, and the interior then fills in to become a third layer (mesoderm) between the others. Like tissues are grouped into organs. The simplest of such animals are the Platyhelminthes (flatworms) which may be paraphyletic to the higher phyla.

The vast majority of the triploblastic phyla form a group called the Protostomia. These forms have a one-way digestive cavity, with a second opening developing to pass waste through. The mesoderm arises as in the flatworms, from a single cell, and then divides to form a mass on each side of the body. Usually there is a hollow space around the gut, called the coelom, arising from a split within the mesoderm, or at least some reduced version thereof (eg a pseudocoelom, where the split occurs between the mesoderm and endoderm).

Incertae sedis (at least I don't know where to put them):

Trochozoa:

Ecdysozoa:

Lophophorata:

Deuterostomia: