KingdomAnimalia

Introduction

Development and evolution

Adult animals are typically diploids, producing small motile sperm and large non-motile eggs. In all forms the fertilized zygote intially 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 Precambrian. These are called Vendians 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 CambrianExplosion.

Systematics and ecology

Phyla of animals

Parazoan and mesozoan animals:

• PhylumPorifera

• PhylumRhombozoa

• PhylumOrthonectida

• PhylumPlacozoa

Lower metazoa:

• PhylumCnidaria (=Coelentrata)

• PhylumCtenophora

• PhylumPlatyhelminthes - probably paraphyletic to higher groups

• PhylumRotifera

• PhylumAcanthocephala

Trochozoa:

• PhylumNemertea

• PhylumMollusca (e.g. AbalonE)

• PhylumSipuncula

• PhylumEchiura

• PhylumPogonophora

• PhylumVestimentifera

• PhylumAnnelida

Ecdysozoa:

• PhylumCephalorhyncha (=priapulids, kinorhynchs, loriciferans)

• PhylumNematoda

• PhylumNematomorpha

• PhylumOnychophora

• PhylumTardigrada

• PhylumArthropoda

Lophophorata:

• PhylumBrachiopoda

• PhylumEctoprocta (=Bryozoa)

• PhylumPhoronida

Deuterostomia:

• PhylumChaetognatha

• PhylumEchinodermata

• PhylumHemichordata

• PhylumChordata (e.g. HomoSapiens, AardvarK, WolF)