Overview of the animal kingdom

For evolution to occur it is not enough to simply succeed - others must fail.

Overview of the Kingdom Animalia

The Kingdom Animalia is divided into Phyla (see below), and is believed to have evolved from single-celled protist ancestors about 1000 million years ago.

There are generally thought to be about 35 extant Phyla, but this number varies according to different workers.

CHARACTERISTICS OF ANIMALS

Multicellular, so different cells can specialise.

Absence of chlorophyll.

Taking into account fossil evidence and the length of time assumed necessary for evolution; it is probable that most of the Phyla were in existence by the Cambrian.

Two of these Phyla, the Porifera and the Placozoa (see right), are sufficiently different from the others to be grouped apart, dividing the Kingdom into two Sub-kingdoms; the Sub-kingdom Parazoa containing the Porifera and the Placozoa, and the Sub-kingdom Eumetazoa containing all the other Phyla.

The Parazoa are evolutionarily the earliest forms of animals, but there is no evidence from the fossil record that any other forms evolved from them.

It is assumed that most of the phyla in the Eumetazoa diversified from a flatworm-like descendant in the Precambrian.

The Eumetazoa is divided according to the symmetry of the animals. Two Phyla are radially symmetrical, the Cnidaria and the Ctenophora. The other Phyla are bilaterally symmetrical. However some animals in the Phylum Echinodermata are secondarily radially symmetrical, e.g. sea urchins and starfish; it is believed that they evolved from bilaterally symmetrical animals.

Animal Kingdom

4 Primary functions an animal requires to exist

Breathing: obtaining a supply of oxygen.

Feeding: obtaining a steady supply of food.

Excretion: getting rid of waste materials.

Reproduction: making sure there are offspring around to replaces it when it dies.

Structural features are associated with bilateral animals:

Elaboration of antagonistic musculature with circular and longitudinal muscles
Acquisition of a body cavity and peristalsis to move food along the gut
Cephalisation, with the mouth and nervous tissues concentrated at the front
Segmentation, this is found in all animals from the Platyhelminthes onwards
Elaboration of the mesoderm with connective tissues, a blood system and muscles.

The Bilateral Phyla can be further subdivided into three groups - see the diagrams on the right.

The Acoelomates have no body cavity surrounding the gut.
Pseudocoelomates have a body cavity surrounding the gut but the gut itself is not surrounded by mesoderm.
Coelomates have a true gut where the whole cavity is lined by mesoderm.

The table below shows how the bilaterally symmetrical phyla are distributed according to the presence or absence of a coelom.

Acoelomate	Pseudocoelomate	Coelomate
Platyhelminthes flatworms, flukes, tapeworms	Rotifera	Priapulida	Chelicerata spiders, scorpions, horseshoe crabs
Nemertea ribbon worms	Gastrotricha	Sipuncula	Pentastomida tongue worms
Gnathostomulida	Kinorhyncha	Mollusca snails, slugs, squid, octopus, clams, ship worms	Phoronida
Mesozoa	Nematoda round worms, hook worms	Echiura	Bryozoa
	Nematomorpha hair worms	Annelida worms, leeches	Brachiopoda lamp shells
	Acanthocephala	Pogonophora	Chaetognatha Arrow worms
	Entoprocta	Tardigrada water bears	Echinodermata star fish, sea urchins, sea cucumbers
	Loricifera	Onychophora velvet worms	Hemichordata
		Crustacea crabs, shrimps, barnacles	Chordata sea squirts, vertebrates
		Uniramia insects, millipedes, centipedes

Three types of animal skeleton

Hydrostatic skeleton. The consists of pressurised fluid within a compartment (think of the air which gives an air bed rigidity). The muscles change the shape of the fluid-filled compartment.

Found in Cnidaria, Platyhelminthes, Nematoda and Annelida, etc.

Exoskeleton. Where the skeleton forms the rigid outer part of the animal. In order to increase in size the animal must moult to expand the shell.

Found in Uniramia, Chelicerata, some Mollusca.

Endoskeleton. Where the skeleton forms inside the body, and the muscles anchor to the outside of the skeleton. To increase body size there is no need to moult.

Found in Chordata, Porifera, Echinodermata.