Overview of the animal kingdom

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. 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 below), 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.

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

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 below.

• 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.

Three types of animal skeleton

Hydrostatic skeleton. This 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.