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.
CHARACTERISTICS OF ANIMALS
Multicellular, so different
cells can specialise.
Absence of chlorophyll.
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
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.
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.
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
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.