In the ETI systems, five kingdoms are distinguished, namely the:
Kingdom Monera (bacteria, blue-green algae)
Kingdom Fungia (mushrooms, fungi, moulds)
Kingdom Plantae (herbs, trees, seaweeds, ferns)
Kingdom Protoctista (unicellular or colonial; partly, entirely heterotrophic organisms)
Kingdom Animalia (multicellular, heterotrophic organisms)
In this classification scheme the traditional Protozoa belong to Kingdom Protoctista; the real animals may be defined as multicellular, heterotrophic, diploid organisms that sometime develop asexually but mostly develop anisogamously from two different haploid gametes; a usually a large egg and a smaller sperm. The product of fertilisation of the egg by the sperm is a diploid zygote that develops by a sequence of normal mitotic cell divisions. These mitoses result first in a solid ball of cells and then in a hollow ball of cells called a blastula. The defining characteristic of all Animalia is their development from a blastula. In most animals, the blastula invaginates, folding inward at a point, to form a gastrula; a hollow sac having an opening at one end. Further growth and movement of cells produce a hollow digestive system called an enteron if it is open at only at one end, and a gut or intestine if it has developed a second opening.
The details of the embryonic development differs widely from phylum to phylum but are fairly constant within each phylum. Such developmental details provide important criteria for determining relationships between the phyla. In many phyla, developmental details are known for very few species and in some phyla none are known. The phyla distinguished in the ETI products are given in the following media clip (Phyla).
Although multicellularity is found in all the kingdoms, it has developed most impressively in the animals- their cells are joined by complex junctions into tissues. Such elaborate joints- desmosomes, gap junctions, and septate junctions, for example, ensure and control communication and the flow of materials between cells. In the phylum Plantae complexity in cells can be very large and cells are surrounded by a non-living cell wall while only a living membrane is found in the Animalia.
Most animals have ingestive nutrition: they take food into their bodies and then either engulf particles or droplets of it into digestive cells by the process of phagocytosis ("cell eating") or pinocytosis ("cell drinking") or absorb food molecules through cell membranes and in between cell walls. Although behaviour of various kinds (attraction to light and avoidance of noxious chemicals, sensing of dissolved gases, and so forth) can be found in members of all five kingdoms, the animals have elaborated upon this theme, far more than members of the other kingdoms. Mammalian behaviour is perhaps the most complex.
The animals are the most diverse in form of all the kingdoms. The smallest are microscopic- smaller than many protoctists- and the largest ever existing on Earth is the Blue Whale, a marine representative of the class Mammalia in the phylum Chordata. The members of most phyla are found in shallow waters. Truly land-dwelling forms are found only in three phyla; Mollusca, Arthropoda and Chordata. Several phyla contain species that live on land and in the soil (for example, earthworms), but these require constant moisture and have not really freed themselves from an aqueous environment throughout their life cycle. In fact, most animal phyla occur in the marine environment and nearly all have aquatic representatives; only the Insecta and Onychophora do not have marine forms.
The roughly 300.000 living species of animals form only a small fraction of the animal kingdom as most species are extinct.
Of all organisms on Earth, only the animals have succeeded in actively invading the atmosphere. Whereas one can find representatives of all five kingdoms that spend significant fractions of their life cycle free flying in the atmosphere (for example, spores of bacteria, fungi, and plants), no organism in any kingdom can spend their entire life cycle free in the atmosphere, only some animals can fly. Active locomotion of animals through the air has been independently achieved several times but only in three phyla: Arthropoda, class Insecta; Mollusca, class Cephalopoda (squids); Chordata, classes Pisces (fishes), Aves (birds), Mammalia (bats), and Reptilia (several extinct flying dinosaurs).
For many years and even today, biologists divided the animals - protozoans and metazoans together into two large groups: the invertebrates, which are those without backbones, and the vertebrates with a backbone. In fact, all animals except the Phylum Chordata, belong to the invertebrate group. This invertebrate/vertebrate dichotomy amply represents the skewed perspective we have as members of Phylum Chordata. Animals closest to our size and those best known to us are members of our own phylum; our pets, beasts of burden, source of food, leather, and bone. We now realise that from a less species-centred point of view, characteristics other than backbones are more basic and reflect much earlier evolutionary divergences.
Two phyla of animals, set apart as Subkingdom Parazoa, lack tissues organised into organs and have an indeterminate shape. These are the newly discovered Placozoa and the well known sponges or Porifera. The other 27 phyla, constituting the Subkingdom Metazoa (true metazoans), have tissues organised into organs and organ systems.
There are two types of the metazoa. One consists of radially symmetrical organisms, the Cnidaria (corals) and the Ctenophora (comb jellies), forming one group and the Echinodermata. The first two phyla have planktonic forms and thus face a uniform environment on all sides; their radial symmetry is both internal and external. The other 24 phyla show bilateral symmetry, at least internally.
The bilaterally symmetrical phyla may be divided into three groups. Since theories on relations of phyla based on symmetry or on the coelom are strongly criticised it is advised not to consider groups based on these characters as phylogenetically related or taxonomic groups; however, for recognition and identification these are valuable characters.
Three types are distinguished: those that lack a coelom, those that have a body cavity but lack a true coelom, and those that develop a true coelom. What is the coelom? During the process of gastrulation three tissue layers develop in animals more complex than the cnidarians and ctenophores. These tissue layers, called the endoderm, mesoderm, and ectoderm (listed from the inside out of the animal), are the layers of cells from which the organ systems of animals develop. In general, the intestine and other digestive and excretory organs develop from endoderm, the muscle and skeletal materials and most of the reproductive system from mesoderm, and the nervous tissue, sense organs and outer integument from the ectoderm. In the coelomates, the mesodermal tissue opens to contain a space that widens to eventually form a body cavity in which the organs develop and from which they are suspended. This true body cavity is called the coelom it is lined with mesodermal tissue. A pseudocoelom (false body cavity) is an internal space that does not develop from mesoderm and it is never lined with mesodermal tissue. For animals of three phyla, Priapulida, Kynorhynchs and Gastrotricha, the nature of the body cavity is controversial.
Two series (called series as they again have no taxonomic or phylogenetic status) of coelomate animals are distinguished according to the fate of an early developmental feature called the blastopore; the hollow ball of cells into which the animal zygote develops by invagination of the blastula. This embryonic structure enlarges as cells divide and may shift in position. In animals of the series Protostoma (the majority), the blastopore eventually becomes the mouth of the adult. In the series the Deuterostoma, the blastopore becomes the anus, the rear end of the intestine; the mouth forms as a secondary opening at the end of the animal opposite from the anus. The four or five deuterostome phyla, Pogonophora, Echinodermata, Hemichrodata, Chordata and probably the Chaetognatha, are thought to have ancestors more recent ancestors than any of the protostome phyla.
Virtually all biologists agree that animals evolved from protoctists. However, which protoctists, when, and in what sort of environments are questions that are still actively debated. The Parazoa, or at least the Porifera, are thought to have evolved from the choanoflagellates (i.e., the choanomastigotes). This is deduced from details of fine structure of the cells. It is possible, in fact likely, that the other animal phyla, especially the Metazoans, had different ancestors among the protoctists.