Metazoa are multicelled animals with three layers of cells. Why multicelled animals evolved is succinctly stated by Meglitsch (1972, p 78): "The highly plastic and successful protozoan organisational plan imposed but one serious limitation - that of size. An undivided mass of protoplasm becomes physiologically and structurally ineffective if too large. It suffers from lack of mechanical strength if flattened or elongated, and from lack of surface area if it approaches a spherical shape. Large animals are of necessity multicellular."
There are colonies of protozoans but their cells are not differentiated except for reproductive cells. In the metazoa the nonreproductive or somatic cells are differentiated during development in order to carry out different functions in the body. The first differentiation is into the cell layers called ectoderm, endoderm and mesoderm.
The simplest forms of multicelled animals are called the Mesozoans. These animals have only a single layer of ectodermal cells on the outside and no inner layer corresponding to endoderm is found. They contain reproductive cells inside the ectodermal ball and so resemble the blastula phase of embryonic development in higher animals. There are only a few species of Mesozoa and they are all parasites. Whether they are related to the ancestors of metazoans or just degenerate forms of some other higher group is not yet established, but they do point to what this stage of evolutionary development might have looked like.
The next level up are the Parazoans, better known as the sponges. These animals have two layers of cells: an outer covering of ectoderm and an inner layer of cavities lined with endodermal cells which resemble protozoan flagellates and which ingest food and push water through the cavities.
All the rest of the animal kingdom consists of Metazoans, where cellular differentiation has led to three cell layers. First the ectoderm and endoderm form. Then, in between these two layers the mesoderm secondarily forms, the cells of which become muscles and other important internal structures.
All Metazoans begin life as a zygote or fertilised egg, which is structurally similar to a protozoan and shows our protozoan roots. Then cleavage begins, when the cell divides and subdivides over and over again until a blastula or ball of cells is formed. In echinoderms and chordates, cleavage is indeterminate, which means that if the cleaved cells separate early in the process, twins, triplets and quadruplets can develop from the individual cells. In the rest of the metazoa, if the early cells are separated they grow into different parts of a larva and then die (determinate cleavage).
Once the blastula is formed, gastrulation can begin. In this phase cells begin to migrate around to form the first two layers, endoderm and ectoderm. An invagination in the ball occurs and this forms the primitive gut with a central cavity and an opening to the outside called a blastopore. In most metazoans the blastopore becomes the mouth but in a few groups, notably the Echinoderms and Chordates again, the blastopore closes and a different opening forms the mouth.
The ectodermal layer gives rise not only to skin and other outer coverings but also forms the nervous system and the sense organs. The endodermal layer gives rise to the lining of the digestive tract plus the glands arising from it. The next layer to develop is the mesoderm and is derived from the first two so is considered a secondary layer while the other two are the primary layers. In the jellyfish and comb jellies the mesoderm is derived entirely from the ectoderm. In all the other metazoans most of the mesoderm comes from the endoderm.
"Metazoan architecture is best understood by considering the relationship of the germ layers and their derivatives to the important body cavities" (Meglitsch, p85). In jellyfish, comb jellies and the flatworms, the only body cavity is the gut. In higher invertebrates the mesoderm does not completely fill the space between the endo and ecto layers and the cavity formed is either a pseudocoelom or a true coelom. In pseudocoelomates the cavity forms between the endo and meso layers, while a true coelom is formed wholly within the mesoderm. Nematodes and other worms that were once lumped in the phylum aschelminthes all have pseudocoeloms. Eventually this phylum was split up when it was discovered that the pseudocoelom formed in different ways in different groups so this phylum was made up of many unrelated types that now warrent their own separate phyla.
The vast majority of metazoans have a true coelom although it may develop in different ways in different groups. One of the important advantages of having this second cavity in the body is that muscles can be moved independently of the gut. In flatworms with no coelom, when the muscles move, the gut moves and vice versa, which places limitations on the animal's functions.
So who are the metazoan coelomates? They can be divided into three major groups by their embryology. The largest group is the Protostomia, whose blastopore becomes a mouth. They are the molluscs, annelids and arthropods, plus the minor coelomate protostomes (priapulids, sipunculids, echiurids, tardigrades, onychophora and pentastomes). Other characteristics of protostomes are that they have spiral determinate cleavage, a trochophore-like larva and the coleom is a schizocoel, where the colom is completely surrounded by mesodermal tissue.
The second major group of metazoan coelomates are the Deuterostomes, in which the blastopore closes and another opening forms which becomes the mouth. Echinoderms and the chordates are deuterostomes as well as some other minor phyla (chaetognaths, pogonophora, hemichordata, enteropneusta, pterobranchs, and ascidians). Other characteristics of deuterostomes are that they have radial indeterminate cleavage, a dipleurula-like larva and the coelom is an enterocoel derived from the primitive gut.
In between is the third group, the lophophorate coelomates, including the phoronids, the ectoprocts and the branchiopods. These animals are not very numerous or well known but "the lophophorates appear to be near the junction of the protostome and deuterostome lines" (Meglitsch p 671). The mouth forms in a typical protostome way from the blastopore. There are both spiral and radial patterns of cleavage and the coeloms show both schizocoelous and enterocoelous development. They may in fact be the most primitive deuterostomes.
To summarise, the metazoa are multicelled animals with a three cell layers, a gut and, in the higher forms, a coelom of some sort. After that, all manner of shapes and sizes, forms and functions can be found. Next time you look at an earthworm, a dog, a snail or an insect, know that these are the characteristics that you share with them and that this can be seen in the embryological development of all Metazoan species.
Reference: P. Meglitsch 1972 Invertebrate Zoology. 2nd ed. Oxford U. Press, London.