The most general trait of animals, and the first to appear during development, is their level of organization. All animals typically begin life as single cells, and some then develop to a tissue level of complexity. But others pass beyond this level and become more complex. Accordingly, Metazoa are considered to include two taxonomic branches.

In the branch Parazoa, the highest level of organization is the tissue. This branch happens to encompass just one phylum, the sponges. All other animals belong to the branch Eumetazoa characterized by the presence of organs and organ systems.

After level of organization, the next most general trait to appear in animal development is that of symmetry. At first all animal embryos are radically symmetrical; they are solid or hollow balls composed of a few cells. Some animal groups then retain this rituality right to the adult stage, but in others the embryos soon become bilateral and the later larval and adult stages usually remain bilateral as well.

On this basis the branch Eumetazoa can be subclassified into two grades, the Radiate and the Bilateria. Radiates are identified by a basic radial symmetry throughout life and also by a structural organization in which the organ is the highest level.

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The grade contains two phyla, the coelenterates and the comb jellies. All other animals belong to the Bilateria, characterized by the presence of organ systems and by bilateral symmetry after early embryonic stages. Most Bilateria retain this symmetry, but a few groups-starfishes, for example- acquire a secondary radiality when the metamorphose into adults; such adults tend to be sluggish or sessile and headless like the Radiata.

After symmetry is established in the embryo, the next most general trait to appear is the pattern of the alimentary structures. Three major patterns are known, and they add to the distinctions between Parazoa, Radiata, and Bilateria.

In sponges, the alimentary structures are unique channel networks: food-bearing water flows through systems of channels that branch throughout the body. In Radiate and one phylum of Bilateria (flatworms), the alimentary pattern is a one-hole sac. A single opening in the sac serves as both mouth and anus, and the layer of cells that forms the sac represents the endoderm, one of the primary germ layers.

In all other Bilateral the alimentary structures form a two-hole tube. Such a tube develops in the embryo from a sac that later acquires a second opening, typically opposite the first one. One opening then specializes as a mouth, the other as an anus, and the tube interconnecting them becomes the alimentary tract in which food passes only one way, from mouth to anus.

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In one group of Bilateral (Protostomia) the first opening becomes the mouth, the second the anus; in another group (Deuterostomia), the first opening forms the anus and the second the mouth.

Taken together, the level of organization, the symmetry, and the alimentary pattern provides a broad outline of the fundamental body form of any animal. Alimentary pattern and symmetry specify the basic interior and exterior architecture, respectively, and the organizational level specifies the complexity of the architectural building blocks. At such a stage of development an embryo is already clearly recognizable as, for example, a billaterial type, and in it at least two of the three primary germ layers are already present; integumentary ectoderm outside and alimentary endoderm inside.

Between these two the mesoderm now develops, from cells produced by ectoderm, endoderm, or both layers as in most cases. The pattern of mesoderm formation is an important criterion that can be used to divide the grade Bilateriainto subgrades.

In one bilaterial group, mesoderm comes to fill completely the available space between ectoderm and endoderm. Animals so constructed lack an internal body cavity; they form the subgrade Acoelomata, and they are exemplified by, for example, flatworms.

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In all other Bilateria mesoderm comes to occupy only part of the space between ectoderm and endoderm, and the remainder later becomes a fluid-filled principal body cavity. The presence of such a cavity makes the motions of the alimentary tract independent of those of the body well and the animal as a whole.

Moreover the cavity permits an animal to attain considerable size, for the fluid in the cavity can provide internal support as a hydraulic “skeleton”. It can also aid in transporting food and wastes, a necessary function that in a large animal could not be accomplished by direct diffusion between surface and deep-lying parts alone.

One group of Bilateria has a body cavity bounded on the outside directly by the body wall and on the inside directly by the alimentary system. The mesoderm here consists of cells and tissues that accumulate in certain restricted regions only.

Animals with body cavities of this type make up the sub grade Pseudocoelomata. It is exemplified by, for example, rotifers and roundworms. In all remaining Bilateria, one portion of the developing mesoderm is situated along the inner surface of the ectoderm and another portion forms around the alimentary tract.

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Most of the free space left between these mesoderm layers typically comes to be enclosed by a mesoderm membrane, the peritoneum. The vertical portions of this membrane represent mesenteries, which suspend the alimentary tract from the body wall.

Any space or body cavity enclosed completely by mesoderm tissues, and especially by a peritoneal membrane, is known as a coelom. Accordingly, animals with a coelom are said to belong to the sugared Coelomata.

The term “acoelomate” and “pseudocoelomate” now become clear. Acoelomates are animals without a coelom and indeed without a body cavity of any kind. Pseudocoelomates have a pseudocoel, or “false” coelom, a body cavity lined directly by ectoderm and endoderm, not by a peritoneum. Such a cavity resembles a true coelom superficially.

Among coelomate Bilateria in turn, subgroups can be distinguished according to how the coelomic cavities develop. In one subgroup, exemplified bymollusks, annelids, and arthropods, all adult mesoderm arises from two endoderm-derived cells, one on each side of the future gut. Each of these so-called teloblast cells then forms a teloblastic band of tissue.

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These band later splits into an outer mesoderm layer that lies along the ectoderm and an inner layer that surrounds the endoderm. Because the coelom here forms by a splitting process, it is called a schizocoel; and animals with colonic body cavities of this type make up the schizocoelomates, an assemblage roughly equivalent to a super phylum.

In another such super phylum, represented mainly by echinoderms and chordates, the mesoderm arises as pair of lateral pouches that grows out from the endoderm. These pouches later separate away from the endoderm as closed sacs, but their inner portions still remain adjacent to the alimentary system and their outer portions come to lie along the body wall.

The final condition is quite similar to that in schizocoelomates. But since the mesoderm and the coelom here are derived from the future gut (‘enteron’), the body cavity is called an intercool; and animals with such cavity are known as enterocoelomates.

In a third group or super phylum, various other, largely unique pattern of coelom formation occur. In one, for example, loose mesoderm cells migrate and eventually arrange themselves as a continuous peritoneal layer.

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Coelome developed in this and various similar ways have not been given any special technical names. The animals in this group include brachiopods (lamp shells) and other so-called lophophorates. The ancestors of this group might have been among the most ancient coelomate animals, from which both the schizocoelomate and the enterocoelomate super phyla later evolved.