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The shape and structure of an animal’s body help us to understand its relationship to other animals. Sponges are unusual because the shape of their body does not have any symmetry. Most animals have a body plan with one of two kinds of symmetry: radial symmetry or bilateral symmetry. Animals with radial symmetry have their body parts arranged regularly around a central axis, like the spokes of a wheel. Animals with bilateral symmetry have right and left halves that are mirror images of each other.
Two ancient phyla of aquatic animals, the cnidarians and the ctenophores, have only radial symmetry. These are the radiate animals. Radiate animals have tissues, but they lack organ systems. The cnidarians include jellyfish, corals, sea anemones, and hydras. Cnidarians have a single opening to their digestive cavity, functioning as both a mouth and an anus, through which food enters and waste is expelled. The opening is surrounded by tentacles, which are used to capture the organisms that cnidarians consume for food. Cnidarians can have the form of a medusa or a polyp.
A medusa is the familiar form of a jellyfish. It’s a bell- or umbrella-shaped form that floats free in the water. Medusas usually reproduce sexually, releasing eggs and sperm. A polyp is something like an upside-down medusa. Polyps are attached to a surface, with their tentacles extending into the water. They usually reproduce asexually. Many cnidarians have both a medusa and a polyp stage in their life cycle.
The ctenophores, or comb jellies, form the other important phylum of radiate animals. Unlike cnidarians, ctenophores have a complete digestive tract with a mouth at one end and anal pores at the other. They swim using comb-like rows of fused cilia.
Some have long sticky tentacles for capturing prey.
Most of the remaining phyla of animals are bilateral. This large category includes organisms from the simplest worms to humans. Besides the symmetry of their bodies, bilateral animals are characterized by cephalization: they have a concentration of sensory organs and nerve tissue, often a brain, at their anterior, or front, end. These are the characteristics of a head. A head is an essential adaptation in an organism that’s moving quickly through its environment. It helps the animal identify food and detect danger.
Groups of bilateral animals can be distinguished by the structure of their body cavities. All bilateral animals have a digestive cavity or digestive tract; the tissue of this cavity is derived from endoderm, the innermost tissue layer of an animal embryo. In addition, many bilateral animals have a fluid-filled cavity called a coelom, which is lined by mesoderm, the middle tissue layer of the embryo.
The coelom strengthens the animal’s body and cushions internal organs. An animal possessing a coelom is called a coelomate animal, or simply a coelomate. Some bilateral animals have a body cavity only partially lined with mesoderm-derived tissue. This cavity is called a pseudocoelom. An animal possessing a pseudocoelom is called a pseudocoelomate animal, or simply a pseudocoelomate.
Other bilateral animals completely lack a body cavity; instead, the space between their digestive cavity and outer layer is completely packed with cells derived from mesoderm. An animal that lacks a coelom is called an acoelomate animal, or simply an acoelomate.
The characteristics of body cavities are distinctive traits, but they aren’t always reliable for establishing phylogenetic relationships between animal phyla. It’s possible that groups of animals modified or lost their coeloms in the course of evolution. The best-known acoelomate animals are the flatworms. Worms of this phylum are anatomically very simple;
they have only one opening to their digestive cavity, if they have one; some lack a digestive cavity entirely! There are both free-living forms and parasites; parasitic flatworms include tapeworms and flukes. As we’ll see, not all worms are alike. The flatworms are but one of many diverse phyla of worms.
Roundworms and rotifers are two important phyla of pseudocoelomate animals. Roundworms, also called nematodes, are extremely widespread. Huge numbers of roundworms live as scavengers in soil and in water. Many others are parasites. Rotifers are tiny animals that live in water or moist soil. They’re characterized by a crown of cilia near their mouths, with which they can swim and collect food. Despite their microscopic size, they have complex organ systems.
The location of the acoelomates and pseudocoelomates in the animal family tree is still a matter of debate among biologists. But it is clear that the bilateral animals can be divided into two large groups, the distinction being clearest for the coelomates. We’ll explore this great divide next.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education