The animal kingdom is the third kingdom of multicellular eukaryotes. There are far more species of animals than of plants and fungi combined. And most animals are anatomically much more complex than any fungi or plants. Animals evolved from single-celled aquatic organisms.

The first animal fossils date to before the Paleozoic era. Early animals were confined to the seas for millions of years, where they evolved into diverse forms, including representatives of most present-day phyla. The greatest diversification of animals in the fossil record occurred during the first period of the Paleozoic era, the Cambrian period, over 500 million years ago. This event is known as the Cambrian explosion.

How do we know whether an organism is an animal, and not a plant or a fungus?

Animals, like fungi, are heterotrophic. But while fungi absorb their food through the surface of their bodies, most animals ingest their food. Once inside the animal’s body, the food is digested and the nutrients are absorbed.

Most animals are diploid organisms that can reproduce sexually via haploid gametes. The animal zygote usually develops into a ball of cells called a blastula. The blastula then develops two layers of cells, usually by folding inward, to form a gastrula. The gastrula has features of layers and cavities that distinguish animals from other organisms, and one animal from another.

Many animals develop gradually through forms resembling an adult. But many others alter their morphology and lifestyle as they develop. Consider the transformation, or metamorphosis, of a caterpillar into a butterfly.

More than any other organisms, animals have a large variety of cells with specialized functions. Unlike the cells of plants and fungi, animal cells have no external walls.

Instead, animal cells are supported and connected by a matrix of special proteins that the cells secrete for this purpose. Animal cells of a given type usually associate tightly to form tissues.

Two particularly important types of tissues in animals are nerve tissue, used for sensing and for processing sensory information, and muscle tissue, used for movement. Tissues of different types may be organized into an organ. An organ is a body part that performs a distinct function that benefits the whole body.

Now let’s look at a phylum of intriguing but simple animals: the sponges. This group was one of the first to diverge from the line leading to the rest of the animal kingdom. Sponges are aquatic animals that live attached to a solid surface. They don’t move about, and their body shape is not symmetrical. The various cells of a sponge are more loosely organized than those of most animals, and don’t form true tissues. Sponges have no organs. A sponge’s body is supported by an internal skeleton of protein fibers or mineral crystals.

The protein fiber skeleton of a living sponge is all that remains of the animal in the familiar bath sponge. Sponges are filter feeders; they obtain food by filtering the surrounding water and trapping small organisms. Water is drawn through small pores into the sponge’s body and expelled through a larger opening. The current of water is generated by the beating flagella of the sponge’s collar cells. Small organisms are trapped on the sticky collars of these cells. The collar cells of sponges reveal a connection between animals and a group of single-celled eukaryotes.

Check the box next to the group of single-celled protists that’s most closely related to animals; then click Submit.

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The choanoflagellates are the protists most closely related to animals. These cells closely resemble the collar cells of sponges, and their flagella and sticky collars work in a similar way when capturing food. Despite their simplicity, sponges have remained successful members of marine communities since they arose in Precambrian times. Most other animals have more complex body plans, however. We’ll explore the basic features of these body plans next.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education