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Jellyfishes Getting to Know Corals Sponges and Other Simple Animals


An animal is a living thing made up of many cells and it is not a plant. Simple animals are those without a fluid-filled body cavity between the outer body wall and the digestive tract. Examples of simple animals are sponges, corals, jellyfishes, sea stars, sea urchins, roundworms, and flatworms such as tapeworms. All simple animals are invertebrates (in-VER-teh-brehts), meaning they do not have a spinal column made up of a series of bones. The simplest animals, sponges, have many cells but do not have true tissues, or groups of cells with the same function. More complex simple animals, such as corals and jellyfishes, do have tissues but no organs. The most complex of the simple animals, sea squirts and lancelets, have a flexible rod that is a simple form of the spinal column of vertebrates (VER-teh-brehts).

Green anemones, California mussels, and ochre sea stars are all examples of simple animals, animals without backbones. (© Stuart Westmorland/CORBIS.)


Almost all simple animals live in water or in other animals. Most simple animals that live in water live in the sea, but some live in the fresh water of ponds and rivers. Some simple sea animals live on surfaces such as rock, shell, sand, and mud. Other simple sea animals, such as jellyfishes, float freely in open water. This anemonefish is protected from many of its predators by living among the stinging tentacles of the anemone. In turn, it keeps the anemone clean, and aggressively keeps other creatures away. (© Tony Wu/www.silent-symphony.com.) Some animals that live in other animals do not harm them; often one or both animals benefit. For example, pearlfish live in the anuses of sea cucumbers without hurting them. Other animals, called parasites (PAIR-uh-sites), harm the animals in which they live, called the hosts. For example, tapeworms are parasites that cause disease in their human hosts.


Some simple sea animals, such as sponges, eat tiny particles of food floating in the water that flows through them. These animals are filter feeders. Some simple animals take in and eat small particles floating, or suspended, in the water. These animals are suspension feeders. Other simple animals, such as jellyfishes, which use their tentacles for this purpose, attack and kill other animals and eat them. These animals are predators (PREH-duh-ters). Animals that live inside or on the surface of another animal and eat its cells, body fluids, food, and waste are parasites.


Behavior is the actions animals take to adjust to and interact with their environment. The animals use behavior to find Some simple animals, such as this octopus, attack and kill other animals and eat them. (JLM Visuals.) The sea urchin can move to a new location by walking on its tube-feet, which are extended here. (Jane Burton. Bruce Coleman, Inc.) food and a place to live, to defend themselves, and to reproduce. As the bodies of animals become more complex, so does their behavior. Sponges, which have the simplest bodies, have the least coordinated behavior. Corals and jellyfishes can contract, or withdraw, to protect their most vulnerable body parts. Animals such sea stars and sea urchins can crawl away to hide under a rock or change direction and swim away to escape predators.


An animal species is a group of individuals that shares a common pool of genes. The genes determine all of the animal's characteristics and are made of DNA. Reproduction is the copying of an individual animal's DNA and the transfer of the copy into a newly formed individual. Reproduction can be asexual (ay-SEK-shuh-wuhl) or sexual (SEK-shuh-wuhl).

In asexual reproduction the offspring develop from a single parent, and the copy of the DNA is nearly identical to the original. No new combinations of genes result from mixing genes from two parents. For example, an animal simply splits in half Hydra reproduce by budding. The brown hydra has three buds and one detached offspring. (© Tom Branch/Science Source, National Audubon Society Collection/Photo Researchers, Inc.) and grows into two full-sized individuals. Or a body part breaks off and develops into a whole new animal. In a third method a bud develops on an animal, grows to full size, and then breaks off to live as a new individual. Asexual reproduction has the advantage of allowing a very fast rate of reproduction with a resulting rapid increase in the population of a species. The primary disadvantage of asexual reproduction is that it does not allow much genetic variation. As a result, the population as a whole becomes unable to adapt to changing environmental conditions.

In sexual reproduction, the DNA copy is not exact, and the genetic makeup of the offspring differs from that of its parents. Each parent's DNA is reduced by half before being joined with the half set of DNA supplied by the other mating partner. Before the DNA is reduced by half, however, gene segments reorganize to form unique combinations of genes. This process is known as crossing over and is the event that distinguishes sexual from asexual reproduction. Crossing over combined with the uniting of DNA from two unique individuals when egg and sperm join during fertilization (FUR-teh-lih-zay-shun) results in the varied offspring that characterize most animal species.

Many animals, particularly simple animals, use both asexual and sexual reproduction at various times. It is rare for both asexual and sexual reproduction to occur at the same time, however. In many simple animals, the asexual and sexual processes occur in different seasons or in different generations. In other groups of simple animals, such as parasite flatworms, asexual and sexual processes are used in two different stages of life.

In most animals, males produce sperm, and females produce eggs. In some animals, however, the same individual produces both sperm and eggs. Some animals produce both sperm and eggs at the same time, and some do so at different stages of life.

Sperm and egg come together in a variety of ways among simple animals. Some animals release their sperm or eggs directly into the surrounding water. In groups such as sponges, corals, and jellyfishes, this method is the only way eggs and sperm come together. In some animals males release their sperm, females release their eggs, and fertilization takes place outside the body. In other animals, the male releases sperm, the female takes up the sperm, and fertilization takes place inside her body. In still other animals, the males transfer sperm directly into the female for fertilization inside her body.


Living things are classified into the following groups: kingdom, phylum (FYE-luhm), class, order, family, genus (JEE-nuhs), and species (SPEE-ceez). Species is the narrowest group. Living things in the same species have the most in common and can reproduce with one another. Kingdom is the broadest group. The kingdoms are animal, plant, fungus, protist (PROH-tist), and moneran (moh-NEER-uhn). Protists are one-celled living things that have a nucleus (NOO-klee-uhs). Examples are amoebas (uh-MEE-buhs) and algae (AL-jee). Monerans are one-celled living things that do not have a nucleus. Bacteria are monerans.

During development cells multiply and organize into layers called germ layers. Each layer then gives rise to certain tissues. A few simple animals do not have specific germ layers. Some scientists place sponges in this group, but other scientists say sponges have two germ layers. Animals in the jellyfishes and corals group and comb jellies have two germ layers. All other animals, simple and complex, have three germ layers. The inner layer typically develops into the digestive system. The outer layer develops into the skin and nervous system. The middle layer develops into the waste disposal system, muscle, and bone.

Mating flatworms (Pseudoceros bilurcus) with insemination marks. (© A. Flowers & L. Newman/Photo Researchers, Inc. )

This West Indies sea egg is releasing its sperm directly into the water. (© Andrew J. Martinez/Photo Researchers, Inc.)


Extinct No longer in existence.

Extinct in the Wild No longer alive except in captivity or through the aid of humans.

Critically Endangered Facing extremely high risk of extinction in the wild.

Endangered Facing very high risk of extinction in the wild.

Vulnerable Facing high risk of extinction in the wild.

Lower Risk/Conservation Dependent If the conservation program were to end, the animal would be placed in one of the threatened categories.

Low Risk/Near Threatened At risk of becoming threatened with extinction in the future.

Least Concern There is no known threat of extinction, and the animal does not qualify for any of the threatened categories.

Data Deficient There is not enough information to make a judgment about the threat of extinction.

Not Evaluated The species has not been evaluated for the threat of extinction.

After hatching or being born, the young of complex animals look like adults but do not have a mature reproductive system. Most simple animals, however, go through a stage called a larva (LAR-vuh)—the plural is larvae(LAR-vee)—before they become adults. The transition from larva to adult, known as metamorphosis (meh-tuh-MOR-fo-sus), requires changes in the form, behavior, and functions of the animal. A larva is a fully functional animal that can feed and move about independently. The most common task of larvae is traveling to colonize new environments for the species. This process is especially important for animals such as corals and sponges, which are fixed in one spot as adults.


Simple animals can be harmful or helpful to people. A huge number of simple animals, such as tapeworms, flukes, roundworms, and heartworms, are parasites that cause disease in humans and domesticated animals throughout the world. And animals such as jellyfish and sea urchins can cause great pain when touched. On the other hand, substances from simple animals such as sponges and sea squirts can be used to make drugs for treating viral diseases and cancer. People also like to visit the seashore and the aquarium to observe the beauty of animals such as corals and sea stars.


Endangered In danger of extinction throughout all or a significant portion of its range.

Threatened Likely to become endangered in the near future.


Simple animals are often taken for granted, but these animals are important in the mechanics of nutrient and energy Aquariums are popular places for people to see and learn about the animals that live underwater. (© Paul A. Souders/CORBIS.) transfer, decomposition, and carbon and nitrogen cycling on which life depends. Each year thousands of species cease to exist. Gone with those species may be life-saving medicines, models for research, and contributions to the balance of the environment. Conservation is the effort to protect species at risk because of harm done by humans. The World Conservation Union (IUCN) is an organization that collects information on the health and status of animals worldwide. The U.S. Fish and Wildlife Service does the same thing in the United States.



Aaseng, Nathan. Invertebrates. New York: Venture, 1993.

Carson, Rachel. The Edge of the Sea. 1955. Reprint, Boston: Mariner, 1998.

Layman, Dale. Biology Demystified. New York: McGraw-Hill, 2003.

Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed. New York: HarperResource, 2000.

Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. Invertebrates. New York: Twenty-First Century, 1996.

Stalcup, Brenda, ed. Endangered Species: Opposing Viewpoints. San Diego, CA: Greenhaven, 1996.

Wells, Sue, and Nick Hanna. The Greenpeace Book of Coral Reefs. New York: Sterling, 1992.

Zimmer, Carl. Parasite Rex. New York: Free Press, 2000.

Web sites:

"Animal." All Science Fair Projects. http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Animal (accessed on January 13, 2005).

"World's Largest Conservation Gathering Opens to Escalating Global Species Extinction Crisis." World Conservation Union. http://www.iucn.org/themes/ssc/red_list_2004/main_EN.htm (accessed on January 13, 2005).

Additional topics

Animal Life ResourceJellyfish, Sponges, and Other Simple Animals