Browse Lessons
Assign Lesson

Help Teaching subscribers can assign lessons to their students to review online!

Assign Lesson to Students

Share/Like This Page

The Simple Animals

The Simple Animals

Introduction: When one thinks of animals, chances are dogs, lions, and whales come to mind. There are also many animals that are far less complex, often lacking the organized internal structures and "intelligence" found in these higher organisms. Some of the organisms in these simple groups include the sponges, mollusks (clams, snails), cnidarians (jellies, sea anemones, corals), roundworms (nematodes), and flatworms (tapeworms). Each of these organisms is multicellular and has cells that are specialized for particular purposes. Most members of these groups live in the water, but there are species that live on land. In the case of the roundworms and tapeworms, many of them are parasitic, living inside other organisms and feeding off of them.

Sponges are the simplest multicellular organisms (See image). They are a collection of cells specifically designed for feeding. Water enters the animal through openings in the sides of the animal. Specialized cells remove any food particles in the water and then pump it out through other openings at the top of the animal. Sponges lack a nervous system, but are very sensitive to changes in the water chemistry.

Molluscs are those animals most people would recognize as clams or snails. They have a hard external shell made form calcium carbonate. While not all molluscs have a shell (e.g. octopus, squid), they all have a structure called a mantle. This mantle is what may or may not produce the shell.

Cnidarians are the jellyfish and their relatives. They all stinging cells called cnidocytes, which they use to paralyze their prey. Sea anemones and corals are also in this group (see image).

Roundworms (nematodes) are most commonly found within the soil, but can also be aquatic. They can number in the millions in just one small shovel of dirt. Most of them are completely harmless, but some can be parasitic. These can often dig their way through an organism's skin and invade its muscle tissue. The image shows what a nematode looks like.

Flatworms are almost always parasitic. They survive by feeding off of a host. In the case of the tapeworm (shown in the image below) the tapeworm lives in the intestines of the host (usually a large mammal - dog, human). They get in there when the host accidentally swallows the eggs, usually from infected water. The tapeworm hatches and then attaches to the wall of the small intestine, where it can grow to be 30+ feet. Since the small intestine is where most nutrient absorption takes place, the food the host consumes does not go into its own bloodstream, but rather feeds the tapeworm. The host can be eating large quantities of food, but still losing weight. Tapeworms can be removed through surgery or certain medications can kill them.

Directions for this Lesson: Answer the practice questions and then watch the video to learn more about the diversity and evolution of the simple animals.

Required Video: