The Animal Kingdom

The Animalia Kingdom is one of 6 Kingdoms of organisms. The other 5 kingdoms are: Plantae, Fungi, Protista, Archaea and Bacteria. Like the name suggests, the animalia kingdom is made up of animals!

Animals, like the flamingo shown here, have a wide variety of characteristics. For example, animals can have 2 legs (birds), 4 legs (dogs), 5 legs (starfish), 6 legs (insects), 8 legs (spiders or octopi),...even no legs (fish)! They can have fur, feathers, scales, skin, shells, or other coverings. Some animals feed their babies milk, others do not. While many animals walk, some fly, swim, slither, or hop! But in spite of all their differences, animals also share many characteristics.

On this page, we'll explore what makes an animal an animal and learn a little about different types of animals too.

• Animals are heterotrophs. That means they don't make their own food, like plants, but instead must find food to eat.
• Digestion takes place inside an animal's body. Once animals have food, they eat it and then digest it inside their bodies. Fungi, like mushrooms, do not do this. Fungi digest their food outside their bodies.
• Mobility. Animals are different from plants in that animals can move.
• Multicellularity. Animals are made up of more than one cell.
• Animals are diploid organisms. Diploid organisms are ones where the adult has 2 copies of each chromosome, but the reproductive cells have only 1 copy of each chromosome.
• No cell walls. Plants have cell walls, but animals do not. The lack of a cell wall in animals helps animals be able to move around!

Blastula

In most animals, when an egg gets fertilized, it undergoes many cell divisions and forms a hollow ball, called a blastula.

- What makes an animal an animal....

• Sexual Reproduction. Most animals engage in sexual reproduction. The egg cells of animals are larger than the sperm cells. The sperm cells get around via a flagella.
• The blastula(pictured above) develops into 2 or 3 layers of cells. These layers eventually develop into all the various types of tissues and organs of the adult body.
  • The echoderm layer of the blastula develops into the sense organs, the nervous system, and the skin.
  • The endoderm layer of the blastula develops into the respiratory system, the bladder, the liver, the digestive organs and the lining of the digestive tract, as well as many glands. (A gland is an organ in the body that secretes chemical substances. You can read more about glands here, if you'd like --> Glands.)
  • The mesoderm layer of cells in the blastula develops into muscles, bones, reproductive organs, excretory organs (the system of the body which helps you get rid of waste), and the circulatory system. (You'll learn more about these various parts of the body in another chapter.)
• Tissues. Almost all animals have tissues. Tissues are groups of cells that work together to do a specific job. Sponges are a type of animal that do not have tissues.

Body Symmetry And An Animal's Body Plan:

An animal's body plan refers to it's shape, internal organization, and type of symmetry.

Symmetry means if you were to divide the object with a line running through it, the object would be the same size and shape on both sides of the dividing line. There can also be symmetry around a central point which means if you rotate the object around it's center, it will have basically the same shape as it did before you turned up (the various legs or other appendages will line up).

Asymmetrical

Animals that are asymmetrical don't have a regular shape. Sponges, shown here, are asymmetrical. That means they are not symmetrical.

Animals that have radial symmetry such as a starfish, have legs that come from a point in the center and branch out.

Most animals that have radial symmetry live in water and either move very slowly, or simply drift along with the water current.

Animals that have a matching right and left side of the body have bilateral symmetry. Bunnies, puppies, kittens, whales, people, and many other types of animals have bilateral symmetry.

Most animals that have bilateral symmetry also have a head on one end of them that has extra nerves in it as well as various sensory structures (sensory structures are what allow us to hear, smell, taste, see, or feel). This is called cephalization. Cephalization helps animals sense danger and find food.

- Bodies With or Without an Coelom (An Internal Body Cavity)

Animals with bilateral symmetry have one of three types of bodies:

• Acoelomate,
• Pseudocoelomate,
• or Coelomate.

Each one of these words has the word "coelom" in it. A coelom means a body cavity or space between the digestive tract (the gut) and the body wall. Inside a coelom, body organs can move freely, grow, and develop independently from the body wall, while fluid inside the coelom cushions and protects the organs.

Acoelomates are animals which do not have a coelom (body cavity). Instead, the space is filled with tissues.

Flatworms are an acoelomate.

In my drawing of the cross section of an acoelomate animal (above), you can see how there are tissues between the skin and the gut. There's no space for a coelom.

Pseudocoelomates are animals which have a "false coelom." This is a body cavity between the endoderm (the gut) and the mesoderm (muscle tissue).

Nematodes (roundworms) are a type of pseudocoelomate.

In the drawing above of a cross section of a pseudocoelomate animal, you can see that there is a cavity - a coelom - that is between the muscle tissue and the gut. It is referred to as a false coelom, rather than a true coelom, because it is not bordered on both sides by muscle tissue.

Coelomates are animals that have a true coelom. The body cavity is located within the mesoderm. The fluid filled coelom cushions the internal organs and allows animals to move around easily without damaging their organs.

Snails, as well as many other animals including humans, have a true coelom.

In this cross section drawing of a coelomate animal, you can see that the coelom is located between two sections of muscle tissue.

- Crustaceans, Spiders, Insects, Earthworms, and Vertebrates all have some form of segmentation during at least one stage in their life cycle.

Segmented animals, like this amphipoda (above), other crustaceans, spiders, earthworms, insects, and others, have a series of similar repeating body parts.

The body of earthworms is segmented. This segmentation provides for greater flexibility as the earthworm moves and burrows. Because each segment contains many of the same organs, an injured earthworm can often still carry out life functions. A circulatory system connects the segments together. Nerves also connect the segments to a brain.

In Vertebrates, the backbone is made up of a series of very similar vertebrae.
This too is a form of body segmentation.

The 2 Main Divisions Of The Animal Kingdom

The animal kingdom is often divided into two main categories: Vertebrates and Invertebrates. Vertebrates are those animals with a backbone. Invertebrates are those animals without a backbone.

Vertebrates

A goat is an example of a vertebrate animal because it has a backbone.

Sponges, jellyfish, sea anemones, sea stars, insects, spiders, crabs, lobsters, snails, clams, squids, and many other types of animals are invertebrates.

97% of all animal species are invertebrates! Invertebrates do not have a backbone. Many invertebrates have an exoskeleton or shell instead of a backbone.

A bee is an example of an invertebrate. Instead of a backbone, a bee has an exoskeleton.

Sea Urchins are also invertebrates. Instead of a heavy backbone, a sea urchin has a porous, light-weight structure called a stereom.

Instead of a backbone, clams have a shell, so they are invertebrates as well.

Even Moon Jellies are invertebrates! Instead of a backbone, they have a hydrostatic skeleton. A hydrostatic skeleton, found in many soft bodied animals, is made up of a fluid-filled cavity (the coelom) surrounded by muscles.

There are over 35 different phlya in the animal kingdom!

Here are a few of the phyla in the animal kingdom:

• Asymmetrical
• No tissues and organs
• Internal cavity has food-filtering cells called choanocytes
• Animals in this phylum are mostly marine animals.
• The bodies of animals in the Porifera phylum are made up of a non-living jelly-like substance (called mesohyl) which is sandwiched between two layers of cells.
• The bodies of animals in this phylum are full of pores and channels which allow water to circulate throughout them.
• There are about 9000 species of animals in the phylum Porifera.

• Radially symmetrical
• Most members of this phylum have tissues
• Animals in this phylum have baglike bodies
• Gelatinous (like jelly) animals
• Live in marine or fresh water
• Have mouths surrounded by tentacles

• Radially Symmetrical
• Transparent
• Gelatinous bodies (jelly like)
• Marine animals

Includes about 20,000 species, including flatworms, tapeworms, planarians, and flukes.

• Bilaterally symmetrical
• Acoelomate (no coelom)
• Flat ribbon-like bodies
• No true segments

- More than 12,000 species, including roundworms

• Parasitic
• long, slender, unsegmented worms
• Pseudocoelomate (false coelom)

- About 110,000 species, including snails, slugs, whelks, clams, oysters, scallops, squids, octopuses, nautilus, and more

• Soft bodied animals
• Have a true coelom
• Have three parts to their bodies: a foot, a visceral mass, and a mantle
• Live on land or in water

- About 12,000 species, including earthworms and leeches

• Bilaterally Symmetrical
• Segmented Worms

- About 1 million species, including spiders, scorpions, insects, horseshoe crabs, sea spiders.

• paired, jointed appendages (legs)
• bilaterally symmetrical
• Exoskeleton
• Segmented bodies
• Live in the air, on land, and in water

- About 6,000 species including sea stars, brittle stars, sea lilies, sand dollars, sea urchins, and sea cucumbers.

• Radially symmetrical (in adult form) - with five main body parts (usually 5 arms)
• tube feet
• marine animals

- About 42,500 species, including lancelets, jawless fish, sharks, rays, skates, bony fish, frogs, toads, salamanders, newts, lizards, snakes, turtles, alligators, birds, mammals, rodents, bats, duckbill platypuses, and more!

• Bilaterally symmetrical
• True Coelom
• Notochord (a flexible rod shaped structure found in the embryos of all chordates)
• Live on land or in water
• Dorsal (meaning located at the back) nerve cord

A List of Pages in This Biology Series

Homepage: Biology: Information, Videos, and Labs

Unit 1 on Cell Biology

Biology Labs, Activities, Videos, and Study Guides About Cells (Photosynthesis, Mitosis, Cell Organelles, and More)

Unit 2 on Genetics

Labs, Information, And Videos For High School Students Studying Genetics.

Unit 3 on The History of Life on Earth

The History Of Life On Earth.

Unit 4 on Ecology

Ecological Principles / Populations

Ecosystems

Biological Communities - Symbiosis, Niches, and Biomes

Global Changes And The Environment

Unit 5 on Diversity

An Introduction to Taxonomy - The Kingdoms and Domains Of Life

Learning About Viruses And Bacteria

Protists: Paramecium, Amoebas, Algae, Diatoms, Euglena, and Others

The Fungi Kingdom

Unit 6 on All About Plants

The Plant Kingdom

Unit 7 on The Animal Kingdom: Invertebrates

The Animal Kingdom - An Introduction

Sponges are Simple Animals

The Cnidarians: Jellyfish, Sea Anemones, Hydrozoans, and Corals

Earthworms, Christmas Tree Worms, Leeches, and Other Annelids

Mollusks Are Invertebrates

Unit 8 on Vertebrates

NEW: Fish: The Characteristics of Jawless Fish, Cartilaginous Fish, and Bony Fish

Check back later for additional biology units!