Characteristics of Life

  • Living things are organized.
  • Living things are made up of cells.
  • Living things metabolize.
  • Living things maintain an internal environment.
  • Living things grow.
  • Living things respond.
  • Living things reproduce.
  • Living things evolve.

Seven Levels of Taxonomic Classification

  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

Kingdom Animalia Characteristics

  • All animals are multicellular, eukaryotic heterotrophs —they have multiple cells with mitochondria and they rely on other organisms for their nourishment.
  • Adult animals develop from embryos: small masses of unspecialized cells
  • Simple animals can regenerate or grow back missing parts
  • Most animals ingest their food and then digest it in some kind of internal cavity.
  • Somewhere around 9 or 10 million species of animals inhabit the earth.
  • About 800,000 species have been identified.
  • Animal Phyla- Biologists recognize about 36 separate phyla within the Kingdom Animalia.
    (We’ll study the 10 major ones!)

Animal Movement

  • Most animals are capable of complex and relatively rapid movement compared to plants and other organisms.
  • Organisms that live rooted to one spot are sessile and those that move around are motile.  Even the most sessile animals can move at lease part of their bodies.  This movement is dependent on how animals obtain food.

Animal Reproduction

  • Most animals reproduce sexually, by means of differentiated haploid cells (eggs and sperm).
  • Most animals are diploid, meaning that the cells of adults contain two copies of the genetic material.

Animal Sizes

  • Animals range in size from no more than a few cells (like the mesozoans) to organisms weighing many tons (like the blue whale).

Animal Habitats

  • Most animals inhabit the seas, with fewer in fresh water and even fewer on land.

Animal Bodies

  • The bodies of most animals (all except sponges) are made up of cells organized into tissues.
  • Each tissue is specialized to perform specific functions.
  • In most animals, tissues are organized into even more specialized organs.
  • Cells form tissues, tissues form organs, and organs form organ systems.  This is how an organism develops.
  • These cells have to differentiate and become specialized in various ways.
  • Cell Structure: The nucleus, nucleolus, ribosomes, smooth ER, rough ER, nuclear membrane, Golgi bodies, lysosomes, mitochondria, centrioles, cytoskeleton, vacuoles.

Animal Systems

  • Skeletal-Support, protection-Bones, shells, cartilage; there are some species that are prokaryotic (invertebrates). Most of them are eukaryotic and have a backbone (vertebrates).
  • Muscular-Movement-Muscles; There are many different body plans. Some have radial (starfish as an example), bilateral (humans), unilateral (earthworms)
  • Digestion-Digestion of food and absorption of nutrients-Mouth, stomach, intestine. This is the process in which the various macronutrients such as carbohydrates, proteins, fats, etc. are broken down and absorbed into the body. There are various sorts of systems that the animals have. Humans have the following (mouth, salivary duct, esophagus, stomach, small and large intestine, pancreas, liver, rectum). Birds for instance have a gullet as part of their digestive system, and then primitive animals such as amoeba, and paramecium have vacuoles on the cell membrane. Then there are sponges that feed for instance, and are filter feeders.
  • Circulatory-Distribution of nutrients and oxygen; removal of wastes-Heart, blood vessels, blood. some animals have an open and closed circulatory system. The open system they exchange wastes over the cell membrane. With closed circulatory systems such as humans it goes through a number of areas to cleanse the blood. The two organs that “bad blood” goes through is the kidneys and the liver, and they detoxify the blood by filtering the impurities and sending it into the appropriate system to be excreted or defecated. Some of the animals have a 2 chambered heart, and some have a four chambered heart. One example of a double chambered heart is a bird, and then the one that has four chambers is a human.
  • Respiratory-Absorption of oxygen; removal of CO2-Lungs, gills. Tthis is known as the “gaseous exchange” system. For instance with humans you take in oxygen, and then it filters through the respiratory system into the alveoli, and the alveoli, filters the wastes from the blood (carbon dioxide) back up the respiratory system through the mouth or nose and back into the air. With fish or marine life it is a bit different with the fact that they have gills so it is an open respiratory system, and they exchange their gases through the gills and through their system, and back out.
  • Excretory-Removal of wastes-Kidneys
  • Nervous-Perception, control of movement, control and coordination of organ system activities-Brain, spinal cord, nerves
  • Endocrine-Control and coordination of organ system activities-Glands
  • Immune-Defense against disease-causing organisms-Blood cells, glands, skin
  • Reproductive-Production of new organisms-Ovaries, testes. Tthere are some invertebrates that do asexual reproduction through means of budding or fission, and then there are many animals that do the sexual reproduction. There are two types of reproduction in the main animal kingdom. One type of fertilization is external this type is like birds and fish where they lay eggs, and then the male fertilizes them. With internal fertilization, that is what many of the chordates such as marsupials, humans etc. have this type.

Animal Symmetry

  • The most primitive animals are asymmetrical.
  • Cnidarians and echinoderms are radially symmetrical.
  • Most animals are bilaterally symmetrical.

Radial Symmetry

  • Forms that can be divided into similar halves by more than two planes passing through it.
  • Animals with radial symmetry are usually sessile, free-floating, or weakly swimming.
  • Radially Symmetrical
  • Like a wheel, animals with this spend most of their time floating like a buoy or attached to rocks.
  • Differences between the dorsal and ventral surfaces allow jellyfish to float upright; sea anemones grip rocks with their ventral surfaces and collect food with their specialized dorsal surfaces.
  • Advantages: Architects and engineers use radially symmetrical designs for structures such as fire hydrants and lighthouses so that the structures will be accessible or visible from any horizontal direction

Bilateral Symmetry

  • Animals with bilateral symmetry are most well-suited for directional movement.
  • Anterior (front end), and posterior (rear) end
  • The left and right sides of most animals are nearly mirrored images.
  • Advantages:

o    This body plan works well for animals, if a body part is damaged, the animal can rely on an identical part on its other side.

o    This symmetry provides balance that aids movement.

o    Anterior and dorsal defenses such as bones, shells, and horns protect delicate internal organs.


  • Bilateral Symmetry usually has led to cephalization
  • the process by which sensory organs and appendages became localized in the head (anterior) end of animals.

Evolutionary Trends

  • If we analyze the basic body plans of animals, we find that they illustrate evolutionary trends.
  • Four major “advances” (in order):
  • Multicellular body plan
  • Bilaterally symmetrical body plan
  • “Tube-within-a-tube” body plan

Pseudocoelomates/ Coelomates

  • Each plan consists of 3 cell layers: endoderm, mesoderm, ectoderm


  • These animals have no other cavity than the gut.
  • They are often called the “solid worms.”


  • These animals have a body cavity (the pseudocoelom) which is not completely lined with mesoderm.
  • The “tube within a tube” body plan.
  • This category is also composed of mostly worms.
  • These animals have a “true coelom” lined with mesodermal peritoneum.
  • Most animals are coelomate (EARTHWORM)

Major Animalia phylums

Phylum Porifera

  • Sponges
  • Very primitive, considered barely animals.
  • Don’t have true organs or nerve or muscle cells

Phylum Annelida

  • Segmented Worms (earthworms, leeches)
  • Segmented Worms
  • Earthworms, leeches, and other segmented worms live in water or damp soil
  • Leeches were once used to suck out people’s “excess” blood and reduce harmful high blood pressure.
  • Leeches are uses today to produce anti-blood-clotting medicines, to suck blood from bruises, and to stimulate blood circulation in severed limbs that have been surgically reattached.
  • Each segment is separated from its neighbors by a membrane and has its own excretory system and branches of the main nerves and blood vessels that run the length of the animal.
  • Both segmented and unsegmented worms have definite anterior and posterior ends.
  • Food travels through the digestive system in one direction; from anterior to posterior.
  • A cluster of nerve cells at the anterior end serves as a simple brain.
  • Reproduction occurs by splitting or by mutual fertilization

Mollusks (Mollusca)

  • Includes snails, clams, slugs, squid, and their relatives.
  • Mollusks have soft bodies with 3 parts
  • A mass that contains most of the organs
  • A muscular “foot” that is used in movement
  • A thick flap called a mantle, which covers the body and in most species produces a heavy shell of calcium compounds.
  • Mollusks pump water through gills
  • This is how food is also ingested for clams and oysters. Squid and octopuses use the pump for jet propulsion through the water in search of prey.

Arthropods (Arthropoda)

  • The largest animal phylum, and have jointed external skeletons.
  • 1 million species, crabs, shrimp, spiders, scorpions and insects make up this phylum
  • Arthropods molt, have heads with many sensory organs.
  • Simple and complex eyes that detect only light intensity and form images
  • Antennae that smell chemical substances in the environment, arthropods also respond to water vapor, like biting mosquitoes.
  • They reproduce sexually, where sperm is released inside the female’s body, not in water.
  • Larvae of many species develop into very different adults, a process called metamorphosis.
  • Arthropods development of resistance to insecticides demonstrates how quickly they adapt to a changing environment.
  • Short generations and many offspring increase the chance that random mutations will produce a few resistant individuals

Echinoderms (Echinodermata)

  • Sea stars and sea urchins.
  • Reproduce sexually. Sperm and eggs are released in water, where they meet and join.
  • Movement by seawater into and out of a system of internal tubes.

Chordates (Chordata)

  • Vertebrates-fish, amphibians, reptiles, birds, and mammals.
  • Four characteristics
  • Stiff dorsal rod helps to organize the embryo’s development.
  • The central nervous system (brain and spinal cord) is tubular
  • Their sides have slits just behind the head. These pharyngeal slits (pharynx means “throat”) becomes gill slits of adult fish. In air-breathing chordates, they develop into various organs such as internal parts of the ears
  • They have a tail; in humans it’s the tailbone, or coccyx, which curls internally.
author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0


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