Animals: Multicellular Creatures That Move (original) (raw)

Animals (Metazoa) are a group of living organisms that includes more than one million identified species and many millions more that have yet to be named. Scientists estimate that the number of all animal species is between 3 and 30 million species.

Animals are divided into more than thirty groups (the number of groups varies based on differing opinions and the latest phylogenetic research) and there are many ways to go about classifying animals. For the purposes of this site, we often focus on six of the most familiar groups; amphibians, birds, fishes, invertebrates, mammals, and reptiles. I also look at many less familiar groups, some of which are described below.

To begin, let's take a look at what animals are, and explore some of the characteristics that distinguish them from organisms such as plants, fungi, protists, bacteria, and archaea.

Animal

Animals are a diverse group of organisms that include many subgroups such as arthropods, chordates, cnidarians, echinoderms, mollusks, and sponges. Animals also include a vast array of lesser-known creatures such as flatworms, rotifers, placazoans, lamp shells, and waterbears. These high-level animal groups may sound rather strange to anyone who has not taken a course in zoology, but the animals that we are most familiar with belong to these broad groups. For example, insects, crustaceans, arachnids, and horseshoe crabs are all members of the arthropods. Amphibians, birds, reptiles, mammals, and fishes are all members of the chordates. Jellyfish, corals, and anemones are all members of the cnidarians.

The vast diversity of organisms that are classified as animals makes it difficult to draw generalizations that are true of all animals. But there are several common characteristics animals share that describe most members of the group. These common characteristics include multi-cellularity, specialization of tissues, movement, heterotrophy, and sexual reproduction.

Animals are multi-cellular organisms, which means their body consists of more than one cell. Like all multi-cellular organisms (animals are not the only multi-cellular organisms, plants, and fungi are also multi-cellular), animals are also eukaryotes. Eukaryotes have cells that contain a nucleus and other structures called organelles that are enclosed within membranes. With the exception of the sponges, animals have a body that is differentiated into tissues, and each tissue serves a specific biological function. These tissues are, in turn, organized into organ systems. Animals lack rigid cell walls that are characteristic of plants.

Animals are also motile (they are capable of movement). The body of most animals is arranged such that the head points in the direction they move while the rest of the body follows behind. Of course, the great variety of animal body plans means that there are exceptions and variations to this rule.

Animals are heterotrophs, meaning they rely on consuming other organisms to obtain their nourishment. Most animals reproduce sexually by means of differentiated eggs and sperm. Additionally, most animals are diploid (the cells of adults contain two copies of their genetic material). Animals go through different stages as they develop from a fertilized egg (some of which include the zygote, blastula, and gastrula).

Animals range in size from microscopic creatures known as zooplankton to the blue whale, which can reach as much as 105 feet in length. Animals live in virtually every habitat on the planet—from the poles to the tropics, and from the tops of mountains to the deep, dark waters of the open ocean.

Animals are thought to have evolved from flagellate protozoa, and the oldest animal fossils date back 600 million years, to the latter part of the Precambrian. It was during the Cambrian period (about 570 million years ago), that most major groups of animals evolved.

Key Characteristics

The key characteristics of animals include:

Species Diversity

More than 1 million species

Classification

Some of the better-known groups of animals include:

Some of the less well-known animal groups include:

Keep in Mind: Not All Living Things Are Animals

Not all living organisms are animals. In fact, animals are just one of several major groups of living organisms. In addition to animals, other groups of organisms include plants, fungi, protists, bacteria, and archaea. To understand what animals are, it helps to be able to articulate what animals are not. The following is a list of organisms that are not animals:

If you're talking about an organism that belongs to one of the groups listed above, then you are talking about an organism that is not an animal.

References