Animals

What is the Definition of Animals in Biology?

When we say animals, you might think of vertebrates (animals with a backbone)--like birds, Mammals, fishes, and reptiles--but vertebrates make up only one phylum, called Chordata.

Many organisms you are familiar with are actually animals. Some of them might be surprising: all insects--from fleas to butterflies--are animals. All worms are animals, too. So are sponges and corals.

In biology, we define animals as follows:

Recall that eukaryotes are organisms whose Cells have nuclei and other membrane-bound organelles. They can be distinguished from prokaryotes like Bacteria which lack membrane-bound organelles. (More on this later!)

While all animals--both extinct and extant--are broadly categorized as a Kingdom, they are further classified into phyla which are subcategories under Kingdom. Zoologists (scientists that study animals) recognize over three dozen phyla of extant animals!

Figure 1 below is a Phylogenetic Tree showing several animal phyla and their evolutionary relationships. These relationships were inferred from various lines of evidence, including morphological traits, developmental characteristics, and Genetic Information.

What are Examples of Common Animals?

Let's take a closer look at some animal groups and examples of common animals within these groups.

Mammals

Mammals are vertebrate animals that nourish their young with milk produced using mammary glands. Mammals are also characterized by hair (whales only have them during their fetal stage). Mammals can also be distinguished from other animal groups by their lower jaw, which is hinged directly to the skull, rather than a separate bone, like other vertebrates.

Mammals can be classified into three groups:

• Monotremes are egg-laying mammals, like platypuses and echidnas.

• Marsupials are mammals whose offspring are born prematurely and continue development outside the mother's body. Many marsupials--like koalas and kangaroos--have a pouch.

• Eutherians are mammals with a placenta (an organ that transports nutrients from the Blood of the mother to that of the fetus) and a long gestation period. Eutherians include dogs, whales, and humans.

Reptiles

Reptiles are vertebrate animals that belong to the class Reptilia, which consists of five orders:

• Squamata consists of lizards and snakes.

• Testudines consists of all turtles.

• Crocodilia consists of all crocodilians.

• Rhynchocephalia consists of tuataras.

• Aves consists of all birds.

Yes, you read that right. Based on their evolutionary history and relationship with other animals, birds are considered Reptiles! Reptiles are generally tetrapods (meaning, they have four limbs) and are covered in scales (birds have scales on their legs and feet!). With the exception of birds, reptiles are ectothermic, meaning they rely on their external environment for thermoregulation. This is why you might see reptiles basking in the sun.

Arthropods

You might not be familiar with the term arthropod, but you've certainly seen many animals from this phylum, as arthropods make up 84% of all known species of animal! Arthropods are invertebrates, meaning they don't have a backbone. Instead, they have an exoskeleton: a skeletal covering made up of chitin (a complex sugar) bound to protein. Arthropods usually have a segmented body with jointed appendages.

Arthropods can be classified into four subphyla:

• Chelicerata consists of Arachnids like spiders, scorpions, mites, and ticks.

• Crustacea consists of crustaceans like crabs, lobsters, and shrimps.

• Hexapoda consist of insects and springtails.

• Myriapoda consists of millipedes and centipedes.

What are the Key Characteristics of Animals? What sets them Apart from other Organisms?

Animals can be distinguished from other organisms by several characteristics, which we will discuss in the following section. Keep in mind, however, that there are exceptions to some of these characteristics.

Animals are Multicellular Eukaryotes

Cells are either prokaryotic or eukaryotic. Prokaryotic cells lack membrane-bound organelles, but eukaryotic cells have them. Animal cells are Eukaryotic Cells. This characteristic distinguishes them from Bacteria and Archaea, which are prokaryotic organisms.

Animals are multicellular organisms. This distinguishes them from protists, which are eukaryotes that are mostly unicellular.

Animal Cells do not have a Cell Wall

Animals can also be distinguished from Plants and Fungi because their cells lack cell walls (Fig. 2). Instead of a cell wall, Proteins outside the cell membrane provide structural support to animal cells and bind them to one another. These Proteins include collagen, which cannot be found in plants or Fungi.

Animals have Complex Tissue Structures

Most animal cells organize into a complex hierarchy of structures:

• Cells come together to form tissues.

• Different tissues come together to form functional organs.

• Organs work together to perform coordinated functions in Organ Systems.

Animals have four major types of tissues:

• Epithelial tissues cover the surface of organs and other structures in the body.

  • Examples include those found in the walls of Kidney tubules, where they help filter Blood.

• Connective tissues bind together cells and organs. They also protect, support, and integrate various parts of the body.

  • Examples of connective tissues include bone and cartilage.

• Muscle tissues enable responses to stimuli and movement.

  • Examples of muscle tissues include skeletal muscles and cardiac muscles.

• Nervous tissues are made up of neurons and glial cells. Neurons receive and transmit electrical signals, enabling communication between various parts of the body. Glial cells support neuron activities.

Muscle and nervous tissues are responsible for movement and coordination, which enable adaptations that set animals apart from plants and fungi (which do not have muscle and nervous tissue).

Note that not all animals have true specialized Tissues and Organs. Some organisms possess specialized cells that perform specific functions, but the cells are not organized into tissues. This is why animals like sponges and corals (which don't have specialized tissues) were previously classified as plants.

So why were they eventually classified as animals? Sponges and corals are considered animals because they are unable to make their own food, a key characteristic which we will discuss next.

Animals Cannot Make their Own Food

Animals differ from plants and fungi based on their nutritional mode. Plants are autotrophic organisms, which means they have the ability to produce Organic Molecules through Photosynthesis. Fungi are heterotrophic, which means they cannot make their own food. Instead, fungi grow on or near their food and take in nutrients through absorption, a process where they release Enzymes that digest the food outside of their bodies.

Animals are heterotrophic, but unlike fungi, they consume other living organisms or dead organic matter using Enzymes that digest food inside their bodies.

Animals can be broadly classified as carnivores, herbivores, or omnivores, based on their feeding habits.

• Carnivores eat other animals.

  • Examples of carnivorous animals are lions, sharks, and spiders.

• Herbivores eat plants or plant materials such as fruits, seeds, nectar, and leaves.

  • Examples of herbivores are deer, crickets, and caterpillars.

• Omnivores eat plants and other animals.

  • Examples of omnivores are bears, crayfish, and humans.

Animals Typically Reproduce Sexually

Most animals undergo sexual Reproduction, where a new organism is formed through the combination of Genetic Information from two individuals of different sexes. This Genetic Information is stored in the Chromosomes in the nucleus of gametes. The fusion of two haploid cells form a diploid zygote which eventually develops into a diploid individual.

Animals are in their diploid stage for most of their life cycle. In their haploid stage, they produce their gametes through Meiosis.

There are some exceptions: for example, male bees, wasps, and ants are haploid because they develop from unfertilized eggs. There are also a few groups–including cnidarians, flatworms, and roundworms–that undergo asexual Reproduction, although most of them also have a sexual phase in their life cycle.

All Animals have developmental genes that control how other genes are expressed

While some animals (including humans) develop directly into adults, most animals go through at least one larval stage in their life cycle.

Although adult animals are morphologically diverse, the Genes that govern animal development are similar across different phyla. The genes that determine animal structure contain DNA sequences known as homeoboxes.

Most animals share the Hox gene family, a gene family that contains homeoboxes. The Hox Genes determine the animal's body plan: the formation of the anterior-posterior axis, the number of body segments, the position of appendages, and even the direction of the head and tail. (We'll discuss the body plan in further detail in the next section!)

The sequence of Hox genes and their positions on chromosomes are similar in most animals, from flatworms to humans. If these genes are shared by most animals, why are we humans so different from the average flatworm?

The varying complexity of animal bodies can be attributed to the duplication of the Hox genes in the course of animal evolution, with the addition of genes leading to the emergence of more complex body types.

What characteristics are used in the classifications of animals?

Key morphological and developmental characteristics described by body plans are used in classifying animals.

Characteristics that describe the body plan of an animal include symmetry and tissue organization, which we will briefly discuss below.

Symmetry

One of the basic features of animal body plans is symmetry, or lack thereof (Fig. 3):

• Bilateral symmetry is where an animal has two axes of orientation: front (anterior) to back (posterior) and top (dorsal) to bottom (ventral). These axes result in two mirror images: right and left halves (think butterflies or crabs!).

  • Animals with bilateral symmetry include mammals (such as humans) and insects.

• Radial symmetry is where the body parts of an animal are arranged around a central axis. While animals with radial symmetry have a top and bottom surface, they do not have a left and right or front and back. Radial symmetry is more common among simple and non-motile animals.

  • Animals with radial symmetry include jellyfish and sea anemones.

• Asymmetry is the lack of symmetry.

  • Asymmetrical animals include sponges.

Tissue organization

Animal body plans also differ in terms of tissue organization. During their embryonic development, tissues separate into layers (called germ layers), forming the different Tissues and Organs of the body.

Animals are either diploblastic or triploblastic:

• Diploblasts have two germ layers.

  • The ectoderm is a germ layer that covers the surface of the embryo. It develops into the outer covering of the animal and, in some cases, into the central nervous system.

  • The inner layer, the endoderm, develops into the lining of the digestive tract and other organs.

• Triploblasts have three germ layers:

  • In addition to the ectoderm and endoderm, triploblasts also have a mesoderm that fills the gap between the ectoderm and endoderm. The mesoderm forms the muscles and most organs between the animal's digestive tract and outer covering.

  • Most triploblasts have a body cavity (or coelom), a fluid- or air-filled space between the digestive tract and the outer covering. It cushions organs to prevent injury and enables internal organs to grow and move independently of the outer body wall. For example, without your coelom, the beating of your heart would warp the surface of your body.