Lesson Video: Classification of Animals Science

Video Transcript

In this video, we will learn how to describe the classification of different types of animals, such as those that have a backbone and those that do not. We will investigate how some animals can even be classified into groups based on the number of legs they have and the arrangement of their teeth.

Our Earth is home to a vast diversity of different animals. We come in a variety of different shapes and sizes. For example, while the tiny bee hummingbird measures on average only 5.3 centimeters, the gigantic blue whale can measure up to around 30 meters long! Some animals like the blue whale live in water, while others, like us humans, live on land. Their structures vary to suit their habitat, which is where they live, and way of life. For example, while the hummingbird has wings to fly in the air, the blue whale has fins to swim in the ocean.

It is really challenging for scientists to predict how many different types of animals there are on Earth, but those that are identified are grouped and named according to their structure and characteristics. This is known as “taxonomy,” the science of organizing living and extinct organisms into groups that have similar features.

Taxonomy makes it easier for scientists to study organisms and share information about them with people all over the world. The arrangement of organisms into specialized groups is called “classification.” In classification, large groups of organisms are broken down into smaller and smaller groups, which we call taxonomic groups, based on similarities in their observable physical characteristics.

All living organisms are made up of cells, the basic unit of life. One of the broadest ways to classify living organisms is by the number of cells they have. Many organisms, like bacteria, are made up of only one cell. These organisms are often described as unicellular, as the prefix uni- means one. There are no unicellular organisms, like this bacterium, that are animals. All animals are multicellular organisms, multi- meaning many. We humans, for example, are made up of trillions of cells.

Let’s take a look at a couple of the different taxonomic groups that organisms can be classified into. One of the largest and broadest groups is the kingdom. Two examples of kingdoms that you may have heard of are kingdom Animalia, which includes all the animals, and kingdom Plantae, which includes all the plants. All the organisms in these two kingdoms are multicellular, and their cells contain a structure called the nucleus. You might be wondering, “but how do plants and animals differ?”

One of the main ways organisms are classified into Animalia or Plantae is by their source of energy. Most plants are described as autotrophs. The prefix auto- means self and -troph means food. What do you think this means about how plants can obtain energy? Well, most plants can make food themselves through a process called photosynthesis that uses light energy, often absorbed from the Sun. Animals, however, need to find food in their surroundings and so are called consumers or heterotrophs. Hetero- means other, so animals feed on other living or once living things. Another difference between animals and plants is that animals are usually motile and move around a lot, whereas plants are often fixed in one place.

Now, how do we sort the organisms further within each group? Well, we can look at their general characteristics to create subgroups. Then, we can create smaller and smaller, more specialized groups, until we get to one of the smallest and most specific groups: the species. “Species” is the term given to a group of organisms that have similar features or characteristics that can breed together to produce fertile offspring. For instance, male horses can breed with female horses to produce offspring called foals that can later breed with other horses. However, although a female horse and male donkey can mate to produce offspring, known as a mule, this mule will not be fertile. This is because a horse and a donkey are not the same species.

Now that we have looked at how organisms can be placed into taxonomic groups, let’s look at the animal kingdom in more detail. The animal kingdom can be subdivided into vertebrates and invertebrates. We, along with all other mammals, birds, reptiles, amphibians, and fish, are vertebrates because we have backbones, which are sometimes called vertebral columns. Our backbones are the basis of our skeleton. We have an internal skeleton, known as an endoskeleton, endo- meaning inner. Invertebrates do not have an internal skeleton and so do not have a backbone. Many of them, however, do have an external skeleton. This is called an exoskeleton, exo- meaning outside.

Both endoskeletons and exoskeletons act as a support structure for the animal’s body, provide somewhere for muscle to attach, allowing the animal to move, and protect the animal’s vital internal organs. Some organisms belonging to the animal kingdom do not have a hard skeleton at all. These soft-bodied animals, like jellyfish, need another form of support. The body of the jellyfish is supported by water, which is why they look rather blob-like if you see one washed up on the beach.

Earthworms are another example of invertebrates with no exoskeleton. Their muscles are organized in a way that supports the body and enables them to move through soil. Mollusks are another group of invertebrates. Many mollusks, like limpets, secrete a hard shell made of calcium which acts as an exoskeleton and protects their soft bodies. Squids and octopuses are examples of mollusks too, but they have lost their external shell during evolution.

Now, let’s look at the largest invertebrate subgroup, the arthropods. Arthropods have three main features in common. They have an exoskeleton, segmented body, and jointed walking legs, which are sometimes called appendages. Arthropods can be split up into smaller subgroups depending on various features, including the number of legs they have. Let’s take a look at some key characteristics of three examples of arthropod subgroups: insects, arachnids, and myriapods.

An insect is an arthropod with three body segments: a head, thorax, and abdomen. They typically have only three pairs of legs, and most adult insects usually have one or two pairs of wings. Some examples of insects include flies, bees, and ants. An arachnid is an arthropod which has a segmented body divided into two regions: the cephalothorax at the front and the abdomen at the rear. We can remember this, as the prefix cephalo- derives from the Greek word for head. The cephalothorax is attached to four pairs of legs and has no antennae. Arachnids do not have wings. Some examples of arachnids include spiders, scorpions, and ticks.

A myriapod is another example of an arthropod subgroup. The individuals that belong to this subgroup do not have wings. Their bodies are made up of a head, joined to numerous similar segments that make up a region called the trunk. Each of these trunk segments has a pair of jointed legs attached to it. Some examples of organisms belonging to the myriapod subgroup include millipedes and centipedes.

Having looked at a group of invertebrates, we will now look at a group of vertebrates that we belong to: mammals. Mammals have fur or hair and can feed their young on milk produced by the mother in her mammary glands. Mammals, like almost all other animals, are consumers. This means that they have to eat food obtained from their surroundings.

One way of organizing animals into groups is by the type of food they eat. As mammals have different diets, their teeth and skull are adapted for each type of feeding. Looking at the dentition, or arrangement of teeth, in an organism can help us work out which classification group they fit into. Let’s take a look at some examples of mammals in each feeding type.

Carnivores, like lions, are animals that only eat other animals. Lions specifically have three pairs of incisors on the top and bottom jaw at the front of their mouths. These incisors are not very developed and point slightly inwards to grip and cut meat. They have one well-developed pair of conical canine teeth on their top and bottom jaw located either side of their incisors to rip and tear meat. All carnivorous mammals need large canine teeth and skulls that can bite down hard in a slicing, scissorlike action. At the back of their mouths, they have some very sharp molars and premolars to slice through meat.

Herbivores, like horses, only eat plants. They need to be able to grind vegetation down, using their premolars and molars. Typically, herbivores have a large gap between the front and back teeth to help them reposition their food as they need to chew it a lot. Omnivores, like humans, are adapted to eat a wide range of foods, both plants and animals. An example of an omnivore with interesting and distinctive dentition is the hedgehog. These mammals have one pair of well-developed incisor teeth. These teeth point forward to scoop up prey and have a gap between them to help them grab slugs and worms to eat. Hedgehogs have two pairs of canines, which are not developed, and premolars and molars behind these canines for chewing their food.

Some mammals, especially herbivores like the rats and the rabbit, have evolved to lose their canine teeth. Let’s compare their teeth. As with most herbivores, both rats and rabbits have a large gap between their front and back teeth. They both have long incisors, and their teeth grow continuously throughout life as they get worn down by all the gnawing they do. However, while rats only have one pair of incisors in their top jaw, a feature common to their rodent subgroup, rabbits have two pairs of incisors in their top jaw, as do all mammals in the subgroup lagomorph.

One group of mammals, called the edentates, have adapted by having very few teeth, like the sloth and armadillo, or no teeth at all, like the anteater. Anteaters eat small insects, so they don’t really need teeth to chew. Instead, they have an extremely narrow and long jaw and tongue so it can get into termite mounds and ant nests easily.

Let’s apply what we’ve learned about the classification of animals to a practice question.

The figure shows Scolopendra, a member of the animal kingdom. To which arthropod subgroup does it belong?

This question is asking about an arthropod subgroup. Arthropods are the largest animal group. They are invertebrate animals with segmented bodies and jointed appendages or legs. They usually also have an exoskeleton, which provides structural support and protection for the vital organs and tissues it covers.

Arthropods can be further divided down into smaller subgroups, mainly based on the number of legs they have. Insects have six legs, in three pairs, and a well-defined head, thorax, and abdomen. The legs and often one or two pairs of wings are attached to the middle segment of the body: the thorax. Some examples of insects include flies and bees. Arachnids do not have wings but do have a segmented body divided into two regions: the cephalothorax and the abdomen. As cephalo- derives from the Greek word for head, we can deduce that this segment is found in front of the abdomen. The cephalothorax has four pairs of legs and no antennae. The arachnids include the spiders and scorpions.

Myriapods are a subgroup of arthropods that include centipedes and millipedes. Their bodies are divided into a head attached to a trunk made up of numerous similar segments, each with a pair of jointed legs attached. Scolopendra, which is the group the organism in the question belongs to, consists of centipedes which live in tropical regions, including the largest centipedes discovered.

In the photograph provided, we can see that the organism has a long, segmented body with pairs of legs attached to each segment. It certainly has more than six or eight legs, suggesting that it is not an insect or an arachnid, but a myriapod. With this information, we can deduce that Scolopendra belongs to the arthropod subgroup myriapod.

Let’s recap some of the key points we have covered in this video about the classification of animals. Taxonomy is the science that arranges organisms into groups according to their structural features, which makes it easier to study them. Animals are either vertebrates, which have backbones, or invertebrates, which do not have a backbone. The backbone forms part of the endoskeleton of vertebrates. Some invertebrates have exoskeletons for protection. Arthropods can be divided into subgroups, depending on the number of legs they have. Mammals are vertebrates that have hair or fur and feed their young on milk from the mother. Mammals have differences in their skull shape, dental arrangement, and teeth structure, depending on their diet.