In this video, we will learn how to recall the characteristics of different
vertebrates within the animal kingdom. We will start by learning what a chordate is and the vertebrates that are a
subclassification of this group before investigating the key features of the animals
that belong to each group of vertebrates, fish, reptiles, amphibians, birds, and
finally mammals. We will also discover the typical characteristics of organisms in the different
subclasses of this incredibly diverse mammalian kingdom.
When studying living organisms, scientists often classify them into distinct groups
that share some key characteristics. This process is known as taxonomy. The taxonomic hierarchy is a ranking system for these different taxonomic groups,
ranging from one of the most general and broad groups, the kingdoms, to one of the
smallest and most specific taxonomic groups, the species. Let’s look at an example of an organism, the crabeater seal, to see how it can be
classified into each taxonomic group in between.
Crabeater seals belong to the Animalia, or the animal, kingdom. This is the same group that includes tiny ants, huge elephants, and even us,
humans. One of the key features that all organisms that belong to the kingdom Animalia share
is that we are all heterotrophs. We all obtain our food by consuming another organism, or a once-living organism’s
organic matter. For instance, the crabeater seal mainly eats tiny living crustaceans called krill,
and ironically not crabs. Animals also tend to be multicellular as our bodies are made up of more than one
cell. We also tend to be much more mobile organisms than those that belong to the other
kingdoms of life. Even though on land, the crabeater seal struggles to move quickly, in water they can
reach swimming speeds of almost 13 kilometers per hour. For comparison, the average human swimmer only reaches speeds of two kilometers per
The next most specific grouping within the animal kingdom is the phylum. And crabeater seals fall into the phylum Chordata. Chordates are organisms that at some point during their development have a notochord,
which is a long elastic rod that provides developmental and sometimes permanent
structural support to chordates. Vertebrates are a subphylum of Chordata. In vertebrates like crabeaters and humans, as they go through embryonic development,
the notochord develops to become part of their vertebral column, otherwise known as
their backbone. The vertebral column acts as the core of the endoskeleton or the internal skeleton of
vertebrates, which is made of bone or cartilage and includes a skull that protects
The other subphylum of Chordata are invertebrates, and this group includes any
organism that do not have an endoskeleton and therefore do not possess a vertebral
column. Vertebrates also have a closed circulatory system containing a heart with multiple
chambers, which is often not the case for invertebrates. Several classes make up each phylum. Along with humans, dogs, and many other species, crabeater seals are in the class
Mammalia, otherwise known as mammals. We will look at the different classes of vertebrates in more detail later on in this
Several orders make up a class. And crabeater seals are in the order Carnivora, which means that they are carnivores
and obtain their nutrition by consuming other animals only. Several families with similar features make up a single order. And crabeaters are in the Phocidae family, sometimes called phocidaes. Several genera make up a single family, with crabeaters belonging to the genus
Lobodon. And a species is a closely related group within a genus, and crabeaters belong to the
species carcinophagus. The genus and the species give the crabeater seal its scientific name,
Let’s dive in and explore the characteristics of the different classes that belong to
the vertebrates subphylum. You may recall that the different classes of vertebrates are referred to as fish,
reptiles, amphibians, birds, and mammals, like the crabeater seal we just
explored. There are actually three large classes of vertebrates that we commonly call fish:
class Agnatha, Chondrichthyes, and class Osteichthyes. Let’s explore the key characteristics of the fish that belong to each class.
Fish like this lamprey belong to the class Agnatha, and some of these fish are
hermaphroditic, which means that they have both male and female reproductive
organs. They are often called jawless fish. This is because instead of a jaw, they have a funnel-shaped mouth that attaches to
the body of a host. Fish belonging to the class Agnatha are parasitic as they rely on larger fish
species, like this trout, to obtain their nutrition. You may recall that a parasite is an organism that lives in or on a host and gets
their food from the host or at the host expense.
As you can see, species that belong to the class Agnatha have long, thin bodies. Unlike the trout that does not belong to this class and has pairs of fins either side
of a line of symmetry along its body, agnathans, like this lamprey, tend to not have
paired fins. Agnathans do not have bones. Instead, they have cartilaginous skeletons, which makes them light and allows them to
move quickly through water. Let’s explore the features of the fish that belong to class Chondrichthyes next.
These fish tend to live in marine saltwater habitats, which means they are usually
found in seas and oceans. And they are unisexual, which means that they tend to be either male or female. Like class Agnatha, Chondrichthyes also have skeletons made of cartilage; sharks
belong to this class, as do rays. Unlike agnathans, which do not have scales, Chondrichthyes have bony scales covering
their bodies, providing them with protection.
As you can see in the diagrams, they also have ventral mouths, which means that their
mouths are located on the underside of their bodies. And if you were to take an ill-advised look into a shark’s mouth, you would see that
they are filled with multiple rows of teeth, which can be very sharp to grip and
tear their prey. We can also see that unlike agnathans, sharks and rays do have paired fins. Most fish have gills that allow them to carry out gas exchange in order to obtain
oxygen from their watery environment. In class Chondrichthyes, these gills are not covered.
The final class of fish are Osteichthyes, which include fish species like salmon,
tuna, and the trout we looked at earlier. They can be found in either saltwater or freshwater habitats, like rivers and
lakes. Like Chondrichthyes, these fish tend to be either male or female, but they differ in
other distinct physical features. For example, Osteichthyes species are sometimes called bony fish as they have a bony
skeleton instead of the cartilaginous skeletons of the other two classes. The scales which cover the bodies of these bony fish are flat, translucent, and
smooth, and is sometimes referred to as tooth-like.
These fish also have terminal mouths, which means that their mouth is located at the
head end or anterior end of their body. Like Chondrichthyes, Osteichthyes also have paired fins. Bony fish also have a protective covering called an operculum over their gills, which
we can see more clearly if we remove the operculum and take a look inside the fish’s
body. The final most distinguishing feature of bony fish is that they possess a gas
bladder, sometimes called a swim bladder, which you can see inside the fish
here. The gas bladder is a beneficial adaptation as it helps the relatively heavier bony
fish remain buoyant and afloat at a constant water depth without expending excess
Now that we know how these three classes differ, let’s see the similar features that
they have that cause them to be grouped together as fish. Most fish lay eggs, which are fertilized externally to the body of the organism. Fish are also ectotherms. The prefix ecto- means outside, and the suffix -therm refers to heat or
temperature. This explains how ectotherms like fish rely on their external environment in order to
regulate their body temperature. A term that was previously used to describe ectotherms, which you may be more
familiar with, is cold-blooded. However, this term is inaccurate as the blood of fish will not actually be cold.
Class Reptilia, more commonly known as reptiles, are also ectotherms. In fact, if you’ve been in a hot country and seen a lizard, snake, crocodile, or
turtle basking in the sun, it’s most likely attempting to warm itself up. A reptile’s body is typically divided into four regions: a head, a neck, a trunk, and
at sometimes a very long tail. They have fixed scales made of keratin, covering most of the dry skin on their
body. You can see the typical shape of these scales in the diagram, which are unlike the
sleeker scales of fish.
Some reptiles have four pentadactyl limbs. This means that there are five finger-like digits at the end of each limb. And these digits often end in sharp claws, as you can see in this lizard to help them
grip onto surfaces and aid the movement of their bodies. This is only in some reptiles, however, as snakes, for example, have no limbs at
all. Unlike fish which obtain oxygen through their gills, reptiles have lungs to inhale
Most reptiles are unisex. And like fish, the females usually lay eggs to reproduce. Unlike fish, however, reptiles reproduce via internal fertilization within the
female’s body. The eggs produced by reptiles may either be thick and calcified or soft and protected
by structures like nests, built by the parents. Let’s take a look at the key features of class Amphibia next, commonly known as
Amphibians are also ectotherms. And like reptiles, they have four pentadactyl limbs. Unlike reptiles, however, amphibians like frogs, toads, salamanders, and newts all
have smooth and moist skin, which secretes mucus. This feature allows some amphibians to exchange oxygen with our environment through
their skin. In their embryonic and young forms, amphibians tend to have gills to obtain their
oxygen as they primarily live in water, like fish. As they grow into adults, however, their bodies change dramatically as they start to
spend part of their lives out of water. In fact, adult amphibians develop lungs to obtain oxygen via gas exchange with
atmospheric air instead of dissolved oxygen in water.
Amphibians also tend to be unisex, but unlike reptiles, they usually lay their eggs
in water. Partly due to this, like fish, their eggs are usually externally fertilized. Let’s look at the next class of vertebrates Aves, more commonly known as birds.
Birds are endotherms. This means that birds have specialized systems in their body to regulate their
temperature and are less reliant on the environment, like fish, reptiles, or
amphibians. Some examples of birds include penguins and flamingos, eagles, parrots, and
pigeons. Like reptiles and amphibians, birds also have four limbs, but they are considerably
different in structure. They have two hind limbs or legs with four digits instead of five for walking and
running on land, gripping onto tree branches, and sometimes to catch their prey. These limbs also tend to have claws. And like reptiles, the skin on these legs is usually scaly.
Birds also have two wings. While in some species, like the flamingo, these wings allow them to fly through the
air, in flightless birds, like the emperor penguin, they may allow the bird to
effectively fly through other mediums, like water. Most bird species are covered in feathers and have hollow bones to keep them light
and aid their movement, especially during flight. They also have strong thoracic muscles attached to their sternum, which is otherwise
known as their breastbone, to help them move their wings.
Birds like this parrot also obtain oxygen via lungs. By looking at part of the internal structure of this parrot, we can see that birds
also have air sacs connected to their lungs. These air sacs act as bellows, providing a flow of air into the birds’ lungs to
supply extra oxygen for the highly energy-demanding movements involved in
flight. And they can even help birds in making vocalizations to communicate with each
other. Birds are also unisex and lay eggs like reptiles, and fertilization takes place
Let’s look at the final class of vertebrates Mammalia, most commonly known as
mammals. Cats, pigs, and humans like us are all examples of mammals. Mammals like birds are endotherms, and we obtain oxygen via our lungs. Like reptiles, we tend to have four pentadactyl limbs, which, depending on the
species, may end in nails, claws like the cat, or hooves like the pig. The body of a mammal is divided into a head, a neck, a thorax, and an abdomen. Instead of feathers or scales, the skin of mammals is covered in hair, which can be a
light thin layer, such as in animals like pigs and humans, or thick and luscious
like the fur of many cats.
Mammals have distinct teeth depending on their diet. For example, cats tend to have sharp, pronounced canines and incisors for gripping
and tearing animal meat, which makes up most of their diet. Mammals also tend to be unisex and carry out internal fertilization. Most mammals, including pigs and cats, are viviparous, which means their young
develop inside their mother’s body. And when their baby is born, the mammalian mother tends to produce milk in her
mammary glands to feed her young.
Mammals are an incredibly diverse group of vertebrates, however, so let’s compare
some subclasses in a table to see how they can be distinguished from each other. One way that mammals can be categorized is according to their embryonic development
and method of birth. The subclass Prototheria are the only mammals that lay eggs and so do not give birth
to live young. The egg incubates the embryo until it hatches, at which stage the mother will still
feed her young milk produced by her mammary glands. An example of a mammal belonging to this subclass is the duck-billed platypus.
Organisms belonging to the subclass Metatheria do not lay eggs, but instead give
birth to live young. These young are also fed on their mother’s milk and are nursed in a pouch, usually
found on her abdomen. An example of a species in the Metatheria subclass is the kangaroo.
Subclass Eutheria are similar in that they also give birth to live young instead of
laying eggs, and their young are also fed milk. Unlike metatherians, however, eutherians, which account for most mammals, including
carnivores, primates like humans, and many others, complete their embryonic
development inside the uterus in their mother’s body, nourished by a structure
called a placenta which attaches to the growing organism by an umbilical cord.
As this subclass is so diverse, let’s take a look at some of the features of
different orders within Eutheria in a phylogenetic tree. A phylogenetic tree is a diagram that shows us the evolutionary relationships between
closely related organisms. The order Proboscidea, which includes animals like elephants, are distinguishable by
their long, muscular trunks and specialized in sizes that elongate into tusks. The order Edentata, which includes armadillos, are characterized by a reduced number
of teeth or no teeth at all and strong curved claws. The order Rodentia includes all rodents like rats and mice and possess one pair of
chisel-like incisors in their jaw, a long tail, and small ears. Animals that belong to the order Lagomorpha, like rabbits, have chisel-shaped
incisors in both their upper and lower jaw. Their hind legs and ears are also longer than rodents, but they have shorter
Primates, like humans and chimpanzees, are characterized by our opposable thumbs, our
highly developed nervous system, and a very large brain. Carnivores belong to the order Carnivora. And like wolves, they have long, pointed canine and premolar teeth and often have
strong curved claws. Horses belong to the order Perissodactyla. And the organisms in this group have hooves with odd numbers of toes. And the herbivorous diet is aided by their large grinding teeth. The order Cetacea includes aquatic mammals, like whales. Species in this order have paddle-like four limbs, but no hind limbs, and a
horizontal tail fin.
Herbivorous species like camels, which have even numbered toes, belong to the order
Artiodactyla. Organisms that feed on insects like hedgehogs are grouped into the order Insectivora,
and their front teeth extend like pincers to help them catch their insect prey.
The final order in subclass Eutheria are chiropterans, like bats. They are distinguished by stretches of skin between their finger bones that allow
them to fly. They are mostly nocturnal and can be carnivorous or herbivorous depending on the
Let’s review the key points that we’ve covered in this video. Kingdom Animalia can be divided into two subphyla, vertebrates and invertebrates. Vertebrates are organisms that have a vertebral column, otherwise known as a
backbone. We looked at the different classes of vertebrates, including the three classes of
fish. We also saw how class Mammalia can be divided into subclasses according to their
methods of reproduction.