From early naturalists to Charles Darwin, many tried to observe the animals, list out how many different types exist, and the relationship between them.
Still, it was a long and tiring process because no matter how many they list out, the unknown animals keep coming.
So, the scientists have decided to divide and list animals into animal classes. And since then, the distinction has been relatively easy regardless of the newfound unknown animals.
Now we can identify which class these unknown animals belong to because of this classification.
Well, there are different classes that we can name to get deeper into zoological studies. But we can divide the animal classes into two types for more straightforward understanding. The simple and basic classification among animals is dividing them by what is common among them. Backbone is common among animals. So, we can safely divide two classes; one type of animals that do not have backbones and the one that has backbones.
Those with backbones are labeled as vertebrates, and those who do not have the backbone are labeled as invertebrates. To be more specific and straightforward, an animal with a spinal cord is a vertebrate. Spiders and other insects; remember how they are so flexible? Because they do not have a bone in the back. They are invertebrates. A straightforward and direct example for a vertebrate is none other than you!
The classification does not end with vertebrates and invertebrates, but this is the highest order of classification. Going further deep, there are many animal classes in both vertebrates and invertebrates. Like how vertebrates are differentiated from invertebrates, deep classifications are put together and separated because of common and uncommon attributes. Let us first dig deep into vertebrates.
We already had a previous understanding that vertebrates are put together in a class because of their common attribute of having a backbone. Vertebrates are called the most significant living animals because of their adaptive immune system.
Their skeleton is so that they are adaptive and fit to survive in different environmental conditions. Many amphibians are vertebrates, and amphibians are animals that can live in both land and water. There are five general classes of invertebrates.
To put it simply, fishes are animals that live underwater. But again, not every living thing that is underwater is a fish. As we have discussed earlier, the classes are commoned using common attributes and separated because of different attributes. The commonalities here in classes are their skeletal structure and their bodies.
Fishes have gills and lack limbs. Fishes need water, and that too in the right temperature to survive. Thus they are called aquatic animals. They are called cold-blooded animals and need the right temperature underwater to survive.
They have fins instead of limbs and gills instead of lungs, and scales instead of skin. They move in water to maintain their body temperature in a way to maintain the cold-blooded nature. They can be found in any water body, from most miniature ponds to the largest oceans. They exist everywhere.
Thus, they are called the most significant living aquatic animals. Fish can breathe air, too; we can see some fish jumping up above the surface level to do that. But their primary source of oxygen is their gills. They take in oxygen from water. Gills are unique to aquatic animals, and there are several aquatic animals other than fishes. Let alone other creatures that fish, only fishes have more than 30,000 species in them.
When we read fish, we assume cute little Nemo was swimming across, but the fun fact here is that even Sharks belong to the Fishes class. However, another fun fact is that even though a whale is an aquatic creature, it is a mammal. And whales have lungs, and they need air to get oxygen. Hence, we cannot just classify all the aquatic creatures into fish. Gills are the main difference between fish and other aquatic creatures.
We have come to know that a Whale is not a fish but a mammal. But what is a mammal? Why is a whale not fish? This class is called Mammalia. The expected attribute that all mammals share is drinking the milk of the mother. The glands that generate this milk are called mammary glands.
And every animal that has mammary glands. Another broad commonality is that mammals do not lay eggs. However, there are two or three mammals that do lay eggs but also have mammary glands. Thus, milk and mammary glands are the best commonalities we can use to classify this category rather than reproduction.
Usually, most mammals are hot-blooded, unlike fish. Thus, they live in hotter temperatures, i.e., on land. But Whales and few other aquatic mammals are an exception. Whales feed their young milk till they reach a particular stage of growth. The direct example of a mammal is a human and another ape. We can see many mammals around us, cats, dogs, cows, horses, and donkeys.
But unlike fish, mammals do not need to try to maintain their body temperature constantly. Warm-blooded mammals have wider acceptance towards environmental conditions and temperature. For example, we can see wolves and mastiffs living comfortably in the snow.
Mammal’s hot blood gives them the adaptability to survive in environments of different temperatures. The fun fact is that not only Whales but even Dolphins are mammals! Yes, dolphins feed their young ones their milk. In most mammals, it is the female of the species that produces milk.
It is easy to spot mammals because of the bony structure too. But we cannot use this as a basis to generalize all mammals. But we can generalize most of the mammals as the primary standard features other than feeding milk are legs, fur/ hair, milk, does not lay eggs. But again, this is the easy way to spot most mammals. But exceptions exist like Platypus, Whales, and other aquatic beings.
Birds can be generalized as vertebrates that can fly. Birds lay eggs, and most of them fly. You can spot a bird just by looking at it. How, may you ask? It is because of the most significant common factor, the feathers! It is only birds that have feathers. Other than these, all the birds have beaks, wings, and claws. One knows and has no difficulty labeling any other animals that belong to this class.
Birds have more than 15,000 species and are pretty diverse. Only birds that do not look like birds are penguins. They belong to the class penguins but do not give you the automatic attribute of a bird when looked at. And Kiwi birds cannot fly either. So, flying here is just a most common attribute but not the one that implies all the animals in this class. One significant difference between birds and mammals is that, of course, that birds do not produce milk for their young.
Birds like hen’s act as a nutrient source for mammals, especially humans. Humans find hens tasty and protein-rich food. Birds, unlike mammals, do not have teeth. It is their beak that helps them catch insects and feed themselves. They have one of the most light-weighted skeletons but have a complex heart of four chambers. Even the metabolism of birds is rapidly faster than any other animal. Their eggs are considered a valuable source of food too.
The tallest bird is an Ostrich. Though they are quite big, they have feathers, produce eggs, and, most importantly, lay eggs! The fun fact is that there are studies of fossils that claim that modern-day birds are descendants of early dinosaurs. And a chicken being the most recent common descendant of dinosaurs. Once apex predator now is the best food available.
We had come across amphibians earlier when we were talking about aquatic animals. That is because amphibians can live both in water and on land. This class of animals is called Amphibia. The direct animal that strikes us when we think about land and water is a frog! You can see frogs having fun in the water and on land.
But other animal species belong to amphibians. But frogs occupy more popularity. To be there are around 4000 species classified under amphibians as of now.
Amphibians and the very name of the class have a more poetic background than biological. The title Amphibia is a poetic and symbolic representation of living a dual life. And that is the exact name for these little creatures because their life is both on water and on the land.
We might also think about Seals that lie around the shores and live in both water and land. But no, they are not amphibians. They are mammals. That is because of the presence of mammary glands and more common attributes to Mammalia than Amphibia.
This indicates that Amphibia, the name might suggest a dual life, but the dual nature is not the only common attribute used to classify these animals. Amphibians have specific attributes of mammals and fishes, but they do not have mammary glands. They, too, like fish, lay eggs. And the fun fact here is, they lay eggs on water. Do you remember seeing little black tadpoles in the water?
Many confuse frog tadpoles that swim around in the water as fish tadpoles. This tadpole can only live inside the water and only come to dry land after its tail transforms into legs. The difference between fishes and amphibians again is the scales! Amphibians do not have scales, whereas fish have scales.
The skin of the Amphibia is dry and surprisingly not wet. And they are not warm-blooded, which is why they still need water to live around. Also, they have both lungs and gills.
Now, this might come as a shock for many to come to know that reptiles are vertebrates! Whenever we listen to the word reptile, we do think of a lizard. And we see that Lizards are so flexible and soft.
It is not logical to assume a backbone just by seeing a Lizard. But even they have backbones and thus fall under vertebrates in animal classes. Imagine all the animals you think that look like a Lizard’s cousin; they all belong to the class of reptiles!
When you pictured a cousin of a Lizard, you would have thought about garden lizards, iguanas, crocodiles, and alligators; yes, they are all reptiles, no matter how big. Crocodiles live in both water and land, but they are not amphibians.
And that is because they are closer to reptile class than Amphibia. And what are these commonalities, if you ask? The primary attribute that puts all the reptiles in one class is that they lay eggs in an enclosed protective sac. This sac consists of several eggs.
Fun fact here, turtles and tortoises that do not look anywhere related to lizards are reptiles! One thing here is that the body of a turtle is encased with a shell, but again the shell is a part of their bodies. Reptiles are cold-blooded animals and can only live in entirely conditional environments and specific temperatures. These reptiles have scales, too, but not like the scales of fishes.
You can see the hard rocky scales on alligators and small shiny scales on snakes. In this way, you can even say that the turtle scales are its horns and rocky parts of the shell. Reptiles have a relatively low metabolism, and this causes them to stay in a single area for long hours to store and preserve their energy.
You can see lizards, snakes waiting at the same spot for hours. Many reptiles like to live in water, but they are not amphibians because they neither have gills nor have lungs.
A simple thing to say is that what is not a vertebrate is an invertebrate. But as we are going deep into classifications, let us again explain these types of animal classes. Invertebrates do not have a spinal column.
This spinal column or backbone is the primary skeletal structure that is seen in invertebrates. For vertebrates, backbones are the support system for their entire body. Imagine a human without a spine; he/ she would not stand because it is the backbone that supports the entire skeletal structure.
But again, looking at the invertebrates, we can say that this rule of backbone only exists for humans. There exist animals that do not have backbones and can have their advantages in life. Invertebrates occupy as much as the population of vertebrates.
You can think of many insects, and they fall into the category of invertebrates. To put it simply, imagine squishy things; snails, squids, worms, and many others are direct examples of invertebrates. You can tell by the look of these animal classes that they do not have a backbone.
Again, several classes are divided into invertebrates because not having a backbone is a far broad class to differentiate. In the usual sense, we do not call insects as animals at all. That is because of the preconceived notion that all animals have backbones. Invertebrates are lacking a backbone and do not give the feel of our preconceived notion.
Arthropods are the largest populated and popular invertebrates that can be found almost anywhere around the planet. Spiders are arthropods. The name arthropod too is poetic but symbolizes the species’ common attribute. That is that all the animals that belong to this class have multiple legs, all joint. Imagine a multi-legged creature other than a spider. What do you see? A crab? Because a crab is an arthropod too.
Arthropods are the greatest in population, as said earlier in all the living animals. Their dense population makes them easy to find. And this easy availability makes them food for many animals. Humans, too, take arthropods as a source of their food. Because it is directly available almost everywhere. Many arthropods live in water too.
Well, there are several other invertebrates, but Arthropods are the only primary ones to know about. Many invertebrates are also called pests by humans. Wasps, termites, bugs, ants, and many other such bugs are usually a nuisance for human lives.
We tend to naturally hate them and exterminate them because they are harmful and destroy our sources of food. It is only ideal for exterminating such ‘pests’ for the well-being and survival of our species.
But some insects help build our ecosystem. Some bees carry seeds and drop them elsewhere for the plants to grow. Without bees, plants’ growth is reduced by more than half, and the rate of oxygen decreases.
We need bees as much as we need oxygen. Our ecosystem is linked with the behavior of bees. And that is why many humans raise and observe bees for natural purposes. This is called beekeeping, and with legal permissions, you can be a beekeeper too.
The most favorite invertebrates to humans are none other than caterpillars. We all like to play with the cute caterpillars on leaves. Even if some of us do not like caterpillars, no one says that butterflies are not beautiful.
These caterpillars transform into beautiful butterflies, and they contribute a lot to the pollination of seeds too. They, too, like bees, carry the pollen grain and help the seeds grow into plants.
No matter how much we hate worms and bugs, they are essential for our ecosystem too. Not all worms are as harmful as ringworms.
Earthworms make the soil fertile for many plants to grow. Using earthworms to fertilize the grounds has always been a practical and most used treatment. Some bugs eat other bugs that destroy plants. This only proves that regardless of our preconceived notion or hatred towards the invertebrates, they are animals too and are as essential as any other animal that exists.
As we can see that these are the different animal classes and different species in them that fall into different categories. Even though all the classes, species, and categories are different, there exists some commonality in certain groups.
And sometimes, there are even common attributes in both different animal classes. That only proves that all these different animals of different classes had one super-class where every animal was one!
And that is true, and that is what is called the evolution of life as devised by Charles Darwin. According to Darwin, natural selection is a process where some species changes their form to survive better causes in creating different or a better version of the species.
Such, from a super-class, all the classes have derived and differentiated. No, we all belong to different classes, but at the beginning of life, or the origin of a single cellular organism, we all were one big class that is a ‘cell.’
All the classification and labeling of animals is to have a better understanding of life. The labeling is fit accordingly so that all the animals can be pointed towards a common origin.
There are still many animals that exist, and we still have not acknowledged them. There is a place called the Mariana Trench, which is the deepest ocean on earth. Deep down in the trench, many aquatic animals live without the need for light as a source of their energy.
This proves that our classifications might fit, but there might be different other classes as per scientific studies. Maybe there is something other than vertebrates, invertebrates, and we still do not know yet.
But science is advancing, and humans are growing towards scientific advancements too. It is only safe to assume that one day we will finally be able to record all the animal classes present in our world.