Animals are absolutely everywhere! Even crowded cities are swarming with animal life, from the smallest creatures such as bugs and mice, to bigger ones such as dogs and birds. According to scientists, there are over tem million types of animals all over the globe, and they are classified into several categories.
To begin with, one must understand that this classification is also based on the origins of animals. Around 3.5 billion years ago, it is believed that life forms started to appear. Later on, they would evolve and become what they are today. From unicellular microorganisms to multicellular life forms, animals have separated into different species and families, differentiated by numerous vital characteristics.
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The sole determination of the animal evolution in time and how exactly they should be put into classes is what biologists nowadays occupy their time with. Although it might seem simple, animal classification needs to take into consideration every minute detail of every member of every species, and only after the thorough analysis of its features can it be put into a certain group. Apart from the physical aspects, animals are also classified according to their behavior, because the way they act defines what they are.
According to Hollar, scientists have separated the entire animal kingdom into several clusters, so that it comes easier to put each animal into a category. The main groups of animals have specific names to reflect their size and importance: the phylum is the largest group of animals, describing those animals with essentially dissimilar body parts. Each phylum is made out of multiple classes, which are divided into orders, and the orders are separated into families. Finally, the families are made out of genera, and each genus divided into species. Each member of a species is closely related to another member of the same species.
Due to this thorough classification, scientists find it easier to put every type of animal into the proper species, then genus, family, order, class and phylum. Although this is maybe the best way to classify animals, it can be rather hard to remember all the scientific names of the classes. This is why there is another type of classification which is more accessible to students and those who have only just started studying about the animal kingdom. This classification divides the entire animal kingdom in two: animals with backbone and animals without backbones.
The animals without backbones are the simplest, most ancient animals that exist in the world. They can be either unicellular or multicellular organisms, and some of them do not even have digestive, circulatory or nervous systems. This class of animals includes the flatworms, the roundworms, the segmented worms, the arthropods, the Mollusca phylum animals and the “spiny-skinned” animals.
The flatworms and the roundworms are harmful organisms that infect other animals as well as the human and cause serious diseases. The segmented worms include some harmful worms as well as those worms that reside in the ground and are harmless to other animals. The arthropods are insects such as the centipedes, the arachnids and the crustaceans (Hollar, 55) the Mollusca phylum includes soft bodied animals such as clams, oysters, octopuses, snails and squids. The “spiny-skinned” animals include sea stars, sea urchins and sea cucumbers and they all live in salty water. As expected for animals that do not have many of the vital systems to live on earth, the animals without backbone live in the water or in the earth, surviving by developing certain special features.
The bigger class of animals is the animals with backbones. Here is where the humans take part, more specifically in the mammals category. Apart from the mammals, the backboned class includes fish, amphibians, reptiles, birds and primitive chordates. They differentiate themselves from the animals without backbone because of their more complex bodily structure.
The primitive chordates include sea dwellers such as the amphioxus and the lamprey. The fish category is a vast one, describing thousands of fish species and members. They all have in common the fact that they are cold blooded, they cannot live outside water and they breathe with the help of their gills. The most well-known amphibian animal is the frog, but there are a few other animals that should be noted: toads and salamanders. Their most important trait is their ability to live both under water and on land. Reptiles are thought to have evolved from amphibians, and birds and mammals have evolved from reptiles. (Hollar, 65) What distinguishes reptiles from other animals is the fact that their body is covered with dry scales and that they have cold blood, thus forcing them to seek the heat of the sun beam in order to surpass certain cold periods of time.
Birds are complex animals, which have appeared on earth by evolving from certain types of amphibians and reptiles. Their wings are anatomically programmed to fly, due to their hallow bones and aerodynamic shape. Lastly, the mammal class is definitely the larges one. It includes animals such as the great blue whale and the dolphin, as well as the gorilla and the human being. What is there to notice about mammals is the fact that they bring to life live cubs, which they feed with their own milk.
To summarize, there are many classes of animals. One just needs to know which one to choose in order to either classify and animal or find it. The animal kingdom is an enormous world filled with wonderful life forms. As Hollar said in his book, there is still much to be discovered about the astoundingly complex world we live in and the secrets that the life of animals share.