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Classification of Animal Kingdom

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The classification of the animal kingdom is the process of classifying animals in a hierarchy. A predetermined number of levels, such as kingdom, family, or genus, form the basis of the ranking system. Depending on the goal of the categorization, the basis for that classification may change. A biological classification typically uses evolutionary and morphological similarities as its foundation.

Animal Kingdom: Basis of Classification

The animal kingdom, which includes all animals, is the largest of the five kingdoms. Animals share the same mechanism of nourishment, known as the heterotrophic mode, as plants do. They are multicellular eukaryotes, lack a cell wall and chlorophyll, and are eukaryotes. They are connected to their cell arrangement, body symmetry, level of organization, coelom, existence or absence of notochord, etc. in addition to these similarities. The animal kingdom has been divided into 11 separate phyla based on these characteristics.

Animals are classified according to a number of fundamental characteristics, which include:

  • Levels of Organization
  • Symmetry
  • Diploblastic and Triploblastic Organization
  • Coelom development
  • Segmentation of the body and
  • Presence or absence of Notochord.

Levels of Organization

In this category, the organisms are divided on the basis of the level of functioning they have in their systems as explained as follows:

  1. Cellular level: This category is for those animals in which cells are arranged as loose aggregates. For e.g. Sponges.
  2. Tissue level: Organisms in which cells perform the same function get arranged as tissues fall under this category. For e.g. Coelenterates.
  3. Organ level: In many organisms, there is an organ level of organizations, which means that tissues performing the same activities are grouped together to form an organ. Each organ has its specific function. For e.g. Platyhelminthes.
  4. Organ system level: This type of organization is seen more commonly in many species. For e.g., Arthropods, Echinoderms, Chordates etc. represent that organs form a group to perform a functioning system where every system (group of organs) is assigned to perform a certain physiological function.

Symmetry 

Symmetry

Symmetry in animals represents the similar parts or proportions formed after the division of the organism by any plane through its axis. On the basis of the symmetry, the animal kingdom is divided into three parts.

  1. Asymmetrical: The organism which shows no symmetry after division through any axis are called asymmetrical organisms. For e.g. Sponges.
  2. Radial Symmetry: The organism which divides into two exactly equal parts when any plane passes through the axis. Such organisms are called radially symmetrical. For e.g. Ctenophores, Coelenterates etc.
  3. Bilateral Symmetry: Only one plane divides the body into two exact, equal halves. Such species are known as bilaterally symmetrical. For e.g. Molluscs, Arthropods etc.

Diploblastic and Triploblastic Organization

At the blastula stage of the organism, the presence of the number of germ layers defines whether the organism is diploblastic or triploblastic.

  1. Diploblastic organisms: If an organism is having two germ layers in the adult stage, i.e. ectoderm and endoderm. Such organisms are called diploblastic organisms. For e.g. Cnidarians and Ctenophores.
  2. Triploblastic organisms: As an adult, the organism is having three embryonic germ layers i.e. ectoderm, mesoderm and endoderm. Such organisms are called triploblastic organisms. For e.g. Platyhelminthes, Chordates etc.

 Coelom Development

Coelom

The main cavity in the body of an organism is called a coelom. Mesoderm lines the coelom. On the basis of the presence or absence of coelom, organisms are divided into three categories as follows.

  1. Acoelomate: Organisms in which coelom or cavity is absent are known as acoelomates. For e.g. Platyhelminthes.
  2. Coelomate: Organisms having proper cavity or coelom are called coelomate. For e.g. Annelids, Molluscs, Arthropods etc.
  3. Pseudocoelomates: These organisms have pseudo coelom i.e. they don’t have a proper cavity, and it’s often known as a false cavity. They have patches of mesoderm, so the cavity is not lined by mesoderm. For e.g. Aschelminthes.

Segmentation of the Body

The organisms which have particular patterns on their body or segments come under this category. For e.g. Arthropods, Chordates, Annelids etc.

Presence or Absence of Notochord 

Rod-like structure originated from mesoderm and provides support to organisms in the embryonic stage and as adults. Organisms are characterized by the presence and absence of notochord.

  1. Chordata -Organisms having notochord are known as chordates. For e.g. Vertebrates, mammals, tunicates etc.
  2. NonChordata – Organisms who lack notochord are called non-chordates. For e.g. Platyhelminthes, Ctenophores, Annelids etc.

Animal Kingdom Classification

The presence or absence of the notochord is one of the most basic criteria utilized to classify animals. Thus, there are two main categories: Chordates and Non-chordates. The flexible notochord is formed of a substance that resembles cartilage. An animal is known as a chordate if it has a notochord at any point in its existence. Contrary to popular assumption, chordates do not only include vertebrates. Invertebrates are categorized as chordates because they may at some point in their lives have a notochord. Thus, all chordates are vertebrates, but not all vertebrates are chordates.

Chordates

All vertebrates, or animals with backbones, as well as several invertebrates, or living organisms without backbones, are members of the Phylum Chordata, which is a subclass of the Kingdom Animalia. They have a bilaterally symmetrical body and are classified into three distinct sub-phyla.

Classification of Chordates

The three following subphyla make up the Phylum Chordata:

  • Urochordata (tunicates)
  • Cephalochordata (lancelets)
  • Vertebrata (vertebrates).

Urochordata

It is also referred to as a “tunicate” because the adult’s body is covered with a tunic made of a cellulose-like substance called tunicin. Only during the larval stage can the notochord be observed; once an adult, it is gone. In adults, a dorsal ganglion takes the role of the nerve cord that exists in larvae. The larva transforms and has the ability to move.

Example: Ascidia, Salpa, and Doliolum.

Cephalochordata

There is an atrium present in cephalochordata. Both the adult and larval stages are movable. Throughout life, there is a tail. They undergo gradual transformation. All throughout life, the notochord is present. They have numerous, well-developed pharyngeal gill slits.

Example: Lancelets are born with the notochord and nerve cord. But unlike Branchiostoma, they don’t have a brain or a skeletal spinal column.

Vertebrata

These evolved chordates have a skull that surrounds their brains. In adults, a spinal column takes the place of the notochord. This explains the expression that “all chordates are vertebrates, but all chordates are not vertebrates”. High levels of cephalization are seen. The epidermis has several layers. They are made up of three different muscle types: cardiac, striped, and unstriped. They have an advanced coelom. The gastrointestinal tract is developed. There are three or four chambers in the heart. Their respiratory and excretory systems have grown considerably. All mammals have endocrine glands. With the exception of hogfish, all are sexually reproducing and unisexual.

Example: Humans.

Subphylum of Vertebrata

Vertebrata is further sub-divided into:

  • Pisces (Fishes): They are aquatic organisms with a sleek body and a pair of fins that are utilized for movement and propulsion. Example: Rohu, Catla.
  • Amphibia: They often consist of animals with cold blood that need an aquatic environment to lay their eggs. These organisms can breathe through their two pairs of limbs and their smooth, moist skin. Example: Frog, Toad.
  • Reptilia: Ectothermic (cold-blooded) species are included in the class Reptilia. They are distinguished by osteoderms, which produce scales, bony plates, or skin scutes. Example: Tortoise, Snake.
  • Aves (Birds): The majority of members have sleek bodies that are especially made to provide little air resistance while flying. Such birds have modified forelimbs that function as wings, with the breast muscles providing the force. Example: Parrot, Pigeon.
  • Mammalia: Mammary glands are a defining characteristic of these organisms. They have two sets of limbs for activities such as walking, grabbing, swimming, and flying. The digits have claws, nails, or hooves. Example: Monkeys, Humans.

Characteristics of Chordates

The Phylum Chordata has the following distinguishing characteristics:

Notochord

It is a long, cartilaginous rod that connects the nervous system and the digestive system. In all vertebrates, it serves as a support for the nerve cord and is eventually replaced by the vertebral column.

Dorsal Nerve Cord

The brain and spinal cord are separated from the bundle of nerves that runs along the “back” of the body. It lies dorsal to the notochord and is hollow.

Pharyngeal Slits

They are the holes that connect the mouth and throat and permit water to enter the body through the mouth without going through the digestive system. These holes on the lateral sides of the pharynx are present in all chordates at some point in their development.

Post Anal Tail

It is the body’s extension to the anus. In chordates, the tail is made up of skeletal muscles that aid in locomotion in species that resemble fish. The majority of adult Chordates lack it.

Non-Chordates

Animals without a notochord, an elastic rod-like structure that supports the body, are known as non-chordates. A tiny group of marine animals that resemble worms and are organized at the organ-system level make up this phylum. Non-chordates comprise the phyla Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and Hemichordata.

Characteristics of Non-Chordates:

Non-Chordates have the following general identifying characteristics:

  • They are animals that are cylindric, triploblastic, coelomate, or pseudocoelomate.
  • These creatures breathe through their gills, trachea, or body surface.
  • Most of the time, members cannot be separated based on gender.
  • Both sexual and asexual reproductive methods are available.
  • Although certain animals also experience internal fertilization, fertilization is external in nature.
  • Non-chordates typically have an open circulatory system in their bodies.

FAQs on Classification of Animal Kingdom

1. What are the 7 classifications of animals?

Answer:

The seven major categories of the scientific classification system are: (1) kingdom, (2) phylum or division, (3) class, (4) order, (5) family, (6) genus, and (7) species. The largest group is a kingdom, while the smallest is a species.

2. What are the 8 ways to classify animals?

Answer:

There are seven levels of classification: domain, kingdom, phylum, class, order, family, genus, and species. With the exception of the domain, Linnaeus created this system of classification based on the morphology (physical characteristics) of animals.

3. Who started classifying animals?

Answer:

By creating a structured system of naming organisms, what we now refer to as a taxonomic nomenclature, in the 18th century, Carolus Linnaeus transformed the study of natural history. He used the five levels of class, order, genus, species, and variation to categorize the natural world into three kingdoms.

4. What are the 11 phyla of kingdom animalia?

Answer:

11 phyla of kingdom animalia are Phylum Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata.

5. What are the classifications of sub phylum Vertebrata?

Answer:

Fish, amphibians, reptiles, mammals, and birds make up the five common classes that make up the subphylum vertebrata under the phylum chordata (animals having backbones).



Last Updated : 08 Sep, 2023
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