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Classification of Animal Kingdom

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Introduction to Kingdom Animalia

IVSAT 2024

The animal kingdom is a noteworthy categorization of living beings on the surface of the planet which is also termed as Kingdom Animalia. The members included in this category are further categorized based on some specific features into various Genus, Family, Order, Class, and phyla. The presence or absence of notochord on the animal body is one of the fundamental means of animal classification. It is to be mentioned here that a notochord is made up of a substance similar to that of cartilage and appears like a flexible rod. It is believed that if an animal gets a notochord at a point in its life, then it will be considered a chordate. 

Non – Chordates and Chordates

1. Non – Chordates

Animals that do not have the rod-like structure called the notochord in their body are termed as non–chordates. Essentially, this phylum includes the animals like marine species as well as the members of the phylum Porifera, Hemichordata, Echinodermata, Mollusca, Arthropoda, Annelida, Aschelminthes, Platyhelminthes, Ctenophora, and Coelenterata. They all are considered to be a part of this non – chordates group. 

Some of the General Features of the Non – Chordates are:

  1. They are basically pseudocoelomate, coelomate, triploblastic, or cylindrical animals.

  2. Usually their body holds a circulatory system, open in form.

  3. Fertilization takes place in an eternal manner, whereas some species are found to have internal fertilization.

  4. The reproduction process includes both sexual as well as asexual means.

  5. Among the members of the group, most of the time the sexual distinction cannot be done accurately.

  6. Through the use of the body surface, the trachea, or gills; the respiration of the non–chordates takes place.

Some of the very common examples of the phylum are Saccoglossus and Balanoglossus.

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2. Chordates

Animal bodies that are featured with the presence of notochord during any point of their lifespan are termed as the chordates. The members of this group are characterized by pharyngeal gill slits and hollow nerve cords. Some of the basic characteristics of these creatures are:

  1. The chordates are divided into large numbers and Protochordates are one of those to be evolved at the earliest times.

  2. In some animals, the presence of chordate can be found in the entire body, from head to tail; whereas in some animals, it is present only in the parts of their larval tail.

  3. Unlike the non–chordates, these creatures have a closed circulatory system.

  4. They are generally bilaterally symmetrical, coelomate, and triploblastic.

  5. Their body holds a post-anal tail.

Some of the important and very common examples of this phylum are Lancelet, Amphioxus, Doliolum, Salpa, and Ascidia.

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Classification of Vertebrates

One of the important factors, taken into account while classifying the different kinds of animals in the world, is the presence of a spine or backbone in their body. An animal that has a backbone is called the vertebrate animal and they are in the phylum Chordata. In terms of the vertebrates, the entire notochord or a part of it is replaced by the hard bones. Vertebrates are generally classified broadly into 7 different parts that include the following:

a) Mammals (Class Mammalia)

b) Birds (Class Aves)

c) Reptiles (Class Reptilia)

d) Amphibians (Class Amphibia)

e) Bony fishes (Class Osteichthyes)

f) Jawless fishes (Class Agnatha)

g) Cartilaginous fishes (Class Chondrichthyes)

  • Class Mammalia

Warm-blooded animals having strings of hair in the body and are able to produce milk, come under this classification. This class of animals is the only one who nourishes the offspring with milk and the world has got almost 4, 000 species of mammals altogether. Human beings, Koalas, cows, etc are considered to be included in this category. 

  • Class Aves (Birds)

These are the cold-blooded and bipedal feathered animals, able to maintain a constant temperature range of their body. The limbs in the front side of their body are shaped like wings and most of the animals of this class are able to fly high in the sky, although there are a few exceptions as well. Class Aves has got almost 9, 000 species altogether.

  • Class Reptilia

All the turtles, crocodiles, lizards, and snakes of the earth come under this classification of animals called reptiles. There are almost 1,000 species on the planet that are considered to be included in the Class Reptilia. They are further divided into the following ways:

  • Testudines (almost 3, 000 species of tortoises, terrapins, and turtles).

  • Squamata (approx. 9, 150 species of worm lizards, snakes, and lizards).

  • Sphenodontia ( 2 tuatara species)

  • Crocodilia (approx. 23 species of gharial, alligators, caiman, and crocodiles).

  • Class Amphibia

Basically, amphibians are the animals that live both on land and in water and are considered to have a “double life”. Ranging to almost 3, 900 species; this class of animals consists of caecilians, toads, frogs, salamanders, etc. The amphibians are generally not able to generate the temperature of their bodies and their skin types limit the distribution of their residence from moist to warm climates. Both the gills and lungs are important for them as during the larval stage they breathe with gills which are replaced with lungs when reaching adulthood. 

  • Class Pisces (Fishes)

Fish can be considered as “the gill-bearing aquatic craniate animals that lack limbs with digits”. This class has the most diverse and wide variety of vertebrates that includes nearly 30, 000 species. 41 percent of the total population lives in freshwater, 58 percent in the marine area, and the rest 1 percent move between these two areas. The body structure of the species generally follows the pattern of head, trunk, and tail. All the fishes do not have fins which can be categorized into two parts: paired (pelvic and pectoral fins) and unpaired (anal, caudal, and dorsal fins). 

  • Bony Fishes (Osteichthyes Class) 

All bony fishes belong to the Osteichthyes class. Osteichthyes, like other fishes, are cold-blooded animals that breathe through gills and swim with fins. Bony fishes have a skeleton made of bone, scales, paired fins, one set of gill openings, jaws, and paired nostrils, among other characteristics.

  • Jawless Fishes (Class Agnatha)

Agnatha is fish with no jaws. This category includes lampreys and hagfish. The agnatha class includes some of the oldest animals. Scientists have discovered agnathan fossils dating back 500 million years from the late Cambrian Period. The jawless fishes, the most basic group of living vertebrates, belong to the Agnatha group. Although the majority of agnathan species are now gone, fossil evidence suggests that the group was once very successful and diverse. Lampreys and hagfish are the two agnathan lineages that have survived to this day.

  • Cartilaginous Fishes (Class Chondrichthyes)

Chondrichthyes are a skeletally cartilaginous group of jawed fish. Sharks, rays, skates, and chimeras are among the many species in this category. The majority of them are marine fish. Bony fishes, which belong to the class Osteichthyes, are the other group of fish.


  • All the animals living on the earth are broadly classified into five types and the animal kingdom is one of them.

  • The presence or absence of notochord on the animal body is one of the fundamental means of animal classification.

  • Vertebrates are generally classified broadly into 7 different parts.

  • Class Pisces (Fishes) has the most diverse and wide variety of vertebrates.

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FAQs on Classification of Animal Kingdom

1. What distinguishes chordates from invertebrates?

Invertebrate chordates are the creatures that have a notochord but no vertebral column (backbone) at some point in their development. The major feature that distinguishes invertebrate chordates from vertebrate chordates, or creatures with a backbone, is this differentiation.

2. Invertebrate chordates include which organisms?

Together with the vertebrates in the Vertebrata/Craniata, Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets). Lancelets are phytoplankton and other microorganism-eating suspension feeders. Tunicates are suspension feeders that live on the ocean floor.

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