Classification of Animal Kingdom | Zoology Optional Notes for UPSC PDF Download

The field dedicated to categorizing living beings is known as taxonomy. All known species have been grouped into five kingdoms, and among these, there is the Animal Kingdom, where organisms are further categorized into various Phyla, Classes, Orders, Families, and Genera based on distinct identifiable characteristics.

One of the most fundamental forms of classification of animals is the presence or absence of the notochord. Hence, two major groups exist, namely: Chordates and Non-chordates.

Non-chordates and the Chordates

The notochord is a pliable rod composed of a cartilage-like substance. An organism is designated as a chordate if it possesses a notochord at any stage of its life. It's important to note that chordates extend beyond vertebrates, contrary to common misconceptions. Some invertebrates have a notochord at some point in their life cycle, qualifying them as chordates. Therefore, while all vertebrates fall under the category of chordates, not all chordates are vertebrates.

Non-chordates

Non-chordates are animals without a notochord – the rod-like elastic structure that supports the body. This phylum consists of a small group of worm-like, marine species with an organ-system level of organization. Members of phylum Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata and Hemichordata fall under Non-chordates.

The general characteristic features of Non-Chordates are:

• They are cylindrical, triploblastic, coelomate, or pseudocoelomate animals.
• Respiration in these animals takes place through gills, trachea or body surface.
• Most of the times, sexes cannot be distinguished among the members.
• Modes of reproduction involve sexual and asexual
• Fertilization is external, though internal fertilization also occurs in some species.
• The body of non-chordates generally includes an open type of circulatory system.

Chordates

Chordates are animals characterized by the presence of notochord at some stage during their development. Members possess a hollow nerve cord and pharyngeal gill slits. The other general characteristic features of Chordates are as follows:

• They are bilaterally symmetrical, triploblastic, and coelomate with the organ-system level of organization.
• They hold a post-anal tail
• The body includes a closed circulatory system.
• In some members of Phylum Chordata, the notochord is present only in the larval tail, and in some, it is present throughout their life from head to tail region.
• Chordates have many sub-divisions and Protochordates are one of the earliest to evolve.

Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata.

• Subphylum – Urochordata
  It is also referred to as Tunicata which are marine animals. The body of these animals is surrounded by a leathery covering (similar to a tunic, hence the name). Larvae are free-swimming, the notochord is present only in the tail of larvae and after settling on the seabed, they get transformed into sessile adults. They are generally hermaphrodites.
  Examples include – Ascidians, Doliolum, Oikopleura, etc.
• Subphylum – Cephalochordata
  It mainly consists of small, fish-like marine animals in which the notochord is extended along the entire body. The animals also have pharynx, which is large with numerous gill- slits. Members of this subphylum have separate sexes.
  Example include – Amphioxus or lancelet.
• Subphylum – Vertebrata
  In this subphylum, the notochord is present in the embryonic stages and is replaced by a vertebral column in the adult. They have 2, 3 or 4 chambered heart, paired appendages for locomotion and kidneys for excretion or osmoregulation.

Vertebrates Classification

The Vertebrata subphylum is further subdivided into five distinct classes of vertebrate animals. These five classes encompass all animal species with a developed vertebral column and an internal skeleton. To date, more than 66,000 vertebrate species have been identified within the Chordata phylum. Vertebrates are characterized by their bilateral symmetry, possession of a coelom, triploblastic organization, and intricate differentiation of body tissues and organs.

Other characteristic features of vertebrates are:

• Presence of a true vertebral column and internal skeleton with muscle attachment points for body movement.
• A front-side muscular heart with two, three or four chambers.
• Kidneys for excretion and osmoregulation
• A paired appendages which may be fins or limbs.
• Possess notochord during the embryonic stage.
• Vertebrates are the only chordates to possess a brain as a part of the central nervous system.

Classification of Vertebrates

Class Pisces (Fishes)

They are aquatic animals, having a streamlined body and a pair of fins which are used for propulsion and movement. Furthermore, fish are cold-blooded, but the discovery of a new species in 2015 has changed this perception. The opah or the moon-fish is a fully warm-blooded fish capable of regulating its body temperature. Endoskeleton may be cartilaginous or bony and respiration occurs through gills. They do not possess eyelids because the surface of the eye is to be kept moist all the time.
Examples of Class Pisces includes dogfish and Rohu.

Class Amphibia 

They usually comprise those organisms which are cold-blooded and require an aquatic habitat to lay eggs. These organisms are mainly characterized by the two pairs of limbs, smooth and moist skin for respiration. They also possess protruding eyes which are protected by usually more than one pair of eyelids. (Frogs have 3).
Examples of Class Amphibia are frog, toad, and salamander.

Class Reptilia 

Class Reptilia comprises those organisms which are ectothermic in nature (cold-blooded). They are characterized by osteoderms which form scales, bony plates or scutes on the skin.  Reptiles also lack an external ear and some reptiles such as snakes are actually “deaf” and instead, pick up vibrations through the ground.  Another amazing sense that only snakes possess is Thermoception. This means that snakes can see infrared radiation emitted by objects or prey.
Examples of Class Reptilia are Tortoise, Wall lizard, Snake, etc.

Class Aves (Birds)

Most members have a streamlined body specially designed to offer low air resistance during flight. In such birds, the forelimbs are modified into wings, with the power coming from breast muscles. Feathers play important roles, from flight, thermal insulation to water-proofing.  All members of this class are warm-blooded and are able to regulate their body temperature. Aves have beaks, which are used for various functions such as preening and feeding. Furthermore, birds are considered to be the living relatives of dinosaurs (evolved from a group of meat-eating dinosaurs called the theropods).
Examples of Class Aves are Parrot, Pigeon, Duck, etc.

Class Mammalia 

These organisms are distinguished by the presence of mammary glands. They have two pairs of limb for walking, grasping, swimming, flying, etc. Digits are provided with claws, nails or hooves. Skin is covered by hair and they have an external ear called pinnae. They are warm-blooded animals.
Examples of Class Mammalia include monkeys, lions, bats, squirrels and humans.