Animal Classification: Basis, and Principles of Binomial Nomenclature

There are approximately 8.7 million species on earth and their variety is simply astonishing. Besides, no two individuals of any animal-form are exactly alike. Hence, there must be some arrangement to group them in orderly categories to study the animal world. The diversity or differences also signify that organisms are really not so different from one another.

Among 8.7 million species, 6.5 million species live on land and 2.2 million in oceans and other water bodies. Zoologists expect that only 15% of species are identified to man from the estimated 8.7 million species.  Only about 953,434 animal species have been catalogued in the Animal Kingdom.

Animal classification helps to know their characteristics, as well as their differentiations with other organisms.

The branch of Biology which deals with the identification, nomenclature and classification of animals is known as taxonomy or systematics. The basis or scheme of classification adopted in animal taxonomy is the same as the one utilized in plants. Modern taxonomy endeavors to synthesize the progress of biology in its all major disciplines and tries to depict the true inter-relationship between animals in evolutionary sequence.  


The term ‘Taxonomy’ (Gr. Taxis-arrangement, nomos-law) was first introduced in 1831 by Swiss Botanist Augustin Pyrame de Candolle in proposing the theory of plant classification. Later, it was introduced also in Zoological classification.

The term may be defined after Simpson as “Taxonomy is the theoretical study of classification, including its bases, principles, procedures and rules”.

Significance of Taxonomy

  • It gives a clear-cut idea about the diversity of animal forms and provides sufficient information permitting a reconstruction of the phylogeny of life.
  • It provides numerous evolutionary phenomena and thus makes them available for causal study by other branches of biology.
  • It gives the classification of animals which are of great hierarchic and explanatory value in most branches of biology.
  • It provides important information which helps in construction of the phylogeny of life. 
  • It is the only branch of biology which contributes relevant information needed for all other branches of biology.
  • It forms the base for the study of principles and fractions of classification and almost reflects the result of biological classification.
  • It contributes to the expanding of biology and maintains balance within biological science as a whole. 

Important Terms and Definition

Classification: Zoological classification may be defined as “the ordering of animals into group or sets on the basis of their relationships, that is, of associations by contiguity, similarity or both” (Simpson).

Systematics: The systematic includes taxonomy, classification and nomenclature. The term is derived from the Latinized Greek word systema. Systematics may be defined as the scientific study of the kinds and diversity of organisms and of any or all relationships among them. In short, systematic is the science of the diversity of organisms.

Nomenclature: The term nomenclature is defined “as the application of distinctive names to each of the groups recognized in classification”. Nomenclature deals with the determination of the correct scientific name to a known plant or animal.

Binomial Nomenclature: Binomial nomenclature is defined as the present method of naming plants or animals scientifically. This system was first invented by Linnaeus. According to this system, every species of plant or animal is given a name in Latin, in two parts (binomial). Among them, one part is the specific epithet or trivial name which is diagnostic of the species and other part is the generic name signifying the genus. The generic name is written first where first word should be written with capital letter and the specific second with a small letter such as Panthera tigris. Here Panthera denotes the generic name while tigris, the specific name.

It may be noted that all scientific names used by various authors before the publication of the twelve edition of Systema Naturae of Carolus Linaeus are described. 

Taxon: Taxon is defined as a general term of taxonomic group irrespective of its rank. It may be noted that the names of taxa above genus are Latin names in plural; the highest unit of classification is the kingdom and the lowest unit is species.

Species: A species may be defined as the smallest unit of classification consisting of identical individuals, young and old, having the same parentage and similar chromosome number that interbreed among themselves.

Sub-species: These are phenotypically similar populations of a species which occupy the same geographic area of the range of a species but differs taxonomically from other populations of the species.

Variety: It is a term which is frequently used in classical taxonomy for a heterogenous group of phenomenon  phenomenon like a non-genetic variations of phenotype, domestic breeds, morphs, etc.

General Classification of the Animal Kingdom

The number of known species of animals is more than 953,434. Some of the species are too numerous to be counted, while others are only a few in number. There are various types of animals inhabiting the earth; all of which taken together form the animal kingdom. They are grouped into several broad divisions called phyla. According to their structural similarities and dissimilarities, the whole animal kingdom can be divided into two main groups:

(A) The Protozoa or Unicellular animals: Their body consists of single cell which performs all the functions of life such as Amoeba, Monocystis, Malarial parasites etc.

image of Amoeba
image of Monocystis

(B) The Matazoa: It is a vast group including all the multi-cellular animals. It is grouped or classification as follows:

On the basis of origin of tissue system, the Matazoa is divided as follows:

(1) Parazoa: The body-cells of these multi-cellular animals do not form definite tissue system. The only phylum under Parazoa is Porifera.

image of Scypha
image of spongilla

(2) Enterozoa: The body-cells of these multi-cellular animals have formed definite and well-defined tissue system.

The basis of Animal Classification

Animals are classified on the basis of some specific and unique features. These features are considered as the basis for the classification of animals. The main basis of animal classification is described below:

A.    Level of organization:

The patterns of organization of cells differ in animals in spite of their multi-cellular nature. In animals, the following cellular organizations are seen:

  • The cellular level of organization: In these types of animals, the cells of the body create loose aggregates. Example: Sponges.
  • Tissue level of organization: In these animals, cells of the animal carrying out a similar function are organized in tissues. Example: Cnidarians.
  • Organ system level of organization: In these animals, all the tissues are united together and form organs. Each organ is specialized for a specific function. Example: Animals under phylum Platyhelminthes and higher phyla.
B. Body Symmetry

The arrangement of body parts around a central point or line determines symmetry. Some animals are asymmetrical which cannot be divided into two equal halves along any plane which pass through the center. Example: Sponges. In the animal kingdom there are two types of symmetry.

image of symmetry types

Types of Symmetry

  • Bilateral symmetry: When the body of an animal can be divided into identical left and right halves along only one plane is known as bilateral symmetry. Example: Annelids, Arthropods, Chordates.
  • Radial Symmetry: When the body of an animal can be divided into two equal halves along any plane passing through the central axis is known as radial symmetry. Example: Cnidarians, Ctenophores, Echinoderms.
C. Metamerism

The body is externally and internally divided into repeated, linear series of body units are called metameres or somites. Metamerism is also known as the segmentation.

image of metamerism

Figure Showing Metamerism or Segmentation

D. Polarity

The polarization of the body parts of an animal on the basis of mouth and head position is known as polarity. The end of the body which bears head is called anterior end and the opposite one is the posterior end. Similarity the end with the mouth is the oral end and an opposite one is the aboral end.

E. Body axes and plane

In bilateral animals, there is a hypothetical line along the body center. This is known as the axis. The axis passes away from the head to tail is called the longitudinal axis and another is the transverse axis that passes transversely in the body. Besides these, there is three planes of the body: median plane, frontal plane, and transverse plane.

image of body plane

Figure Showing Body Plane and Axis 

F. Germ Layers

Germ layers are the collection of cells, formed during animal embryogenesis, found in the gastrula stage. The gastrula may be of two or three layers. These layers of gastrula are called germ layers which play a significant role in the primary classification of animals. According to the basis of the number of germ-layers, animals are of two types:  

Image of Germ layers

Figure Showing Germ Layers

Diploblastic animals: These are enterozoans with loosely differentiated two layers of tissue cells-outer ectoderm and inner endoderm. They are separated by a non-cellular partition. The diploblastic animals are placed in two phyla: (i) Cnideria and (ii) Ctenophora.

image of Moon Jellyfish
Aurelia aurita (Cnidarians)
image of Beroe cucumis
Beroe cucumis (Ctenophorans)

Triploblastic animals: These entoerozoans with three layers of tissue: outer ectoderm, middle mesoderm and inner endoderm. Example: Platyhelminthes to Chordates.

G. Coelom

The liquid filled cavity between the alimentary canal and body wall of the triploblastic animals is known as a body cavity. The body cavity that is lined by the mesodermal peritoneal membrane is called coelom. According to Hymen (1951), “ Coelom is the hollow space between alimentary canal and body wall which is lined by mesodermal peritoneal tissue”. The animals are of three categories based on the presence or absence of coelom:

image of coelom types
Figure Showing Coelom Types
  • (i) Acoelomate animals: In these animals, coelom is absent. The body cavity is filled by mesenchyme and muscles. They are found in phylum Platyhelminthes, Nemartea..
  • (ii) Psedocoelomate animals: In these animals, the body cavity is not filled by mesodermal peritoneal tissue. These animals are found in phylum Nematoda, Acanthocephala, Rotifera, Entroprocta.
  • (iii) Coelomate animals: These are animals having true coelom which is lined by a mesodermal peritoneal membrane and include the phyla like Annelida, Mollusca, Echinodermata, and Chordata.

You might also reas: Coelom Vs Pseudocoelom:  Definition, Types, Differences, Functions and Examples

image of starfish
Starfish (Echinoderms)
image of Ampullaria globosa
Ampullaria globosa (Molluscs)
Image of Taenia
Taenia (Platyhelmints)
image of Ascaris
Ascaris (Nemathelminths)
H.  Notochord

The notochord is a flexible rod-shaped structure composed of cells derived from the mesoderm found in the embryo of all higher animals. On the basis of the presence or absence of notochord animals are classified into two groups:

  • Non-chordates: These are animals without a notochord. Example: Phylum Porifera to Echinodermata
  • Chordates: These are animals with the presence of notochord. Example: Phylum Chordata

Principles of Animal Classification

A. Taxonomic Categories

During biological classification, animals are categorized into different groups. These groups are known as taxa (singular taxon). Carolous Linnnaeus (1953) described six taxa in animal classification but at present, we use seven main taxa. In the biological classification, the taxonomic hierarchy is the process where various taxonomic categories, groups, and class are arranged into the successive level. In this case, the sequence should be either decreasing or increasing order from kingdom to species or vice versa. Here, each level is known as taxonomic rank or category where rank for the kingdom is high and species is always lowest.

Taxonomic Hierarchy

1. Kingdom







B. Species and Sub Species

Species is the lowest rank in the taxonomic hierarchy and it is the basic unit of classification. According to Earnest Mayr (1969) ‘Species are a group of interbreeding natural population that are reproductively isolated from other such groups’.

Sub Species: In biological classification, a subspecies is a subdivision that ranks immediately below a species which capable of interbreeding successfully, usually a fairly stable geographically isolated population.

C. Nomenclature of Animal

In biological classification, nomenclature is the system of giving a scientific name of animals or plants. Animal species get its name in two ways:

(a) Binomial nomenclature: Binomial nomenclature is a system for naming animals by means of two Latin names: the first indicating the genus and the second the species to which the organism belongs. Such a name is called a binomial name, a binomen or a scientific name. According to this rule, the scientific name of Royal Bengal Tiger is Panthera tigris where Panthera denotes generic name and tigris denotes specific name.

(b) Tri-nomial nomenclature: Trinomial nomenclature refers to names for taxa below the rank of species. This name has three parts. Here only one rank is permitted below the rank of species such as subspecies. For example, Panthera leo persica (found in India) is one of the subspecies of the lion species, Panthera leo (found in Africa).

Rules of Binomial nomenclature

Swidish naturatist Carolous Linnaeus introduced binomial nomenclature in his 10th edition of the book Systema Naturae published in 1758. At present the International Committee has been established to frame the rules and regulations regarding binomial nomenclature for plants and animals. It is known as the International Council for Binomial Nomenclature (ICBN). Following are some of the major guidelines for scientific naming of animals:

  • Each animal should have only one scientific name; the same name should not be used for two or more species under the same genus.
  • Each binomial name should have two parts. The first part refers to the name of the genus and the second part to the name of the species.
  • Every scientific name should have words either in Latin or to be Latinized.
  • Both the name should be printed in letters or else they should be underline separately. For example, Felis leo or Felisleo.
  • The name of the genus is noun which should start with a capital letter and the name of the species is adjective and start with a small letter.
  • Name of the scientist who first identified and described the species should be abbreviated and written after the species name, preferably in brackets. For example, Homo sapiens Linnaeus is written as Homo sapiens (L.).
  • According to the law of priority that scientific name of an animal should be acknowledged which first published in an international journal.
  • The generic and specific name of an animal may be of the same. For example, Catla catla. This provision is called tautonym.
  • The specimen used as a sample during identification and naming should be preserved as a type specimen in the laboratory.
D. Validity of Name

Any name proposed for an animal according to the rules of nomenclature is called available name. But it is actually not a valid name, because this name may be null and void because of the following two reasons:

  • If the proposed name is identical in spelling to another such name, that belongs to a different taxon. This is called a homonym.
  • If the proposed animals have an earliest valid scientific name, this name is known as a synonym. Both homonym and synonym have their senior and junior entity. The earliest such name is called the senior while the later name is the junior. According to the law of priority only senior homonym or synonym approved as a valid name for an animal.
E. Law of Priority

Law of priority is a principle in a taxonomy where the first properly published name of a species or genus takes preference over any subsequently published. According to this law, the valid name of a taxon is the oldest available name applied to it, unless that name has been invalidated.

Importance of Animal Classification

In prehistoric time the man had learned to identify animals to meet their requirements. Now a day`s scientist uses classification systems to help them make sense of the animal world around them. Animal science would be a really crazy place if we did not have a way of organizing animals. Some importance of animal classification mention below:

  • A classification system is a useful tool which helps to identify the animal taxa on the basis of the interrelationship among different groups of animals.
  • It helps to know the animal kingdom. In this case, by studying classified animals anyone can gather knowledge about animal kingdom with minimum time and less labor.
  • The classification systems provide relatively stable, unique and unequivocal names for newly identified animals.
  • The classification system is also a very useful tool for understanding the phylogenic relationships between different groups of animals.
  • Classification system provides information to identify beneficiary animal species and thus help in the selection of economically important animals.
  • Classification system helps in pest control. Application of pest control tactics totally depends on the classification of pests, biological control agents and other components of the ecosystem.
  • Knowledge of classification is essential for hybridization and artificial breeding of animals to produce improved quality poultry-dairy animals.
  • Classification system provides the basic foundations of conservation practice and sustainable management of the world biodiversity.
  • The animal classification serves as a base for the development of other biological sciences such as bio-geography etc.