The characteristics on the basis of which organisms are classified are called taxonomic bases. The Greek philosopher Aristotle divided red-blooded animals into two groups called Enaima and non-red-blooded animals. Enaima is divided into two parts. Ovaries and uterus. The main basis of classification is-
1. Number of Cells
(i) Unicellular Animal: They are protocellular and their body is made up of only one cell. Protozoa are unicellular organisms. Such as Amoeba (Amoeba proteus), Malaria parasite (Plasmodium vivax), Entamoeba histolytica etc.
(ii) Multicellular Animal: They are true cells and their body is made up of multiple cells. Animals of the order Metazoa, Porifera and Chordata are multicellular. Such as Hydra- (Hydra vulgaris), Ilisha (Tenualosa ilisha), Doel (Copsychus saularis) etc.
2. Body Shape
(i) Micro animal: Those animals which are very small and microscopic are called Micro animals. For example, malaria bacteria (Plasmodium vivax), dysentery bacteria (Entamoeba histolytica) etc.
(ii) Macro animal: Those animals which are large in size and can be seen with the naked eye are called macro animals. For example, guinea pig (Cavia porcellus), Indian elephant (Elephas indicus), blue whale (Balaenoptera musculus) etc.
3. Levels of organization
(i) Cell level structure: The body structure in which some cells combine to perform specific functions is called cellular level structure. Cells perform reproductive functions, nutrient collection, metabolism etc. For example, Porifera animals.
(ii) Cell-planar structure: The structure in which some similar cells are arranged in a specific layer to form a tissue is called cell-planar structure. E.g. Cnidaria animals. Dr. Siddiq Publications
(iii) Organ-level structure: The body structure in which multiple tissues combine to form organs is called organ-level structure. E.g. Platyhelminthes.
(iv) Structure of organ-tantra level: The body structure in which more than one organ forms a system is called structure of organ-tantra level. Such structures first appeared in Nemertean sea creatures. Such as Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata and Chordata.
4. Way of living
(i) Free living: Animals that move freely. For example – pigeon, cow
(ii) Parasite: Those organisms which live in the body of another organism and take food from it are called parasites. For example, worms, lice etc. (iii) Symbiont: When two different species live together and benefit both, it is called symbiont. For example – Chlorohydra viridissima and Zoochlorella.
(iv) Commensal: Two different species of organisms living together are called symbionts, neither benefiting nor being harmed by the other. For example, the sucking fish called Remora shows symbiosis by attaching to the body of Shark or any other fish.
5. Embryo layer
After fertilization the zygote divides by cleavage process to form blastomere. Blastomere cells are arranged to form a distinct morula. Later the cells of the morula dasa give rise to the hollow blastula. Blastula cells develop into bilayer or trilayer gastrula. Scientist Heinz Christian Pander discovered the embryonic layer. Multicellular organisms are divided into two groups based on the embryonic stage.
(i) Diploblastic animal: Those animals which have two layers in the embryo are called diploblastic animals. Their embryos have two layers called ectoderm and endoderm. Between the ectoderm and endoderm is an acellular layer called mesogia. Animals in the order Cnidaria are bivalves. Such as Hydra (Hydra vulgaris), Jellyfish (Aurelia aurita), Obelia (Obelia geniculata) etc.
(ii) Triploblastic animals: Animals which have three layers in their embryo are called triploblastic animals. The outer layer is the ectoderm, the middle layer is the mesoderm and the inner layer is the endoderm. The order Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata and Chordata are phyla.
6. Notochord
(i) None chordata: Those animals which do not have notochord are called non-chordata. Protozoa, Metazoa, Porifera, Cnidaria, Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda I Echinodermata are non-chordata. Such as Hydra (Hydra vulgaris), cockroach (Periplaneta americana), tapeworm (Taenia solium), snail (Pila globosa), sea star (Asterias rubens) etc.
(ii) Chordata: All animals which have notochord in their body are called Chordata. All animals in the order Chordata have notochords. Animals in the order Chordata are divided into three suborders.
• Urochordata: Urochordata only have a notochord in the tail region in the larval stage. E.g. Ascidia.
• Cephalochordata: In Cephalochordata the notochord extends from the apex of the head to the caudal region. Their notochord remains for life. For example, Branchiostoma.
• Vertebrata: In vertebrates the notochord is replaced by the spinal cord. E.g. Ascidia.
7. Vertebral column
(i) Invertebrate: Those animals which do not have backbone are called invertebrates. Protozoa, Metazoa, Porifera, Cnidaria, Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda I Echinodermata are invertebrates. For example – snail (Pila globosa), fly (Musca domestica), earthworm (Metaphire posthuma) etc.
(ii) Vertebrate: All the animals which have backbone in their body are called vertebrates. Animals in the order Chordata are vertebrates. For example, lion (Panthera leo), tiger (Panthera tigris), cow (Naja naja), cow (Bos indicus) etc.
8. Symmetry
Balanced distribution of different parts of the body is called symmetry.
(i) Asymmetry: If the body of an animal is not divided into two equal parts along the center or axis, it is called asymmetric. For example – Snail (Pila globosa), Amoeba (Amoeba proteus), Spongilla (Spongilla proliferens), Sponge (Cliona celata) etc.
(ii) Bilaterally symmetrical: When the body of an animal is divided along the center into two equal parts, it is called bilaterally symmetrical. Their taxonomic level is Bilateria. Animals of the order Platyhelminthes, Arthropoda I Chordata are bilaterally symmetrical. For example, humans (Homo sapiens), cockroaches (Periplaneta americana), frogs (Duttaphrynus melanostictus), butterflies (Pieris brassicae), Inlish (Tenualosa ilisha), pigeons (Columba livia), liver worms (Fasciola hepatica) etc.
(iii) Biradial symmetrical: If the body of an animal is divided longitudinally along the axis to get two equal parts, it is called biradial symmetrical. In this case four equal parts are obtained as the cutting planes are at right angles. Such symmetry is called Tetramerous symmetry. Such as Anthozoa, Ctenophora, Ceoloplana etc.
(iv) Radial symmetrical: If the body of an animal is divided along the center to get more than two equal parts, it is called radial symmetry. Their taxonomic level is radiata. Such as Hydra (Hydra vulgaris), Jelly fish (Aurelia aurita), Sea anemone (Metridium dianthus), Sea snake (Astropecten auranciacus).
(v) Spherical symmetry: If the body of an animal is divided repeatedly along the center and two equal parts are obtained each time, it is called spherical symmetry. Like Volvox globactor, Radiolaria (Acrosphaera trepanata), Haliozoa (Gymnosphaera albida) etc.
9. Segmentation/Metamerism
When the body of an organism is made up of similar parts, it is called fragmentation or metamerism. Each segment is called a metamere or somite.
(i) Asegment: Those animals which have no segmentation in their body are called non-segmented animals. Chordata are unsegmented animals. For example, humans (Homo sapiens), starfish (Asterias ruben) etc.
(ii) Homonomous metamere: Animals whose body is made up of similar segments are called homosegmented animals. Annelida is a group of animals. For example, earthworm (Metaphire posthuma), cockroach (Hirudinaria medicinalis) etc.
(iii) Heteronomous metamere: Those animals whose body is made up of unequal or different types of segments are called heteronomous animals. Animals of the order Arthropoda are unclassified. For example – cockroach (Periplaneta americana), Prajaprati (Pieris brassicae and Palilio krishna) etc.
10. Coelom
In animals, the body cavity between the alimentary canal and the body wall is covered by a membrane called peritoneum and filled with chyloemic juice. It arises from embryonic mesoderm. According to the nature of Siloam, animals can be divided into 4 categories.
(i) Acoelomate: Animals that do not have a coelom are called acoelomates. Their spaces are filled by spongy parenchyma cells. Animals of the order Porifera, Platyhelminthes, Cnidaria and Ctenophora are asilomates. For example – Tapeworm (Taenia solium), Hydra (Hydra viridis), Liverworm (Fasciola hepatica) etc.
(ii) Pseudocoelomate: The body cavity between alimentary canal and body wall is not covered by peritoneum in animals called pseudocoelomate. Animals of the order Nematoda, Rotifera, Acanthocephola, Ectoprocta and Kinorhyncha are pseudocylomates. Such as round worm (Ascaris lumbricoides), eye worm (Loa loa), hook worm (Ancylostoma duodenale) etc.
(iii) Coelomate: The body cavity between alimentary canal and body wall is covered by peritoneum in animals called coelomate. Animals belonging to the order Annelida, Arthropoda, Echinodermata, Mollusca, Hemichordata and Chordata are coelomates. Such as earthworm (Metaphire posthuma), human (Homo sapiens), mosquito (Culex pipiens), sea star (Asterias vulgare), hilsa (Tenualosa ilisha), oyster (Unio marginalis) etc.
Silom is divided into two parts based on embryonic development.
(i) Psyzocilous chyloem: The chyloem that arises from embryonic mesodermal tissue is called zygosylous chyloem. For example Annelida, Arthropoda and Mollusca.
(ii) Enterocillus chyloem: The chyloem arising from the embryonic mesodermal sac is called enterocylus chyloem. For example, Echinodermata and Chordata.
Importance of coelom
(i) Cells help in independent growth and movement of various organs.
(ii) It protects the various parts of the animal body from friction.
(iii) Siloam helps transport waste materials, nutrients and gaseous materials.
(iv) It acts as a hydrostatic skeleton during locomotion of the animal.
11. Alimentary canal
Animals are divided into two groups based on alimentary canal.
(i) Parazoa: All the animals which do not have alimentary canal are called Parazoa. Animals of the order Porifera are Parazoa. For example, Cycon (Scypha gelatinosum), Spongilla (Spongilla lacustris), Cliona (Cliona celata) etc.
(ii) Enterozoa: All organisms having alimentary canal are called Enterozoa. Enterozoa are animals of the order Annelida, Arthropoda, Nematoda, Platyhelminthes, Echinodermata, Cnidaria, Mollusca and Chordata. Asexual reproducers and sexual reproducers.
12. Reproductive characteristics
Animals are divided into 2 groups based on reproductive characteristics.
(i) Asexual reproducer: Those animals which do not have any specific reproductive organs and reproduce without the help of opposite sex are called asexual reproducers. Asexual reproduction is of three types. These are-
* Binary fission: The process in which an organism divides to produce two offspring is called bisegmentation. Such as Euglena (Euglena viridis), Hydra (Hydra vulgaris) etc.
* Multiple division: The process in which a single-celled organism divides repeatedly to produce numerous offspring is called multiple division. For example – Entamoeba histolytica, Paramecia (Paramecium caudatum) etc.
* Budding: The process by which new organisms are created by budding from the mother body is called budding. Such as Hydra (Hydray viridis).
(ii) Sexual reproducer: The process of reproduction through the union of sperm and egg of two animals of opposite sex of the same species is called sexual reproduction.
* Oviparous: The animals that lay fertilized or unfertilized eggs and the eggs hatch outside the mother’s body to develop into young animals are called oviparous. For example – fish, birds, snakes, poultry, turtles, frogs etc.
* Endo-viviparous: The animals that lay eggs and after fertilization the eggs develop in the mother’s womb and become babies are called endo-viviparous. For example, sharks, some amphibians etc.
* Viviparous: Those animals whose eggs are fertilized inside the mother’s body and the embryo receives nutrients from the mother’s body through immortality and develops into a full-fledged baby animal are called in utero. For example, mammals.
13. Tagmatization
The process by which the animal body is divided into larger segments or tagmas is called regionalization or tagmatization. Each region is called a tagma. The evolutionary process by which regionalization is accomplished is called tagmosis. The bodies of animals of the class Insecta of the order Arthropoda are divided into three tagmas. Head, thorax and abdomen. For example – grasshopper (Poekilocerux pictus).
14. Polarity
The polarization of the animal body based on the position of the head and mouth is called marginality. There are five types of limitations in the animal body.
(i) Anterior end: The end of the animal where the head is is called the anterior end. This is called the oral edge.
(ii) Posterior end: The end opposite to the head of the animal is called the posterior end. called the paraoral edge.
(iii) Dorsal end: The upper surface of the animal’s body is called the dorsal end.
(iv) Ventral end: The bottom of the animal’s body is called the ventral end.
(v) Lateral end: The bottom of both sides of the animal body is called lateral end.
15. Planes
The region along which the animal body can be divided into right and left or longitudinally and transversely or anteriorly and posteriorly is called floor. There are generally three types of floor in the animal body.
(i) Sagittal plane: The plane by which the body can be divided into right and left is called midline plane.
(ii) Frontal plane: The plane by which the animal body can be divided into digital and dorsal parts is called frontal plane.
(iii) Transverse plane: The plane by which the animal body can be divided into anterior and posterior parts is called transverse plane.
16. Cleavage and development
The process by which the zygote divides repeatedly to form a multicellular embryo is called cleavage. If the cleavage is complete it is called holoblastic and if it is partial it is called meroblastic. At the time of cleavage, the end of the egg containing the yolk is called the vegetative pole and the end containing the nucleus is called the animal pole.
Cleavage is of two types on the basis of swelling. These are-
(i) Indeterminate Cleavage: In embryonic development where every cell produced in the initial stage of cleavage has the capacity to form an embryo is called indeterminate cleavage.
(ii) Determinate cleavage: In embryonic development, the specific cells produced in the initial stage of cleavage possessing the ability to form an embryo is called determinate cleavage. Dr. Siddique Publications
Cleavage is of three types based on the cleavage plane. These are-
(i) Radial Cleavage: The cleavage process in which the cells of the zygote divide evenly and radially is called radial cleavage. Auricular cleavage occurs in animals of the order Arthropoda.
(ii) Bilateral cleavage: The process in which the zygote cleaves anteriorly up to the second cleavage and transversely along the midline from the third cleavage is called bilateral cleavage. This results in two rows of four cells each. Bilateral cleavage occurs in animals of the order Chordata.
(iii) Spiral Cleavage: After the anterior and transverse divisions of the zygote, during the third division, when the blastomeres of the animal pole change slightly with the blastomeres of the vegetal pole, it is called spiral cleavage. Spiral cleavage occurs in animals of the order Annelida and Mollusca.
Bilaterally symmetrical animals are divided into two groups based on cleavage and embryonic development.
(i) Protostomia: Cleavage in protostomia animals is spiral and definite. Their embryonic blastopore develops into a pore. Such as Annelida, Arthropoda and Mollusca.
(ii) Deuterostomia: Cleavage in Deuterostomia animals is spiral and indeterminate. Their embryonic blastopore develops into an anus. E.g. Echinodermata and Chordata.
17. Vascular system: The system through which water and blood flow is called circulatory system. Water is the main means of transportation for underdeveloped animals. Animals of the group Periphera have a vascular system and animals of the group Echinodermata have a water circulatory system. The main means of circulation in advanced animals is blood or liquid connective tissue. There are two types of circulatory system.
(i) Closed circulatory system: The circulatory system in which blood always flows through the vessels is called closed circulatory system. The circulatory system of animals of class Annelida and Chordata is closed type.
(ii) Open circulatory system: The circulatory system in which blood flows through different parts of the body is called open circulatory system. The circulatory system of Arthropoda and Mollusca animals is open type.
18. Skeleton
The system that structures the body, carries the weight of the body and preserves the body is called the skeleton. There are two types of skeletons.
(i) Exoskeleton: The skeleton that forms the outer structure of the body is called exoskeleton. Exoskeleton of vertebrates like hooves, horns, claws, scales, feathers, beaks etc. The exoskeleton is the chitinous plate of arthropods and the calcareous shell of molluscs.
(ii) Endoskeleton: The skeleton which forms the internal structure of the body is called endoskeleton. The bones and cartilage of vertebrates are the endoskeleton. Invertebrates have endoskeleton. Spicules of Parifera, calcareous plate of Echydermata etc. are endoskeleton. Molluscs do not have skeletons. Earthworms, worms, cockroaches, butterflies etc. do not have skeletons.