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.

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.

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.

(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.

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.

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.

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.

(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.

(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.

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.