Kingdom Animalia

Kingdom Animalia:

Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom Animalia includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.
Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.
Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa.
Humans make use of many other animal species for food, including meat, milk, and eggs; for materials, such as leather and wool; as pets; and as working animals for power and transport. Dogs have been used in hunting, while many terrestrial and aquatic animals are hunted for sport. Non-human animals have appeared in art from the earliest times and are featured in mythology and religion.
Kingdom Animalia is composed of all animals. The animal kingdom is the largest kingdom among the five kingdoms. Animals are multicellular eukaryotes. But they don’t have a cell wall or chlorophyll like plants. Hence, members of the animal kingdom have a heterotrophic mode of nutrition. Kingdom Animalia has been classified into 10 different subphyla based on their body design or differentiation.

The different subphylum of the animal kingdom are as follows:

1. Porifera
2. Coelenterata (Cnidaria)
3. Platyhelminthes
4. Nematoda
5. Annelida
6. Arthropoda
7. Mollusca
8. Echinodermata
9. Protochordata
10. Vertebrata


Subphylum Porifera:
Porifera means organisms with holes. They are commonly known as Sponges. Features of the poriferan are:
1. Non-motile, multicellular organisms with the hard outer skeleton.
2. Have a porous body.
3. Pores on the bodies create a canal system which helps in the circulation of substances.
4. Not differentiated into head and tail; don’t have a well-developed organ or organ system.
5. Include MarineHabitat.

Example of subphylum Porifera includes- Spongilla, Sycon, etc.

Subphylum Coelenterata (Cnidaria)
The term Coelenteratais derived from the Greek word “kilos” which means hollow-bellied. Their features are:
1. Have a hollow body cavity.
2. The body is differentiated into two ends.
3. Includes all aquatic animals.
4. The body is made of two layers of cells: inner and outer linings.
5. Live in colonies (corals) as well as solitary (Sea anemone).

Example of subphylum Coelenterata includes – Hydra, Jellyfish, etc.
Subphylum Platyhelminthes:
Platyhelminthes are commonly known as flatworms. Their features are:
1. Dorsoventrally flattened body.
2. Complex and have differentiated body structure.
3. Tissues are differentiated from three layers of cells and are triploblastic.
4. Don’t have true internal cavity or coelom.
5. Have bilateral symmetry.
6. Either free-living (Planaria) or parasitic (Liver flukes).

Example of subphylum Platyhelminthes includes -Tapeworm, Planaria, etc.
Subphylum Nematoda:
Phylum Nematoda consists of nematodes or roundworms. Their features are:
1. Nematodes have a cylindrical body.
2. Bilaterally symmetrical and triploblastic.
3. Have pseudocoelom, a false body cavity.
4. Parasitic and causes diseases such as elephantiasis, ascariasis, etc.

Example of subphylum Nematoda includes – Ascaris, Wuchereria, etc.
Subphylum Annelida:
Annelids are commonly known as segmented or ringed worms. They have the following features:
1. Have a segmented cylindrical body.
2. The body is differentiated into head and tail.
3. Bilaterally symmetrical and triploblastic.
4. Have a true body cavity.
5. Habitat: marine, freshwater, and land.

Example of subphylum Annelida includes – Earthworm, Leech, etc.
Subphylum Arthropoda:
Arthropod means jointed legs. Animals which have jointed appendages belong to this phylum. This is the largest phylum in the animal kingdom. Other features are:
1. They are bilaterally symmetrical.
2. Have jointed appendages, exoskeleton, and a segmented body.
3. Have well-differentiated organ and organ system.
4. Have an open circulatory system, but don’t have differentiated blood vessels.

Example of subphylum Arthropoda includes – Spiders, butterflies, and mosquitoes.
Subphylum Mollusca:
Phylum Mollusca consists of a large group of animals. Features are:
1. Bilaterally symmetrical and triploblastic.
2. Less segmented body.
3. Well-developed organ and organ system.
4. Open circulatory system.
5. Limbs are present.

Example of subphylum Mollusca includes- Snails and octopus.
Subphylum Echinodermata:
The term Echinodermata is derived from the Greek words, echinos meaning hedgehog and derma meaning skin. Thus, echinoderms are spiny-skinned animals.
1. Radial symmetry and triploblastic.
2. Have true coelom.
3. Have hard calcium carbonate skeleton structure.
4. Free-living marine animals.

Example of subphylumEchinodermata includes- Sea urchins, starfish, etc.
Subphylum Protochordate:
Protochordates have the following features:
1. Bilaterally symmetrical and triploblastic.
2. Have true coelom.
3. Habitat: marine.
4. The notochord is present at some stages of lives.

Example of subphylum Protochordates includes- Balanoglossus, etc.
The notochord is a long supporting structure that separates the nervous tissues from the gut. It runs along the back of an animal and is a place for muscle attachment that helps in movement.
Subphylum Vertebrata:
Phylum Vertebrata consists of animals with a true vertebral column. They have an internal skeleton where muscles are attached and help in movement. Other features are:
1. Bilaterally symmetrical, triploblastic, coelomates and the segmented body.
2. The body design is complex and well-differentiated.
3. The body has an organ and organ system level of organization.
4. Possess notochord.
Vertebrates are further grouped into five classes. They are-Pisces, Amphibia, Reptilia, Aves, Mammalia.

All animals are members of the Kingdom Animalia, also called Metazoa. This Kingdom does not contain prokaryotes (Kingdom Monera, includes bacteria, blue-green algae) or protists (Kingdom Protista, includes unicellular eukaryotic organisms). All members of Animalia are multicellular, and all are heterotrophs (that is, they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.

Animal cells lack the rigid cell walls that characterize plant cells. The bodies of most animals (all except sponges) are made up of cells organized into tissues, each tissue specialized to some degree to perform specific functions. In most, tissues are organized into even more specialized organs. Most animals are capable of complex and relatively rapid movement compared to plants and other organisms. Most reproduce sexually, by means of differentiated eggs and sperm. Most animals are diploid, meaning that the cells of adults contain two copies of the genetic material. The development of most animals is characterized by distinctive stages, including a zygote, formed by the product of the first few division of cells following fertilization; a blastula, which is a hollow ball of cells formed by the developing zygote; and a gastrula, which is formed when the blastula folds in on itself to form a double-walled structure with an opening to the outside, the blastopore.

It is estimated that around 9 or 10 million species of animals inhabit the earth; the exact number is not known and all estimates are rough. Animals range in size from no more than a few cells to organisms weighing many tons, such as blue whales and giant squid. By far most species of animals are insects, with groups such as mollusks, crustaceans, and nematodes also being especially diverse. By this measure our own group, the vertebrates, is relatively inconsequential from a diversity perspective.

Research continues on the evolutionary relationships of the major groups of animals. For the sake of convenience, the Animal Diversity Web follows the system outlined in Hickman and Roberts (1994). For some groups we incorporate the results of current research in our classification and discussion.

Levels of Organisation:
• Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisation of cells.
• For example, in sponges, the cells are arranged as loose cell aggregates, i.e., they exhibit cellular level of organisation. Some division of labour (activities) occur among the cells.
• In coelenterates, the arrangement of cells is more complex. Here the cells performing the same function are arranged into tissues, hence is called tissue level of organisation.
• A still higher level of organisation, i.e., organ level [organ level of organisation] is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised for a particular function.
• In animals like Annelids, Arthropods, Molluscs, Echinoderms and Chordates, organs have associated to form functional systems, each system concerned with a specific physiological function. This pattern is called organ system level of organisation.
• Organ systems in different groups of animals exhibit various patterns of complexities.
• For example, the digestive system in Platyhelminthes (incomplete digestive system) has only a single opening to the outside of the body that serves as both mouth and anus, and is hence called incomplete. A complete digestive system has two openings, mouth and anus.
• Similarly, the circulatory system may be of two types: open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it and closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).
Symmetry:

• Animals can be categorised on the basis of their symmetry.
• Sponges are mostly asymmetrical, i.e., any plane that passes through the centre does not divide them into equal halves.
• When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry. Coelenterates, Ctenophores and Echinoderms have this kind of body plan.
• Animals like Annelids, Arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.

Diploblastic and Triploblastic Organisation:
• Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., Coelenterates. An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm.
• Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (platyhelminthes to chordates).

Figure: Showing germinal layers : (a) Diploblastic (b) Triploblastic
Coelom:
• Presence or absence of a cavity between the body wall and the gut wall is very important in classification.
• The body cavity, which is lined by mesoderm is called coelom.
• Animals possessing coelom are called coelomates, e.g., Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates & Chordates.
• In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g., Aschelminthes.
• The animals in which the body cavity is absent are called acoelomates, e.g., Platyhelminthes.

Segmentation:
• In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs.
• For example, in earthworm, the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.
Notochord:
• Notochord is a mesodermally [the middle layer of cells or tissues of an embryo, or the parts derived from this (e.g. cartilage, muscles, and bone)] derived rod-like structure formed on the dorsal side [posterior] during embryonic development in some animals.
• Animals with notochord are called chordates and those animals which do not form this structure are called non-chordates, e.g., Porifera to Echinoderms.

Etymology:
The word “animal” comes from the Latin animalis, meaning having breath, having soul or living being. The biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals.
Characteristics

Animals are unique in having the ball of cells of the early embryo (1) develop into a hollow ball or blastula (2).
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, and unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrient animals are heterotrophic, feeding on organic material and digesting it internally. With very few exceptions, animals respire aerobically. All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later become sessile. The blastula is a stage in embryonic development that is unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
Structure:
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as shells, bones, and spicules. In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.
With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues. These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
Reproduction and development:

Sexual reproduction is nearly universal in animals, such as these dragonflies.

Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm, also develops between them. These germ layers then differentiate to form tissues and organs.
Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren (Malurus splendens), females benefit by mating with multiple males, thus producing more offspring of higher genetic quality.
Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.
Ecology:
Predators, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other animals.
Animals are categorised into ecological groups depending on how they obtain or consume organic material, including carnivores, herbivores, omnivores, detritivores, and parasites. Interactions between animals form complex food webs. In carnivorous or omnivorous species, predation is a consumer-resource interaction where a predator feeds on another organism (called its prey). Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations. Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts’ living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours, such as hawksbill sea turtles that primarily eat sponges.

Hydrothermal vent mussels and shrimps

Most animals rely on the energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher trophic levels, typically acquire energy (in the form of reduced carbon) by eating other animals. The carbohydrates, lipids, proteins, and other biomolecules are broken down to allow the animal to grow and to sustain biological processes such as locomotion. Animals living close to hydrothermal vents and cold seeps on the dark sea floor do not depend on the energy of sunlight. Rather, archaea and bacteria in these locations produce organic matter through chemosynthesis (by oxidizing inorganic compounds, such as methane) and form the base of the local food web.
Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510–471 million years ago during the Late Cambrian or Early Ordovician. Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth’s habitats and microhabitats, including salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of animals, plants, fungi and rocks. Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F). Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.
Diversity:

The blue whale is the largest animal that has ever lived.

Largest and smallest:
The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 190 metric tonnes and measuring up to 33.6 metres (110 ft) long. The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes. Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 µm, and one of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown.
Numbers and habitats:
The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species, along with their principal habitats (terrestrial, fresh water, and marine), and free-living or parasitic ways of life. Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.
Phylum Example No. Of
Species Land Sea Fresh
water Free-
living Parasitic
Annelids 17,000[61] Yes (soil)[63] Yes[63] 1,750[62] Yes 400[64]
Arthropods 1,257,000[61] 1,000,000
(insects)[69] >40,000
(Malac-
ostraca)[70] 94,000[62] Yes[63] >45,000[b][64]
Bryozoa 6,000[61] Yes[63] 60–80[62] Yes
Chordates 65,000[61]
45,000[71] 23,000[71] 13,000[71] 18,000[62]
9,000[71] Yes 40
(catfish)[72][64]
Cnidaria 16,000[61] Yes[63] Yes (few)[63] Yes[63] >1,350
(Myxozoa)[64]
Echinoderms 7,500[61] 7,500[61] Yes[63]
Molluscs 85,000[61]
107,000[73] 35,000[73] 60,000[73] 5,000[62]
12,000[73] Yes[63] >5,600[64]
Nematodes 25,000[61] Yes (soil)[63] 4,000[65] 2,000[62] 11,000[65] 14,000[65]
Platyhelminthes 29,500[61] Yes[74] Yes[63] 1,300[62] Yes[63] >40,000[64]
Rotifers 2,000[61] >400[75] 2,000[62] Yes
Sponges 10,800[61] Yes[63] 200-300[62] Yes Yes[76]
Total number of described species as of 2013: 1,525,728[61]
Evolutionary origin:

Dickinsonia costata from the Ediacaran biota (c. 635–542 MYA) is one of the earliest animal species known.
The first fossils that might represent animals appear in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as most probably being early sponges.
The oldest animals are found in the Ediacaran biota, towards the end of the Precambrian, around 610 million years ago. It had long been doubtful whether these included animals, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes that these were indeed animals. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.

Anomalocaris canadensis is one of the many animal species that emerged in the Cambrian explosion, starting some 542 million years ago, and found in the fossil beds of the Burgess shale.
Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 542 million years ago, in beds such as the Burgess shale. Extant phyla in these rocks include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, along with numerous now-extinct forms such as the predatory Anomalocaris. The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously.
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. Trace fossils such as tracks and burrows found in the Tonian period may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced today by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution. Around the same time, another line of evidence may indicate the appearance of grazing animals: the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing.
Phylogeny:
Further information: Lists of animals
Animals are monophyletic, meaning they are derived from a common ancestor. Animals are sister to the Choanoflagellata, with which they form the Choanozoa. The most basal animals, the Porifera, Ctenophora, Cnidaria, and Placozoa, have body plans that lack bilateral symmetry. Their relationships are still disputed; the sister group to all other animals could be the Porifera or the Ctenophora, both of which lack hox genes, important in body plan development.
These genes are found in the Placozoa and the higher animals, the Bilateria. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body’s system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development.
The phylogenetic tree (of major lineages only) indicates approximately how many millions of years ago (mya) the lineages split.
Choanozoa   Choanoflagellata

Animalia   Porifera

Eumetazoa   Ctenophora

ParaHoxozoa     Placozoa

Cnidaria

Bilateria   Xenacoelomorpha

Nephrozoa Deuterostomia   Chordata

Ambulacraria

Protostomia Ecdysozoa   Scalidophora

Arthropoda and allies

Nematoda and allies
>529 mya
Spiralia Gnathifera   Rotifera and allies

Chaetognatha

Platytrochozoa   Platyhelminthes and allies

Lophotrochozoa   Mollusca andallies

Annelida and allies
550 mya

580 mya

 

610 mya

650 mya

Triploblasts

680 mya

 

760 mya

950 mya
Non-bilaterian animals

Non-bilaterians include sponges (centre) and corals (background).
Several animal phyla lack bilateral symmetry. Among these, the sponges (Porifera) probably diverged first, representing the oldest animal phylum. Sponges lack the complex organization found in most other animal phyla; their cells are differentiated, but in most cases not organised into distinct tissues. They typically feed by drawing in water through pores.
The Ctenophora (comb jellies) and Cnidaria (which includes jellyfish, sea anemones, and corals) are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into organs. They are diploblastic, having only two main germ layers, ectoderm and endoderm. The tiny placozoans are similar, but they do not have a permanent digestive chamber.
Bilaterian animals:


Main articles: Bilateria and Symmetry (biology) § Bilateral symmetry

Idealised bilaterian body plan. With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.
The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria. The body is triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is an internal body cavity, a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan and a tendency to move in one direction have a head end (anterior) and a tail end (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.
Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth. Many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis. They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures.
Genetic studies have considerably changed zoologists’ understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the protostomes and the deuterostomes. The basalmost bilaterians are the Xenacoelomorpha.
Protostomes and deuterostomes:

Further information: Embryological origins of the mouth and anus

The bilaterian gut develops in two ways. In many protostomes, the blastopore develops into the mouth, while in deuterostomes it becomes the anus.

Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage. Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.
The main deuterostome phyla are the Echinodermata and the Chordata. Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers. The chordates are dominated by the vertebrates (animals with backbones), which consist of fishes, amphibians, reptiles, birds, and mammals. The deuterostomes also include the Hemichordata (acorn worms).
Ecdysozoa:

Ecdysis: a dragonfly has emerged from its dry exuviae and is expanding its wings. Like other arthropods, its body is divided into segments.
Main article: Ecdysozoa
The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting. They include the largest animal phylum, the Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water; some are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.
Spiralia
Main article: Spiralia

Spiral cleavage in a sea snail embryo
The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo. The Spiralia’s phylogeny has been disputed, but it contains a large clade, the superphylum Lophotrochozoa, and smaller groups of phyla such as the Rouphozoa which includes the gastrotrichs and the flatworms. All of these are grouped as the Platytrochozoa, which has a sister group, the Gnathifera, which includes the rotifers.
The Lophotrochozoa includes the molluscs, annelids, brachiopods, nemerteans, bryozoa and entoprocts. The molluscs, the second-largest animal phylum by number of described species, includes snails, clams, and squids, while the annelids are the segmented worms, such as earthworms, lugworms, and leeches. These two groups have long been considered close relatives because they share trochophore larvae.
History of classification
Further information: Taxonomy (biology), History of zoology (through 1859), and History of zoology since 1859

Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus’s “Vermes” into 9 phyla by 1809.
In the classical era, Aristotle divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then arranged on a scale from man (with blood, 2 legs, rational soul) down through the live-bearing tetrapods (with blood, 4 legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously-generating creatures like sponges (no blood, no legs, vegetable soul). Aristotle was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch, and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about.
In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae. In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes une espèce de chaos (a chaotic mess) and split the group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians.
In his 1817 Le Règne Animal, Georges Cuvier used comparative anatomy to group the animals into four embranchements (“branches” with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and zoophytes (radiata) (echinoderms, cnidaria and other forms). This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen in 1860.
In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges. The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.
In human culture:

Sides of beef in a slaughterhouse

The human population exploits a large number of other animal species for food, both of domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species. Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially. Invertebrates including cephalopods, crustaceans, and bivalve or gastropod molluscs are hunted or farmed for food. Chickens, cattle, sheep, pigs and other animals are raised as livestock for meat across the world. Animal fibres such as wool are used to make textiles, while animal sinews have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats.Dyestuffs including carmine (cochineal), shellac, and kermes have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the first days of agriculture.
Animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models. Animals have been used to create vaccines since their discovery in the 18th century. Some medicines such as the cancer drug Yondelis are based on toxins or other molecules of animal origin.

A gun dog retrieving a duck during a hunt
People have used hunting dogs to help chase down and retrieve animals, and birds of prey to catch birds and mammals, while tethered cormorants have been used to catch fish. Poison dart frogs have been used to poison the tips of blowpipe darts. A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including praying mantises, reptiles such as snakes and chameleons, and birds including canaries, parakeets, and parrots all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits. There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own. A wide variety of terrestrial and aquatic animals are hunted for sport.

Artistic vision: Still Life with Lobster and Oysters by Alexander Coosemans, c. 1660
Animals have been the subjects of art from the earliest times, both historical, as in Ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer’s 1515 The Rhinoceros, and George Stubbs’s c. 1762 horse portrait Whistlejacket. Insects, birds and mammals play roles in literature and film, such as in giant bug movies. Animals including insects and mammals feature in mythology and religion. In both Japan and Europe, a butterfly was seen as the personification of a person’s soul, while the scarab beetle was sacred in ancient Egypt.
Among the mammals, cattle, deer, horses, lions, bats, bears, and wolves are the subjects of myths and worship. The signs of the Western and Chinese zodiacs are based on animals.

Animal Kingdom is classified into:

Phylum – Porifera
Phylum – Coelenterata (Cnidaria)
Phylum – Ctenophora
Phylum – Platyhelminthes
Phylum – Aschelminthes (Nemotoda) Annelida
Phylum – Arthropoda
Phylum – Mollusca
Phylum – Echinodermata
Phylum – Hemichordata
Phylum – Chordata

Phylum – Porifera
• Phylum – Porifera includes organisms with holes.
• They are primitive multicellular animals and have cellular level of organisation.
• They are non-motile animals attached to some solid support.
• The body design involves very minimal differentiation and division into tissues.
• They are commonly called sponges.
• They are generally marine and mostly asymmetrical animals.
• Sponges have a water transport or canal system.
• Water enters through minute pores (ostia) in the body wall into a central cavity, spongocoel, from where it goes out through the osculum.
• This pathway of water transport is helpful in food gathering, respiratory exchange and removal of waste.
• The body is supported by a skeleton made up of spicules or spongin fibres.
• Sexes are not separate (hermaphrodite), i.e., eggs and sperms are produced by the same individual.
• Sponges reproduce asexually by fragmentation and sexually by formation of gametes.
• Fertilisation is internal and development is indirect having a larval stage which is morphologically distinct from the adult.

Figure: Examples of Porifera : (a) Sycon (b) Euspongia (c) Spongilla
• Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia (Bath sponge).
Phylum – Coelenterata (Cnidaria)
• The name cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocytes) present on the tentacles and the body.
• Cnidoblasts are used for anchorage, defense and for the capture of prey.
• Coelenterata (Cnidaria) are aquatic, mostly marine sessile or free-swimming radially symmetrical
• They exhibit tissue level of organization [have more body design differentiation than sponges].
• They have a central gastro-vascular cavity with a single opening.
• They are diploblastic.
• Some of these species live in colonies (corals).
• Some have a solitary [living alone] like–span (hydra).
• Some of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate.
• Cnidarians exhibit two basic body forms called polyp and medusa. The former is a sessile and cylindrical form like Hydra, Adamsia (Sea anemone), etc. whereas, the latter is umbrella-shaped and free-swimming like Aurelia or jelly fish.
• Those cnidarians which exist in both forms exhibit alternation of generation (Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
• Jellyfish and sea anemones are common examples.
• Digestion is extracellular and intracellular.
• Examples: Aurelia (jelly fish), Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

Phylum – Ctenophora
• Ctenophora are commonly known as sea walnuts or comb jellies.
• They exclusively marine, radially symmetrical, diploblastic
• They exhinit tissue level of organisation.
• The body bears eight external rows of ciliated comb plates, which help in locomotion.
• Digestion is both extracellular and intracellular.
• Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores.
• Sexes are not separate and reproduction takes place only by sexual means.
• Fertilisation is external [fertilization occurs outside the body] with indirect development [zygote → larvae → animal].
• Examples: Pleurobrachia and Ctenoplana.

Phylum – Platyhelminthes
• Platyhelminthes are more complexly designed than the earlier groups.
• They are bilaterally symmetrical.
• They are triploblastic. This allows outside and inside body linings as well as some organs to be made. There is thus some degree of tissue formation [organ level of organisation].
• The body is flattened dorsiventrally, meaning from top to bottom, which is why these animals are called flatworms.
• They may be freeliving or parasitic. Hooks and suckers are present in the parasitic forms.
• Some examples are freeliving animals like planarians, or parasitic animals like
• Parisites are mostly endoparasites found in animals including human beings. Some of them absorb nutrients from the host directly through their body surface.
• Acoelomate: There is no true internal body cavity or coelom, in which well developed organs can be accommodated.
• Specialised cells called flame cells help in osmoregulation and excretion.
• Sexes are not separate.
• Fertilisation is internal and development is indirect.
• Some members like Planaria possess high regeneration capacity.

Phylum – Aschelminthes (Nemotoda)
• Body in aschelminthes (Nemotoda) is cylindrical [bilaterally symmetrical] rather than flattened.
• They exhibit organ-system level of body organization [there are tissues, but no real organs].
• They are triploblastic. A sort of body cavity or a pseudocoelom, is present.
• They are freeliving, aquatic, terrestrial or parasitic in plants and animals.
• These are very familiar as parasitic worms causing diseases, such as the worms causing elephantiasis (filarial worms) or the worms in the intestines (roundworm or pinworms).
• The body is circular in cross-section, hence, the name roundworms.
• Alimentary canal is complete.
• An excretory tube removes body wastes from the body cavity through the excretory pore.
• Sexes are separate (dioecious), i.e., males and females are distinct.
• Often females are longer than males.
• Fertilisation is internal and development may be direct (the young ones resemble the adult) or indirect.

Phylum – Annelida
• Annelida are aquatic [marine and fresh water] or terrestrial; free-living, and sometimes parasitic.
• Their body surface is distinctly marked out into segments or metameres [metamerically segmented] and, hence, the phylum name Annelida (Latin, annulus: little ring).
• They exhibit organ-system level of body organization.
• They are coelomate [true body cavity]. This allows true organs to be packaged in the body structure.
• They are bilateral symmetric and triploblastic.
• They possess longitudinal and circular muscles which help in locomotion.
• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
• A closed circulatory system is present.
• Nephridia (sing. nephridium) help in osmoregulation and excretion.
• Neural system consists of paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral nerve cord.
• Nereis, an aquatic form, is dioecious [Sexes are separate], but earthworms and leeches are monoecious [having both the male and female reproductive organs in the same individual].
• Reproduction is sexual.

Phylum – Arthropoda
• Insects, arachnids and crustaceans are members of the largest category of creatures on the planet: arthropods.
• Arthropods have hard, external shells called “exoskeletons,” segmented bodies and jointed legs.
• Some familiar examples are prawns, butterflies, houseflies, spiders, scorpions and crabs and some
• They exhibit organ-system level of organisation.
• They are bilaterally symmetrical, triploblastic, segmented and coelomate The coelomic cavity is blood-filled.
• The body of arthropods is covered by chitinous The body consists of head, thorax and abdomen.
• There is an open circulatory system, and so the blood does not flow in well defined blood vessels.
• Respiratory organs are gills, book gills, book lungs or tracheal system.
• Sensory organs like antennae, eyes (compound and simple), statocysts or balance organs are present.
• Excretion takes place through malpighian tubules.
• They are mostly dioecious.
• Fertilisation is usually internal.
• They are mostly oviparous.
• Development may be direct or indirect.

Arachnids
• Spiders, harvestmen, mites, ticks and other arachnids are members of the class Arachnida.
Crustaceans
• Crustaceans make up a large group of arthropods that includes animals such as crabs, lobsters, crayfish and shrimp. They breathe with gills and have two pairs of antennae.
Insects
• In general, insects have three-part bodies, six jointed legs, compound eyes and two antennae.
• Bees, wasps, beetles, mosquitoes, flies, grasshoppers, ants, butterflies and moths, and dragonflies and damselflies are common types of insects.
Phylum – Mollusca
• Mollusca are the second largest animal phylum. They are terrestrial or aquatic.
• They exhibit organ-system level of organization.
• They are bilaterally symmetrical, triploblastic, coelomate animals. There is little segmentation.
• They have an open circulatory system and kidney-like organs for excretion. The anterior head region has sensory tentacles.
• The mouth contains a file-like rasping organ for feeding, called radula.
• They are usually dioecious and oviparous with indirect development.
• Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump.
• Examples are octopus, snails and mussels.

Phylum – Echinodermata
• These animals have an endoskeleton of calcareous ossicles [calcium carbonate structures] and, hence, the name Echinodermata (spiny skinned organisms).
• They are exclusively free-living marine animals with organ-system level of organisation.
• They are triploblastic with a coelomic cavity [coelomate animals]. The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.
• Water-driven tube system [water vascular system] are used for locomotion, capture and transport of food and respiration.
• They are triploblastic and coelomate animals.
• Digestive system is complete. An excretory system is absent.
• Sexes are separate. Reproduction is sexual. Fertilisation is usually external.
• Development is indirect with free-swimming larva.
• Examples: Star fish, Sea urchin, Sea lily, Sea cucumber, Brittle star.

Phylum – Hemichordata
• Hemichordata was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordata.
• This phylum consists of a small group of worm-like marine animals with organ-system level of organisation.
• They are cylindrical [bilaterally symmetrical], triploblastic, coelomate animals.
• The body is Circulatory system is of open type.
• Respiration takes place through gills.
• Excretory organ is present.
• Sexes are separate. Fertilisation is external. Development is indirect.
• Examples: Balanoglossus and Saccoglossus.
Phylum – Chordata
• Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
• They are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation.
• They possess a post anal tail and a closed circulatory system.
• Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
• Subphyla Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine.
• In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life.
• Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Amphioxus or Lancelet.

All chordates possess the following features:
1. Have a notochord
2. Have a dorsal nerve cord
3. Are triploblastic
4. Have paired gill pouches
5. Are coelomate.

Vertebrata:
These animals have a true vertebral column and internal skeleton, allowing a completely different distribution of muscle attachment points to be used for movement.
The members of subphylum Vertebrata possess notochord during the embryonic period.
The notochord is replaced by a cartilaginous or bony vertebral column in the adult.
Thus all vertebrates are chordates but all chordates are not vertebrates.
Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs.
Vertibrates are bilaterally symmetrical, triploblastic, coelomic and segmented, with complex differentiation of body tissues and organs.


Comparison of Chordates and Non-chordates
S.No. Chordates Non-chordates
1. Notochord present. Notochord absent.
2. Central nervous system is dorsal, hollow and single. Central nervous system is ventral, solid and double.
3. Pharynx perforated by gill slits. Gill slits are absent.
4. Heart is ventral. Heart is dorsal (if present).
5. A post-anal part (tail) is present. Post-anal tail is absent.
Phylum – Chordata
• Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal [relating to the pharynx] gill slits.
• They are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation.
• Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
• Subphyla Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine.
• In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life.
• Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Amphioxus or Lancelet.
Comparison of Chordates and Non-chordates
S.No. Chordates Non-chordates
1. Notochord present. Notochord absent.
2. Central nervous system is dorsal, hollow and single. Central nervous system is ventral, solid and double.
3. Pharynx perforated by gill slits. Gill slits are absent.
4. Heart is ventral. Heart is dorsal (if present).
5. A post-anal part (tail) is present. Post-anal tail is absent.
Vertebrata:

• These animals have a true vertebral column and internal skeleton, allowing a completely different distribution of muscle attachment points to be used for movement.
• The members of subphylum Vertebrata possess notochord during the embryonic period.
• The notochord is replaced by a cartilaginous or bony vertebral column in the adult.
• Thus all vertebrates are chordates but all chordates are not vertebrates.
• Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs.
• Vertibrates are bilaterally symmetrical, triploblastic, coelomic and segmented, with complex differentiation of body tissues and organs.

Division In Vertebrata:

Basic Concepts
Viviparous and Oviparous Animals:

• We have learnt that some animals give birth to young ones while some animals lay eggs which later develop into young ones.
• The animals which give birth to young ones are called viviparous animals.
• Those animals which lay eggs are called oviparous animals.
• In some animals, the young ones may look very different from the adults. Recall the life cycle of the silkworm (egg → larva or caterpillar → pupa → adult) (egg → tadpole (larva) → adult). The transformation of the larva into an adult through drastic changes is called metamorphosis.
Warm Blooded vs. Cold Blooded Animals
Warm Blooded or Endotherms or Homoiothermous animals:
• All mammals and birds with few exceptions are warm blooded. [Bats, Echidnas, Mole Rats etc. cannot regulate their body temperature]
• They maintain a constant internal body temperature irrespective of external environment. [Can regulate their body temperature by generating their own heat when they are in a cooler environment, and by cooling themselves when they are in a hotter environment]
• They can survive in a wide of environments as they are able to regulate their body temperature.
• They require a lot of food for their survival. Most of the food consumed is utilized to maintain a constant body temperature.
• They are active in both warm and cold environments.
• To stay cool, warm-blooded animals usually sweat. Animals like elephants use their ears to cool their body [large, thin ears which loose heat quickly].
• Some warm-blooded animals, especially birds, migrate from colder to warmer regions in the winter.
• Mammals have hair, fur and birds have feathers to help keep them warm.
• Warm-blooded animals can also shiver to generate more heat when they get too cold.
• Constant body temperature provide a nice warm environment for viruses, bacteria and parasites to live in.
Hibernation:

• Hibernation is a state of inactivity and metabolic depression in few endotherms [warm blooded animals – bear, rodents] and ectotherms [many reptiles like snakes, turtles and amphibians like frogs]. Snakes, lizards, toads, frogs, salamanders and most turtles will mostly hibernate during harsh winters.
• Hibernating animals usually retreat to a den, a burrow, or a hollow log for protection and shelter.
• During “true hibernation,” the animal’s body temperature drops, and its rate of breathing slows down. These hibernating animals are very difficult to awaken.
• Some warm-blooded animals such as bears, rodents etc. hibernate during extreme weather seasons and unfavorable conditions.
• During hibernation these animals live off of stored body fat and can drop their body temperatures significantly.
• Most animals will eat large amounts of food before hibernating.
Class – Cyclostomata:

• All living members of the class Cyclostomata are ectoparasites [ives on the outside of its host] on some fishes.
• They have an elongated body bearing 6-15 pairs of gill slits for respiration.
• Cyclostomes have a sucking and circular mouth without jaws.
• Their body is devoid of scales and paired fins.
• Cranium and vertebral column are cartilaginous.
• Circulation is of closed type.
• Cyclostomes are marine but migrate for spawning [release or deposit eggs] to fresh water.
• After spawning, within a few days, they die. Their larvae, after metamorphosis [transformation from an immature form to an adult form in two or more distinct stages. Example: Larvae → Tadpole → Frog], return to the ocean.
• Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

Class – Pisces:
• These are fish. Their skin is covered with scales/plates. They lay eggs [oviporous].
• They obtain oxygen dissolved in water by using gills.
• The body is streamlined, and a muscular tail is used for movement.
• They are cold-blooded and their hearts have only two chambers, unlike the four that humans have.
• Some fish skeletons are made entirely of cartilage [Chondrichthyes], such as sharks, and some with a skeleton made of both bone and cartilage [Osteichthyes].

Chondrichthyes:

• They are marine animals with streamlined body and have cartilaginous endoskeleton. Mouth is located ventrally.
• Notochord is persistent throughout life.
• Gill slits are separate and without operculum (gill cover).
• The skin is tough, containing minute placoid scales.
• Teeth are modified placoid scales which are backwardly directed.
• Their jaws are very powerful.
• These animals are predaceous [shark].
• Due to the absence of air bladder, they have to swim constantly to avoid sinking.
• Heart is two-chambered (one auricle and one ventricle).
• Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon).
• They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature.
• Sexes are separate. In males pelvic fins bear claspers.
• They have internal fertilisation and many of them are viviparous [give birth to young ones].
• Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carchaiodon (Great white shark), Trygon (Sting ray).

Osteichthyes:

• It includes both marine and fresh water fishes with bony endoskeleton.
• Their body is streamlined. Mouth is mostly terminal.
• They have four pairs of gills which are covered by an operculum on each side.
• Skin is covered with cycloid/ctenoid scales.
• Air bladder is present which regulates buoyancy.
• Heart is two- chambered (one auricle and one ventricle).
• They are cold-blooded
• Sexes are separate.
• Fertilisation is usually external.
• They are mostly oviparous and development is direct.
• Examples: Flying fish, Sea horse, Fighting fish, Angel fish etc.

Class – Amphibia:
• As the name indicates (Gr., Amphi : dual, bios, life), amphibians can live in aquatic as well as terrestrial habitats.
• The amphibian skin is moist without scales [mucus glands in the skin]. The eyes have eyelids. A tympanum represents the ear.
• Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca which opens to the exterior.
• They have a three-chambered heart (two auricles and one ventricle). These are cold-blooded
• Respiration is through gills, lungs and through
• Respiration is by gills, lungs and through skin.
• Sexes are separate. Fertilisation is external.
• They are oviparous and development is indirect.
• Examples: Toad, Frog), Tree frog, Salamander, Limbless amphibia.

Class – Reptilia:
• The class name refers to their creeping or crawling mode of locomotion (Latin, repere or reptum, to creep or crawl).
• They are mostly terrestrial animals and their body is covered by dry and cornified skin, epidermal scales or scutes. Snakes and lizards shed their scales as skin cast.
• They do not have external ear openings. Tympanum represents ear. Limbs, when present, are two pairs.
• Heart is usually three-chambered, but four-chambered in crocodiles.
• Reptiles are poikilotherms [cold-blooded animals].
• They lay eggs with tough coverings and do not need to lay their eggs in water, unlike amphibians.
• Sexes are separate.
• Fertilisation is internal.
• They are oviparous and development is direct.
• Examples: Turtle), Tortoise, Chameleon (Tree lizard), Garden lizard, Crocodile, Alligator, Wall lizard, Poisonous snakes – Naja (Cobra), Bangarus (Krait), Vipera (Viper).

Class – Aves:

• They have a four-chambered heart. They breathe through lungs. All birds fall in this category.
• The characteristic features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich). The forelimbs are modified into wings.
• The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches.
• Skin is dry without glands except the oil gland at the base of the tail.
• Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic).
• The digestive tract of birds has additional chambers, the crop and gizzard.
• They are warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature.
• Respiration is by lungs. Air sacs connected to lungs supplement respiration.
• Sexes are separate. Fertilisation is internal. They are oviparous and development is direct.
• Examples : Crow, Pigeon, Ostrich), Neophron (Vulture) etc..

Class – Mammalia:

• Mammals are warm-blooded animals with four-chambered hearts.
• Most mammals familiar to us produce live young ones. However, a few of them, like the Platypus and the Echidna lay eggs, and some, like kangaroos give birth to very poorly developed young ones.
• They are found in a variety of habitats – polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some of them have adapted to fly or live in water.
• The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished.
• They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying.
• The skin of mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw.
• Heart is four-chambered. They are homoiothermous [warm-blooded]. Respiration is by lungs.
• Sexes are separate and fertilisation is internal.
• They are viviparous with few exceptions and development is direct.
• Examples: Oviparous – Platypus; Viviparous – Kangaroo, Flying fox), Delphinus (Common dolphin), Balaenoptera (Blue whale), etc.

Animal Classification Summary:

• Porifera includes multicellular animals which exhibit cellular level of organisation and have characteristic flagellated choanocytes.
• The coelenterates have tentacles and bear cnidoblasts. They are mostly aquatic, sessile or free-floating. The ctenophores are marine animals with comb plates.
• The platyhelminths have flat body and exhibit bilateral symmetry. The parasitic forms show distinct suckers and hooks.
• Aschelminthes are pseudocoelomates and include parasitic as well as non-parasitic round worms.
• Annelids are metamerically segmented animals with a true coelom.
• The arthropods are the most abundant group of animals characterised by the presence of jointed appendages.
• The molluscs have a soft body surrounded by an external calcareous shell. The body is covered with external skeleton made of chitin.
• The echinoderms possess a spiny skin. Their most distinctive feature is the presence of water vascular system.
• The hemichordates are a small group of worm-like marine animals. They have a cylindrical body with proboscis, collar and trunk.
• Phylum Chordata includes animals which possess a notochord either throughout or during early embryonic life. Other common features observed in the chordates are the dorsal, hollow nerve cord and paired pharyngeal gill slits.
• Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possess jaws (Gnathostomata). Agnatha is represented by the class, Cyclostomata. They are the most primitive chordates and are ectoparasites on fishes. Gnathostomata has two super classes, Pisces and Tetrapoda.
• Classes Chondrichthyes and Osteichthyes bear fins for locomotion and are grouped under Pisces. The Chondrichthyes are fishes with cartilaginous endoskeleton and are marine.
• Classes, Amphibia, Reptilia, Aves and Mammalia have two pairs of limbs and are thus grouped under Tetrapoda. The amphibians have adapted to live both on land and water.
• Reptiles are characterised by the presence of dry and cornified skin. Limbs are absent in snakes. Fishes, amphibians and reptiles are poikilothermous (cold¬blooded).
• Aves are warm-blooded animals with feathers on their bodies and forelimbs modified into wings for flying. Hind limbs are adapted for walking, swimming, perching or clasping.
• The unique features of mammals are the presence of mammary glands and hairs on the skin. They commonly exhibit viviparity.

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