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27.1: Features of the Animal Kingdom - Biology

27.1: Features of the Animal Kingdom - Biology



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27.1: Features of the Animal Kingdom

Features of the Animal Kingdom

Even though members of the animal kingdom are incredibly diverse, most animals share certain features that distinguish them from organisms in other kingdoms. All animals are eukaryotic, multicellular organisms, and almost all animals have a complex tissue structure with differentiated and specialized tissues. Most animals are motile, at least during certain life stages. All animals require a source of food and are therefore heterotrophic, ingesting other living or dead organisms this feature distinguishes them from autotrophic organisms, such as most plants, which synthesize their own nutrients through photosynthesis. As heterotrophs, animals may be carnivores, herbivores, omnivores, or parasites (Figureab). Most animals reproduce sexually, and the offspring pass through a series of developmental stages that establish a determined and fixed body plan. The body plan refers to the morphology of an animal, determined by developmental cues.

All animals are heterotrophs that derive energy from food. The (a) black bear is an omnivore, eating both plants and animals. The (b) heartworm Dirofilaria immitis is a parasite that derives energy from its hosts. It spends its larval stage in mosquitoes and its adult stage infesting the heart of dogs and other mammals, as shown here. (credit a: modification of work by USDA Forest Service credit b: modification of work by Clyde Robinson)


Facts About the Five Kingdoms

  • Everything that lives on the earth belongs in one of the Five Kingdoms.
  • There are different features that decide what kingdom an organism fits in.
  • Remember, to fit in a kingdom, the organism has to have like characteristics with the other organisms.
  • Scientists and biologists can tell how to identify an organism to a kingdom by studying it under a microscope.

What Did You Learn?

  • What are the Five Kingdoms? The Five Kingdoms are Animal, Monera, Fungi, Plant and Protist.
  • What is the difference between a Monera and a Protist? Even though both the Monera and Protist are single celled organisms, the Monera does not have a nucleus and the Protist does.
  • What is similar to the Plant and Animal Kingdom? Both of these kingdoms contain organisms that are found all over the world in different shapes and sizes.
  • What does unicellular mean? Unicellular means having one cell.
  • Why are fungi not considered part of the Plant Kingdom? Even though a Fungi has similar characteristics of a plant, it cannot make its own food which makes is a requirement to be part of the Plant Kingdom.
  • Back to – Biology

Art connection

Hox genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters: Hox-A , Hox-B , Hox-C , and Hox-D . Genes within these clusters are expressed in certain body segments at certain stages of development. Shown here is the homology between Hox genes in mice and humans. Note how Hox gene expression, as indicated with orange, pink, blue and green shading, occurs in the same body segments in both the mouse and the human.

If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?


Major Animalia phylums

Phylum Porifera

  • Sponges
  • Very primitive, considered barely animals.
  • Don’t have true organs or nerve or muscle cells

Phylum Annelida

  • Segmented Worms (earthworms, leeches)
  • Segmented Worms
  • Earthworms, leeches, and other segmented worms live in water or damp soil
  • Leeches were once used to suck out people’s “excess” blood and reduce harmful high blood pressure.
  • Leeches are uses today to produce anti-blood-clotting medicines, to suck blood from bruises, and to stimulate blood circulation in severed limbs that have been surgically reattached.
  • Each segment is separated from its neighbors by a membrane and has its own excretory system and branches of the main nerves and blood vessels that run the length of the animal.
  • Both segmented and unsegmented worms have definite anterior and posterior ends.
  • Food travels through the digestive system in one direction from anterior to posterior.
  • A cluster of nerve cells at the anterior end serves as a simple brain.
  • Reproduction occurs by splitting or by mutual fertilization

Mollusks (Mollusca)

  • Includes snails, clams, slugs, squid, and their relatives.
  • Mollusks have soft bodies with 3 parts
  • A mass that contains most of the organs
  • A muscular “foot” that is used in movement
  • A thick flap called a mantle, which covers the body and in most species produces a heavy shell of calcium compounds.
  • Mollusks pump water through gills
  • This is how food is also ingested for clams and oysters. Squid and octopuses use the pump for jet propulsion through the water in search of prey.

Arthropods (Arthropoda)

  • The largest animal phylum, and have jointed external skeletons.
  • 1 million species, crabs, shrimp, spiders, scorpions and insects make up this phylum
  • Arthropods molt, have heads with many sensory organs.
  • Simple and complex eyes that detect only light intensity and form images
  • Antennae that smell chemical substances in the environment, arthropods also respond to water vapor, like biting mosquitoes.
  • They reproduce sexually, where sperm is released inside the female’s body, not in water.
  • Larvae of many species develop into very different adults, a process called metamorphosis.
  • Arthropods development of resistance to insecticides demonstrates how quickly they adapt to a changing environment.
  • Short generations and many offspring increase the chance that random mutations will produce a few resistant individuals

Echinoderms (Echinodermata)

  • Sea stars and sea urchins.
  • Reproduce sexually. Sperm and eggs are released in water, where they meet and join.
  • Movement by seawater into and out of a system of internal tubes.

Chordates (Chordata)

  • Vertebrates-fish, amphibians, reptiles, birds, and mammals.
  • Four characteristics
  • Stiff dorsal rod helps to organize the embryo’s development.
  • The central nervous system (brain and spinal cord) is tubular
  • Their sides have slits just behind the head. These pharyngeal slits (pharynx means “throat”) becomes gill slits of adult fish. In air-breathing chordates, they develop into various organs such as internal parts of the ears
  • They have a tail in humans it’s the tailbone, or coccyx, which curls internally.

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27.1: Features of the Animal Kingdom - Biology

By the end of this section, you will be able to do the following:

  • List the features that distinguish the kingdom Animalia from other kingdoms
  • Explain the processes of animal reproduction and embryonic development
  • Describe the roles that Hox genes play in development

Two different groups within the Domain Eukaryota have produced complex multicellular organisms: The plants arose within the Archaeplastida, whereas the animals (and their close relatives, the fungi) arose within the Opisthokonta. However, plants and animals not only have different life styles, they also have different cellular histories as eukaryotes. The opisthokonts share the possession of a single posterior flagellum in flagellated cells, e.g., sperm cells.

Most animals also share other features that distinguish them from organisms in other kingdoms. All animals require a source of food and are therefore heterotrophic, ingesting other living or dead organisms. This feature distinguishes them from autotrophic organisms, such as most plants, which synthesize their own nutrients through photosynthesis. As heterotrophs, animals may be carnivores, herbivores, omnivores, or parasites ((Figure)a,b). As with plants, almost all animals have a complex tissue structure with differentiated and specialized tissues. The necessity to collect food has made most animals motile, at least during certain life stages. The typical life cycle in animals is diplontic (like you, the diploid state is multicellular, whereas the haploid state is gametic, such as sperm or egg). We should note that the alternation of generations characteristic of the land plants is typically not found in animals. In animals whose life histories include several to multiple body forms (e.g., insect larvae or the medusae of some Cnidarians), all body forms are diploid. Animal embryos pass through a series of developmental stages that establish a determined and fixed body plan. The body plan refers to the morphology of an animal, determined by developmental cues.

Figure 1. Heterotrophy. All animals are heterotrophs and thus derive energy from a variety of food sources. The (a) black bear is an omnivore, eating both plants and animals. The (b) heartworm Dirofilaria immitis is a parasite that derives energy from its hosts. It spends its larval stage in mosquitoes and its adult stage infesting the heart of dogs and other mammals, as shown here. (credit a: modification of work by USDA Forest Service credit b: modification of work by Clyde Robinson)

Complex Tissue Structure

Many of the specialized tissues of animals are associated with the requirements and hazards of seeking and processing food. This explains why animals typically have evolved special structures associated with specific methods of food capture and complex digestive systems supported by accessory organs. Sensory structures help animals navigate their environment, detect food sources (and avoid becoming a food source for other animals!). Movement is driven by muscle tissue attached to supportive structures like bone or chitin, and is coordinated by neural communication. Animal cells may also have unique structures for intercellular communication (such as gap junctions). The evolution of nerve tissues and muscle tissues has resulted in animals’ unique ability to rapidly sense and respond to changes in their environment. This allows animals to survive in environments where they must compete with other species to meet their nutritional demands.

The tissues of animals differ from those of the other major multicellular eukaryotes, plants and fungi, because their cells don’t have cell walls. However, cells of animal tissues may be embedded in an extracellular matrix (e.g., mature bone cells reside within a mineralized organic matrix secreted by the cells). In vertebrates, bone tissue is a type of connective tissue that supports the entire body structure. The complex bodies and activities of vertebrates demand such supportive tissues. Epithelial tissues cover and protect both external and internal body surfaces, and may also have secretory functions. Epithelial tissues include the epidermis of the integument, the lining of the digestive tract and trachea, as well as the layers of cells that make up the ducts of the liver and glands of advanced animals, for example. The different types of tissues in true animals are responsible for carrying out specific functions for the organism. This differentiation and specialization of tissues is part of what allows for such incredible animal diversity.

Just as there are multiple ways to be a eukaryote, there are multiple ways to be a multicellular animal. The animal kingdom is currently divided into five monophyletic clades: Parazoa or Porifera (sponges), Placozoa (tiny parasitic creatures that resemble multicellular amoebae), Cnidaria (jellyfish and their relatives), Ctenophora (the comb jellies), and Bilateria (all other animals). The Placozoa (“flat animal”) and Parazoa (“beside animal”) do not have specialized tissues derived from germ layers of the embryo although they do possess specialized cells that act functionally like tissues. The Placozoa have only four cell types, while the sponges have nearly two dozen. The three other clades do include animals with specialized tissues derived from the germ layers of the embryo. In spite of their superficial similarity to Cnidarian medusae, recent molecular studies indicate that the Ctenophores are only distantly related to the Cnidarians, which together with the Bilateria constitute the Eumetazoa (“true animals”). When we think of animals, we usually think of Eumetazoa, since most animals fall into this category.

Link to Learning

Watch a presentation by biologist E.O. Wilson on the importance of diversity.

Animal Reproduction and Development

Most animals are diploid organisms, meaning that their body (somatic) cells are diploid and haploid reproductive (gamete) cells are produced through meiosis. Some exceptions exist: for example, in bees, wasps, and ants, the male is haploid because it develops from unfertilized eggs. Most animals undergo sexual reproduction. However, a few groups, such as cnidarians, flatworms, and roundworms, may also undergo asexual reproduction, in which offspring originate from part of the parental body.

Processes of Animal Reproduction and Embryonic Development

During sexual reproduction, the haploid gametes of the male and female individuals of a species combine in a process called fertilization. Typically, both male and female gametes are required: the small, motile male sperm fertilizes the typically much larger, sessile female egg. This process produces a diploid fertilized egg called a zygote.

Some animal species—including sea stars and sea anemones—are capable of asexual reproduction. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where part of a parent individual can separate and grow into a new individual. This type of asexual reproduction produces genetically identical offspring, which would appear to be disadvantageous from the perspective of evolutionary adaptability, simply because of the potential buildup of deleterious mutations.

In contrast, a form of uniparental reproduction found in some insects and a few vertebrates is called parthenogenesis (or “virgin beginning”). In this case, progeny develop from a gamete, but without fertilization. Because of the nutrients stored in eggs, only females produce parthenogenetic offspring. In some insects, unfertilized eggs develop into new male offspring. This type of sex determination is called haplodiploidy, since females are diploid (with both maternal and paternal chromosomes) and males are haploid (with only maternal chromosomes). A few vertebrates, e.g., some fish, turkeys, rattlesnakes, and whiptail lizards, are also capable of parthenogenesis. In the case of turkeys and rattlesnakes, parthenogenetically reproducing females also produce only male offspring, but not because the males are haploid. In birds and rattlesnakes, the female is the heterogametic (ZW) sex, so the only surviving progeny of post-meiotic parthenogenesis would be ZZ males. In the whiptail lizards, on the other hand, only female progeny are produced by parthenogenesis. These animals may not be identical to their parent, although they have only maternal chromosomes. However, for animals that are limited in their access to mates, uniparental reproduction can ensure genetic propagation.

In animals, the zygote progresses through a series of developmental stages, during which primary germ layers (ectoderm, endoderm, and mesoderm) are established and reorganize to form an embryo. During this process, animal tissues begin to specialize and organize into organs and organ systems, determining their future morphology and physiology.

Animal development begins with cleavage, a series of mitotic cell divisions, of the zygote ((Figure)). Cleavage differs from somatic cell division in that the egg is subdivided by successive cleavages into smaller and smaller cells, with no actual cell growth. The cells resulting from subdivision of the material of the egg in this way are called blastomeres. Three cell divisions transform the single-celled zygote into an eight-celled structure. After further cell division and rearrangement of existing cells, a solid morula is formed, followed by a hollow structure called a blastula. The blastula is hollow only in invertebrates whose eggs have relatively small amounts of yolk. In very yolky eggs of vertebrates, the yolk remains undivided, with most cells forming an embryonic layer on the surface of the yolk (imagine a chicken embryo growing over the egg’s yolk), which serve as food for the developing embryo.

Further cell division and cellular rearrangement leads to a process called gastrulation. Gastrulation results in two important events: the formation of the primitive gut (archenteron) or digestive cavity, and the formation of the embryonic germ layers, as we have discussed above. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called organogenesis. Diploblastic organisms have two germ layers, endoderm and ectoderm. Endoderm forms the wall of the digestive tract, and ectoderm covers the surface of the animal. In triploblastic animals, a third layer forms: mesoderm, which differentiates into various structures between the ectoderm and endoderm, including the lining of the body cavity.

Figure 2. Development of a simple embryo. During embryonic development, the zygote undergoes a series of mitotic cell divisions, or cleavages, that subdivide the egg into smaller and smaller blastomeres. Note that the 8-cell stage and the blastula are about the same size as the original zygote. In many invertebrates, the blastula consists of a single layer of cells around a hollow space. During a process called gastrulation, the cells from the blastula move inward on one side to form an inner cavity. This inner cavity becomes the primitive gut (archenteron) of the gastrula (“little gut”) stage. The opening into this cavity is called the blastopore, and in some invertebrates it is destined to form the mouth.

Some animals produce larval forms that are different from the adult. In insects with incomplete metamorphosis, such as grasshoppers, the young resemble wingless adults, but gradually produce larger and larger wing buds during successive molts, until finally producing functional wings and sex organs during the last molt. Other animals, such as some insects and echinoderms, undergo complete metamorphosis in which the embryo develops into one or more feeding larval stages that may differ greatly in structure and function from the adult ((Figure)). The adult body then develops from one or more regions of larval tissue. For animals with complete metamorphosis, the larva and the adult may have different diets, limiting competition for food between them. Regardless of whether a species undergoes complete or incomplete metamorphosis, the series of developmental stages of the embryo remains largely the same for most members of the animal kingdom.

Figure 3. Insect metamorphosis. (a) The grasshopper undergoes incomplete metamorphosis. (b) The butterfly undergoes complete metamorphosis. (credit: S.E. Snodgrass, USDA)

Link to Learning

Watch the following video to see how human embryonic development (after the blastula and gastrula stages of development) reflects evolution.

The Role of Homeobox (Hox) Genes in Animal Development

Since the early nineteenth century, scientists have observed that many animals, from the very simple to the complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog embryo, at a certain stage of embryonic development, look remarkably alike! For a long time, scientists did not understand why so many animal species looked similar during embryonic development but were very different as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo would take. Near the end of the twentieth century, a particular class of genes was discovered that had this very job. These genes that determine animal structure are called “homeotic genes,” and they contain DNA sequences called homeoboxes. Genes with homeoboxes encode protein transcription factors. One group of animal genes containing homeobox sequences is specifically referred to as Hox genes. This cluster of genes is responsible for determining the general body plan, such as the number of body segments of an animal, the number and placement of appendages, and animal head-tail directionality. The first Hox genes to be sequenced were those from the fruit fly (Drosophila melanogaster). A single Hox mutation in the fruit fly can result in an extra pair of wings or even legs growing from the head in place of antennae (this is because antennae and legs are embryologic homologous structures and their appearance as antennae or legs is dictated by their origination within specific body segments of the head and thorax during development). Now, Hox genes are known from virtually all other animals as well.

While there are a great many genes that play roles in the morphological development of an animal, including other homeobox-containing genes, what makes Hox genes so powerful is that they serve as “master control genes” that can turn on or off large numbers of other genes. Hox genes do this by encoding transcription factors that control the expression of numerous other genes. Hox genes are homologous across the animal kingdom, that is, the genetic sequences of Hox genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor, from worms to flies, mice, and humans ((Figure)). In addition, the order of the genes reflects the anterior-posterior axis of the animal’s body. One of the contributions to increased animal body complexity is that Hox genes have undergone at least two and perhaps as many as four duplication events during animal evolution, with the additional genes allowing for more complex body types to evolve. All vertebrates have four (or more) sets of Hox genes, while invertebrates have only one set.

Art Connection

Figure 4. Hox genes. Hox genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters on different chromosomes: Hox-A, Hox-B, Hox-C, and Hox-D. Genes within these clusters are expressed in certain body segments at certain stages of development. Shown here is the homology between Hox genes in mice and humans. Note how Hox gene expression, as indicated with orange, pink, blue, and green shading, occurs in the same body segments in both the mouse and the human. While at least one copy of each Hox gene is present in humans and other vertebrates, some Hox genes are missing in some chromosomal sets.

If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?

Two of the five clades within the animal kingdom do not have Hox genes: the Ctenophora and the Porifera. In spite of the superficial similarities between the Cnidaria and the Ctenophora, the Cnidaria have a number of Hox genes, but the Ctenophora have none. The absence of Hox genes from the ctenophores has led to the suggestion that they might be “basal” animals, in spite of their tissue differentiation. Ironically, the Placozoa, which have only a few cell types, do have at least one Hox gene. The presence of a Hox gene in the Placozoa, in addition to similarities in the genomic organization of the Placozoa, Cnidaria and Bilateria, has led to the inclusion of the three groups in a “Parahoxozoa” clade. However, we should note that at this time the reclassification of the Animal Kingdom is still tentative and requires much more study.

The animal might develop two heads and no tail.

Section Summary

Animals constitute an incredibly diverse kingdom of organisms. Although animals range in complexity from simple sea sponges to human beings, most members of the animal kingdom share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile creatures with a fixed body plan. A major characteristic unique to the animal kingdom is the presence of differentiated tissues, such as nerve, muscle, and connective tissues, which are specialized to perform specific functions. Most animals undergo sexual reproduction, leading to a series of developmental embryonic stages that are relatively similar across the animal kingdom. A class of transcriptional control genes called Hox genes directs the organization of the major animal body plans, and these genes are strongly homologous across the animal kingdom.

Art Connections

(Figure) If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?


Animal Kingdom

Let us begin with the very basics. Think of any creature – bugs, birds, animals, fishes, or anything else. I can tell you that the creature you just thought of belongs to a group called the Animal Kingdom.

The Animal Kingdom is divided into two groups:

Invertebrates – All animals without back bones are called Invertebrates. Earthworms, starfishes, squids, snail fall in this category.
Vertebrates – All animals with back bones are called Invertebrates.

Now all the animals with backbones, vertebrates, are further split into smaller groups amphibians, reptiles, birds, fish and mammals. Are you curious to know what animals fall under which group? Let us explore, slowly and steadily.

The word amphibian means two lives – one on land, one in water. Look below to learn more about amphibians.

Did you know that the largest amphibian is Chinese Giant Salamander?

You might ask me a question about amphibians. What does coldblooded mean? Coldblooded animals are those whose bodies don’t automatically regulate their temperature. Thus, their body temperature depends on the temperature of their surroundings. On the other hand, warm-blooded animals can regulate their body temperatures. When it is cold outside, they can make their body heat to keep their bodies warm. When it very hot they can maintain their body temperature at a certain level.

Reptiles are often referred to as ‘creeping or crawling animals’. They have a bony external plate such as a shell and are all covered in scales. Tortoise, lizards, snakes, are all reptiles. The largest species of lizard Komodo dragon is also a reptile. Did you know that the saliva of Komodo dragon is so poisonous that no prey that’s been licked or bitten can get away?

Do I have to tell you a thing or a two about birds? All birds have wings, even the birds that can not fly. In the case of flightless birds, these redundant wings are often known as ‘flippers’.

These underwater rulers constitute a major part of the animal kingdom. Guess who are they? Fishes! Did you know that there are more than 30,000 known species of fish? Fish are covered in scales. These are often covered in a layer of slime to help their movement through water.

Mammals are a very special group of vertebrates. Do you know why? Humans are mammals!! And so are dogs, whales, elephants, horses! The unique thing about mammals is that they have special glands that produce milk for their young ones. Ever seen a calf sucking milk from mama cow’s udder?

Here is a challenge for you now. All the animals that you see from now on, why don’t you try to guess which animal group they fall into? Shall we start with your pet dog? :)


ANIMAL KINGDOM: Part 1

1. The characteristic features of an invertebrate is given.

2. Fill in the blanks with appropriate word: (Score 1)

3. Name the phyla in which the following cells/ structures/ organs are present. (Score 2)

2015 SEPTEMBER (IMPROVEMENT)

1. Observe the following features of animals and answer the following questions. (Score 1)

2. Assign the following features of animals given in column A to the most appropriate animal phylum given in column B. (Score 2)

3. The diagrammatic sketch given below represents a hypothetical chordate.

1. Prawns and insects are included in phylum Arthropoda while they have different habits and habitats. Justify your answer. (Score 1)

2. Figures (X) & (Y) are the fish of two different classes. Identify them and differentiate between these classes. (Score 2)

3. Birds are well adapted for flying. Give any three of such adaptations. (Score 3)

All vertebrates are chordates, but all chordates are not vertebrates. Justify this statement with an example.


Animal Kingdom Classification

In the study of zoology there has a vital part of animal kingdom classification. Numerous living organisms are exist on the earth. Out of those organisms a large number of animals are there. All these animals are collectively formed the animal kingdom. Still today more than a million species of animal have been identified. Their structure, nature and life processes a full of diversified. Some of the animal organisms are living in water, some are on land and maximum of them are living in the air. In this way they occupy more or less all the media of the earth. Some of them act like an enemy to the human being but some other are acting like a friend. To realise and study this wide nature of animals the animal kingdom classification becomes very essential. The classification has been made on the basis of their resemblances and relationships.

What is classification?

Classification is defined as an orderly or ranking of organisms into groups or sit on the basis of their relationship.

What is the necessity of animal kingdom classification?

a) By classifying the organisms, they are divided into large as well as small groups so that one can easily get an idea about the groups and the individual type.
b) Scientific identification and naming of organisms is possible here.
c) Classification helps to understand the process of organic evolution.

Types of animal kingdom classification: -

It has become a formidable task to classify the animal kingdom. Because, different scientists have been classified it differently. Some of the scientists classified the animal kingdom on the basis of the numbered of cells. On this basis the animal kingdom is classified image your sub-kingdoms, such as protozoa and metazoa. The sub- kingdom is again classified into one phylum which is also known as protozoa but metazoa is divided into many image your phyla. In this type of classification protozoan animals are called single celled organisms and metazoan are all multicellular animals.

Animal Kingdom (On the basis of number of cells)

Some scientists have made the animal kingdom classification on the basis of absence and presence of notochord into two groups, such as Non-chordata or Achordata and Chordata. The animals which have no notochord are placed in non –chordata group and the animals that possess the notochord in the body are placed under the group of chordate.

On the Basis of Absence and Presence of Notochord

The notochord in majority of chordates is subsequently replaced by vertebral columns, which are known as vertebrates. In the case of small number of chordates, the notochord is not being replaced by vertebral column rather notochord remains as a notochord, and they are regarded as protochordates. But in non-cordates, notochord and vertebral column both are absent.

On the Basis of Absence and presence of Vertebral Column

The vertebral column is absent in the invertebrates but presents in all animals of the vertebrate group.
For animal kingdom classification the modern zoologist have elevated some of the previously accepted classes to phylum status. Many invertebrate animals have been searched out and many of them have now been placed in the new phyla yet their position in the animal kingdom remains n uncertainty. So the classification is becoming more and more complex day by day. By eliminating the complexity in modern animal kingdom classification, the animal kingdom is divided into few major bigger groups is known as phylum. Each phylum is divided into classes, each class into orders, each order into families, and family into genus. Each genus consist of one or more species and it is the smallest unit of classification.


Section Summary

Animals constitute a diverse kingdom of organisms. Although animals range in complexity from simple sea sponges to human beings, most members share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile creatures with a fixed body plan. Most members of the animal kingdom have differentiated tissues of four main classes—nervous, muscular, connective, and epithelial—that are specialized to perform different functions. Most animals reproduce sexually, leading to a developmental sequence that is relatively similar across the animal kingdom.

Organisms in the animal kingdom are classified based on their body morphology and development. True animals are divided into those with radial versus bilateral symmetry. Animals with three germ layers, called triploblasts, are further characterized by the presence or absence of an internal body cavity called a coelom. Animals with a body cavity may be either coelomates or pseudocoelomates, depending on which tissue gives rise to the coelom. Coelomates are further divided into two groups called protostomes and deuterostomes, based on a number of developmental characteristics.