We live in a beautiful world. In our surroundings, we can see a diverse range of animals including birds, insects, reptiles, amphibians etc. All animals have their own specialised characteristic features. Can you name some animals from your area? Now you understand that all the animals have different names and they are placed in different groups. Have you ever thought why the animals are different from one another and how they are grouped in different categories?
Fig: Kingdom Animalia
Animalia is a broad Kingdom that is classified on the basis of different characteristic features, such as levels of organisation, symmetry, germ layer organisation, coelom, segmentation, body plan, presence notochord etc. By using these many features as a base, animals are classified into various phyla.
Do you know how many phyla there are? There are 11 phyla in the Animal Kingdom. In every phylum, related animals are placed. Now let’s take a look at all these phyla in detail in this article.
Table of contents
All animals are included in the Kingdom Animalia. It is the largest Kingdom among all the five Kingdoms. Members of the animalia are eukaryotic and mostly multicellular. However, unlike plants, they lack a cell wall and chlorophyll. Therefore, members of the Animal Kingdom show a heterotrophic mode of nutrition. They are consumers. Approximately, around 7.8 million species of animals are present on the planet Earth.
Fig: Kingdom Animalia
Animals are classified into eleven different phyla based on different features as follows:
Animals are classified into several phyla based on the following features:
According to the number of cells present, organisms are classified into two types such as unicellular and multicellular organisms.
Unicellular organisms are those in which all functions are performed by a single cell. Examples include the phylum Protozoa.
Fig: Paramoecium - Unicellular
Multicellular organisms or metazoans are those organisms whose body is made up of many cells. Therefore, all the body functions are performed at tissue, organ, and organ system levels in them. Examples include phylum Porifera to Chordata.
GIF: Human lice - Multicellular
According to levels of organisation, organisms are classified into four types as follows:
Fig: Levels of organisation
It is the lowest level of organisation and is commonly seen in protozoans. Protoplasm consists of cell organelles, nucleus and cytoplasm. Cytoplasm is defined as the complex, semifluid, translucent material that makes up a live cell's inner matter and is made up of proteins, lipids, and other molecules suspended in water. Here the body of the animal consists of a single cell which performs all the vital activities of the body. Examples include Paramoecium.
Fig: Paramoecium - protoplasmic level
The body structure of these organisms are composed of cells that are loosely arranged. These cells are responsible for performing different functions but the cells are not organised into tissues and organs. Between the cells, there is a clear division of labor. The members of the phylum Porifera possess a cellular level of organisation.
Fig: Sponges (Phylum Porifera) - Cellular level
The body of the organism is multicellular here. The specialised cells that perform similar functions are organised into tissues. The members of phylum Coelenterata (Cnidaria) and Ctenophora show tissue level of organisation.
Fig: Jellyfish (Phylum Cnidaria) - Tissue level
Organs are formed when tissue performing similar functions aggregates and they perform respective physiological functions. The members of phylum Platyhelminthes (flatworms) show organ level of organisation.
Fig: Phylum Platyhelminthes - Organ level
In this type the body possess tissues, organs and organ systems. The organ system is made up of the organs necessary to carry out a specific function. The members of phylum Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata show organ system level of organisation.
Fig: Humans (Phylum Chordata) - Organ system level
In terms of size, shape, and relative position, symmetry is the distribution of body parts around an axis or on either side of a line of division. It is further classified into three types as follows:
In this symmetry the body of an animal can be divided into two equal halves by cutting it in any plane passing through the central axis. The animals showing radial symmetry are called radiata. These animals possess an oral end and an aboral end. It is commonly seen in phylum Ctenophora, Coelenterata and Echinodermata.
Fig: Radial symmetry in starfish
In this type of symmetry, the body can be divided into two equal halves only through one plane (the median sagittal plane). The animals that possess bilateral symmetry are called bilateria. These animals possess an anterior and posterior end. The side of the body that is kept forward during locomotion is called the anterior side and the other is called the posterior side. The surface of the body facing the substratum is called the belly or ventral surface. The opposite side is called the back or dorsal surface. It is commonly seen in phylum Chordata, Aschelminthes, Platyhelminthes etc.
Fig: Bilateral symmetry in goat
Body cannot be divided into two equal halves in any plane in this model. Therefore, the body has an irregular shape. Phylum Porifera shows this model. Examples include sponges.
Fig: Asymmetry in sponges
The different layers of cells in an embryo formed in the gastrula stage is known as the germ layer. It aids in the development of all tissues and organs. There are three germ layers, ectoderm, mesoderm, and endoderm. Based on the number of germ layers present, organisms are classified into two categories as follows:
The organisms possess two germ layers, external ectoderm, and an internal endoderm are called diploblastic. An undifferentiated layer called mesoglea is present between ectoderm and endoderm here. It is present in the phylum Coelenterata and Ctenophora. A common example of a diploblastic animal is Hydra.
Fig: The two germinal layers in Hydra
The triploblastic organisms possess three germ layers, external ectoderm, middle mesoderm, and an internal endoderm. The embryos of all animal phyla from Platyhelminthes to Chordata possess this model. Humans have triploblastic organisation.
Fig: Three germinal layers in humans
Coelom is the body cavity present between the alimentary canal and the body wall. It is lined by mesoderm or coelomic epithelium on all sides and is only present in triploblastic animals. Coelom provides protection to the internal organs from accidental shock. It allows changes in the shape and size of the internal organs. Depending on whether there is a cavity between the body wall and the alimentary canal, the coelom is categorised into three types as follows:
Acoelom is a condition in which the organism lacks a body cavity or coelom. The organisms are known as acoelomates. It is shown by flatworms or Platyhelminthes, which are bilaterally symmetrical and triploblastic.
In this type of condition, between the ectoderm and the endoderm, mesoderm is located in scattered pouches. Mesoderm does not line the body cavity. Examples include roundworms or Aschelminthes which are bilaterally symmetrical and triploblastic.
These organisms are known as coelomates because they have a true body cavity between the alimentary canal and the body wall. The cavity is lined by the mesoderm from all sides. Examples include humans that are symmetrical and triploblastic. The coelomate phyla include Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata.
Based on the method of formation of coelom, it is of two types as follows:
It develops as a split in the mesoderm sheet. It is found in the phylum Annelida, Arthropoda and Mollusca.
Here in this model the mesoderm arises from the wall of the embryonic gut as outgrowths or pouches. These pouches pinch off and enlarge until they squeeze out the blastocoel. It is seen in the phylum Echinodermata and Chordata.
Segmentation is described as the divisibility of the body into similar constituent units. On the basis of segmentation, animals are divided into three categories as follows:
In this type, the body is not divided into segments. The members of the phylum Porifera and Coelenterata are included in this category.
Fig: Non-segmented body of sponges
The body is segmented only externally here and the internal segmentation does not form. The members of the phylum Platyhelminthes (Tapeworm) are included in this category.
Fig: Pseudometamerism in tapeworm
The body is divided into segments, with some organs repeating in succession here. For example, the segments contain serial repetitions of excretory organs (nephridia) in earthworms. The members of the phylum Annelida possess this model.
Fig: Metameric segmentation in earthworm
The term ‘body plan’ refers to the collection of physical characteristics shared by all members of a given phylum. There are three types of body plan as follows:
This type of body plan includes cells that are arranged as loose cell aggregates. Tissues and organs are absent in the organism. This type of body plan is seen in the phylum Porifera (sponges).
Fig: Cell aggregate body plan
In this model the body is like a sac with a single cavity or digestive tract having a single outlet or mouth. The one external hole called the blastopore that functions as both the anus and the mouth in an incomplete digestive system. This type of body plan is seen in the phylum Coelenterata, Ctenophora, and Platyhelminthes.
Fig: Blind sac body plan
In this model the body consists of two tubes, outer body wall and inner digestive tract. The digestive tract or the alimentary canal runs through the body from the mouth to the anus in this model. Ingestion of food occurs through the mouth and the egestion of the undigested waste materials takes place through the anus. The outer tube consists of skin, shells, horns etc. This appears to be a tube inside of a tube with cavities that are filled with fluid in between the two. This type of body plan is seen in Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata.
Fig: Tube within a tube body plan
The digestive system is made up of various organs that work together to digest food materials, absorb nutrients and eject undigested materials. There are two distinct digestive system types:
A complete digestive system is one that contains two different openings, one for ingestion of food (mouth) and the other for egestion of waste materials (anus). The digestive system of humans is a complete type.
Fig: Complete digestive system in humans
A digestive system is said to be incomplete if it only has one opening and serves as both the mouth and the anus. The digestive system in Platyhelminthes is an incomplete one.
Fig: Incomplete digestive system in flatworm
Digestion is of two types as follows:
It occurs within the cell
It occurs outside the cell in a cavity
Digestive enzymes are poured into the food vacuoles here and the digestion takes place inside the food vacuoles
The digestive enzymes are poured into the cavity where digestion of food takes place
The products of digestion enters into the cytoplasm
The products of digestion enters into the wall of the cavity
Examples include phylum Protozoa and Porifera
Examples include higher invertebrates and Chordata
GIF: Intracellular digestion
Fig: Extracellular digestion
A circulatory system is a group of organs that aids in the movement of different metabolic byproducts throughout an animal body. The circulatory system is of two types as follows:
In an open circulatory system, blood immediately bathes the cells and tissues as it passes through the sinuses and body cavities. The circulatory system in phylum Arthropoda and Mollusca is open type. The body cavity of these animals remains filled with blood always.
Fig: Open circulatory system in arthropods
In this type of circulatory system, blood travels through blood vessels and capillaries. It is seen in phylum Annelida and Chordata.
Fig: Closed circulatory system in humans
Notochord is a rod-like structure present on the posterior surface of an embryo. It is present only during embryonic development and is derived from mesoderm.
Fig: Notochord during embryonic development
Animals are categorised into two types based on the presence or absence of notochord as follows:
Animals that possess notochord during the embryonic development are known as chordates. Examples include humans.
Animals that lack a notochord during embryonic development are known as non-chordates. Examples include sponges.
Porifera includes organisms with pores. The name is derived from the two Greek words Porous means pore and pherein means bearing. These are commonly known as sponges.
The characteristic features of poriferans are as follows:
Based on their exoskeleton, phylum Porifera is classified into three different classes as follows:
The Greek word ‘kilos’ means hollow-bellied from where the word ‘Coelenterata’ originated. They are also called Cinidaria. The Greek word ‘knide’ means ‘nettle or sting cells’.
Some of the characteristic features are listed below:
Based on the dominance of medusoid or polyploid phase, phylum Cnidaria is classified into three different classes as follows:
Fig: Examples of Cnidaria
The name ‘Ctenophora’ comes from the Greek words ‘ktene’ means ‘comb’ and ‘phors’ means ‘bearing.’ Sea walnuts and comb jellies are other names for ctenophores.
The common characteristic features are listed below:
Based on the presence or absence of tentacles, they are classified into two classes:
GIF: Pleurobrachia GIF: Ctenoplana
The term Platyhelminthes is derived from the Greek words ‘platys’, which means broad or flat, and ‘helmin’, which means worm. Members of Platyhelminthes are hence also known as flatworms.
The characteristic features of the flatworms are listed below:
Based on the mode of living, they are classified into three classes as follows:
Fig: Examples of Platyhelminthes
The name Aschelminthes is derived from the Greek words ‘nema’, which means thread, and ‘helmin’, which means worm. Therefore, the Aschelminthes members are also known as nematodes.
Some of the characteristic features of Aschelminthes are listed below:
Phylum Aschelminthes is classified into two classes:
Fig: Examples of Aschelminthes
The Latin words ‘annulus’ which means little ring, and ‘lidos’ which means form, are the roots of the word Annelida. They are called segmented worms.
Some of the characteristics of Annelida are listed below:
On the presence or absence of setae, they are classified into four classes as follows:
Fig: Examples of Annelida
The greek word ‘Arthropoda’ has been derived from two words ‘arthron’ meaning joint and ‘podos’ meaning foot.
Some of the characteristic features of Arthropoda are listed below:
Arthropods are divided into seven separate classes based on the body divisions and presence or absence of particular appendages.
Fig: Examples of Arthropoda
The word Mollusca comes from the Latin words ‘mollis’ which means ‘soft.’ The creatures with soft bodies belong to this phylum.
Some of the features of Mollusca are listed below:
The phylum Mollusca is classified into six classes:
Fig: Examples of Mollusca
The word Echinodermata is derived from the Greek words ‘echinos’ which means spiny, and ‘derma’ which means skin or covering. Like porcupines, echinoderms have spiny skin.
Some of the features of Echinoderms are listed below:
The animals in this phylum Echinodermata are divided into five classes. They are as follows:
Fig: Examples of Echinodermata
Hemichordata was formerly included under the phylum Chordata but is now considered to be a non-chordate. The terms ‘hemi’ which means ‘half’ and ‘chordata’ which means ‘notochord’ are combined to form the word ‘hemichordata.’ They are therefore called half chordates and they resemble little worm-like creatures.
Some of the features of Hemichordata are listed below:
The members of the phylum Hemichordata are classified into three classes:
Fig: Examples of Hemichordata
The phylum Chordata possess animals that have notochords.
Some of the defining features of phylum Chordata are listed below:
Phylum Chordata is further classified into three subphyla as follows:
Subphylum Urochordata is distinguished by the exclusive presence of notochord in the larval tail. In adulthood, it fades away. The nerve cord, which is present in larvae, is replaced by a dorsal ganglion in adults. Examples of urochordates include Doliolum, Ascidia, Salpa, Herdmania, etc.
Fig: Examples of Urochordates
A notochord that runs from the head to the tail region is a distinguishing feature of subphylum Cephalochordata. The subphylum is known as Cephalochordata because the notochord extends to the body's anterior tip. In cephalochordates, the notochord remains throughout life. Examples include Branchiostoma (Sea lancelet or Amphioxus).
Fig: Example of Cephalochordata - Branchiostoma
The highest order of chordates are vertebrates. It is distinguished by the single notochord present during the embryonic stage of life, which eventually develops into a cartilaginous or bony vertebral column in the adult form. An anterior notochord segment is replaced with a skull or cranium to protect the brain. The vertebrates are also referred to as Craniata for this reason.
Fig: Development of notochord into a vertebral column in vertebrates
Fig: Common examples of Chordata
Solution: The largest phylum in the Animal Kingdom is called Arthropoda. Approximately 84% of the animal species on Earth are represented by it. Mollusca is the second largest phylum of the Kingdom Animalia. There are numerous species in this phylum that are freshwater, marine, and terrestrial. About 100,000 live species are present there. Hence, the correct option is d.
2. In the animal kingdom, how many phyla have radial symmetry?
Solution: Any plane passing through the centre of the body which splits the organism into two identical halves is called radial symmetry. The organisms that possess radial symmetry are called radiata. In the animal kingdom, radial symmetry is exhibited by three phyla as follows:
Hence, the correct option is c.
3. From the given options, which is the most distinctive feature that distinguishes non-chordates from chordates?
Solution: Those animals which do not possess a notochord at least some stage in their life cycle are referred to as non-chordates. Examples include starfish, cockroaches, earthworms, etc. Those animals which possess a notochord at least some stage in their life cycle are known as chordates. It comprises fishes, amphibians, reptiles, birds, mammals, etc. The vertebral column replaces the notochord in higher chordates or vertebrates. Hence, the correct option is b.
4. Which taxonomic unit is exclusively used for the classification of the Animal Kingdom?
Answer: The exclusively used taxonomic unit for the classification of the Animal Kingdom is phylum. Phylum is described as a group of related animals.
Answer: There is only one species that has been called biologically immortal and this is the jellyfish, Turritopsis dohrnii. These tiny, translucent creatures, which are found in waters all over the world, have the ability to go back in time and resume an earlier phase of their life cycles.
Answer: The arthropods have multipurpose appendages that allow more mobility on land. Arthropods may be found in almost every environment and niche on the Earth because of the evolution of a wide variety of appendages, including antennae, claws, wings, and mouthparts. They have a hard exoskeleton of chitin that protects the body.
Answer: Hippopotamus, saltwater crocodiles, great white sharks, etc., have the most powerful bites in the world.
Answer: A sloth needs a minimum of two weeks to digest food. This is because it has the lowest metabolic rate which means that it takes a long time to digest its meal.
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