Animal Kingdom

Animal kingdom also known as the kingdom Animalia, is a diverse and vast biological kingdom consisting of multicellular, eukaryotic organisms. The classification of animals is based on various criteria, including arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory, or reproductive systems. The animal kingdom is primarily classified into vertebrates (animals with a backbone) and invertebrates (animals without a backbone), with further subdivisions into various phyla based on anatomical and genetic traits.

What is Animal Kingdom?

The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etc., have enabled us to broadly classify the animal kingdom. The animal kingdom contains all the animals from the simplest to the most complex. Animals are multicellular organisms that are capable of movement, either actively or passively.

They are heterotrophic, i.e., they obtain their energy by consuming other organisms or organic matter. The animal kingdom is divided into many phyla, each with its unique characteristics and adaptations. Some of the most well-known phyla include the arthropods, which include insects, spiders, and crustaceans, and the chordates, which include vertebrates like fish, amphibians, reptiles, birds, and mammals.

Animal Kingdom Classification

Animals included in the animal kingdom are eukaryotic organisms. Animals have been categorized into two primary categories in the animal kingdom based on the presence or absence of a backbone or spinal column:

Invertebrates

Invertebrates are animals that lack a vertebral column or backbone, and they make up the vast majority of animal species on Earth. They lack a hard body structure and blood flows in an open cavity in most invertebrates. They have gill or trachae for respiration. Most invertebrates have an exterior skeleton to protect their soft, inner body which is made up of Chitin, a glucose derivative. This diverse group includes arthropods (insects, spiders, crustaceans), mollusks (snails, clams, octopuses), annelids (earthworms, leeches), and many more.

Vertebrates

Vertebrates are a subphylum of animals within the phylum Chordata and are characterized by the presence of a vertebral column or backbone, which contains and protects the spinal cord. They exhibit bilateral symmetry and are generally more complex than invertebrates. Vertebrates include several major groups: fish (with both jawless and jawed varieties), amphibians (such as frogs and salamanders), reptiles (including turtles, snakes, and lizards), birds, and mammals.

The animals are further classified into various phyla depending on their characteristics:

Phylum Porifera

In the animal kingdom, the Phylum Porifera has the lowest multicellular animals. The name Porifera itself indicates the pores which show that the organisms under this phylum have pores in their bodies that are known as Ostia. It includes simple, aquatic animals known as sponges. This phylum has more than 5000 species in it. The characteristics of the organism are as follows:

• These are the first multicellular organism that has pores on their body.
• They lack true tissues and organs
• The poriferans have loosely organized cells. 
• Mostly, found in marine water.
• In ancient times, these were recognized as plants because of their green color and their relation to algae. Later on, after the discovery of their life cycle and feeding habits, they were put under the animal kingdom. 
• Poriferans don’t have any organs in their body. 
• Examples are Sycon, Hylonema, Spongilla, and Euplectella.

Phylum Coelenterata

The simplest form of tissue organization is found in Phylum Coelenterata which has only two layers of cells with radial symmetry. They have the following characteristics:

• They do not have an organized circulatory system. Circulation happens through diffusion between the layers of the tissues.
• They exhibit radial symmetry and possess specialized stinging cells called cnidocytes.
• They have a central gastrovascular cavity with a single opening serving as both the mouth and anus.
• These are mostly found in marine water.
• Most coelenterates have tentacles that are armed with cnidocytes to capture and immobilize prey.
• Coelenterates have both modes of digestion i.e., intracellular and extracellular.
• Respiration and excretion are done by simple diffusion. 
• Reproduction can be asexual through budding or fragmentation, and sexual reproduction occurs in some species through the release of gametes.
• They have the ability to regenerate lost body parts, making them resilient to physical damage.
• Examples are Moon jelly, Lion’s mane jellyfish, Barrel jellyfish, and Portuguese.

Phylum Ctenophora

Ctenophora are the largest animals to swim with cilia. They show the following characteristics:

• Ctenophores exhibit radial symmetry, with their body parts arranged in a circular or comb-like pattern.
• These organisms have two layers of cells which are thicker at the outer part and their bodies consist of jelly mass. 
• Many ctenophores are bioluminescent, producing light to attract prey or deter predators.
• Different types of body forms are found in this phylum like egg-shaped cydippids with retractable tentacles, flat, large-mouthed beroids, etc. 
• They have a simple digestive system with a mouth and an anal pore, and some species have a branched, tree-like digestive system.
• Ctenophores are carnivorous and feed on zooplankton, small fish, and other small aquatic organisms.
• They possess a simple nerve net that allows for basic coordination and responses to stimuli.
• The distinguishing feature of ctenophores is that they have colloblasts i.e., sticky and adhere to the prey but some species lack these colloblasts. 
• They reproduce through external fertilization, releasing gametes into the water.
• They are diploblastic. But according to some, they are known as triploblastic because they have complex muscles which arise from middle cell layers. 
• Examples are Nuda, Tentaculata, Bathocyroe, Dryodoridae, etc.

Phylum Platyhelminthes

Phylum Platyhelminthes organism shows bilateral symmetry. They are soft-bodied invertebrates and are known as flatworms. They are primarily parasitic or free-living and thrive in aquatic or moist environments. They show following characteristics:

• They have a flat, ribbon-like body shape, which gives them their common name, “flatworms.”
• Flatworms lack a coelom (body cavity),and they don’t have any specialized organs for respiration and circulation. 
• They possess a branched, sac-like gut with a single opening that serves as both the mouth and anus.
• Mostly Platyhelminthes are free-living or parasitic in nature. 
• Flatworms have a ladder-like nervous system with paired longitudinal nerve cords and ganglia (clusters of nerve cells) that allow for basic sensory perception and coordination.
• They are triploblastic and don’t have cilia for movement. 
• Both male and female organs are present in the same body so are hermaphrodites. They can reproduce asexually as well as sexually. Sexual reproduction is done by the fusion of gametes and asexually it is done by fission and regeneration. 
• For excretion, they have flame cells. Flames cells are also helpful in osmoregulation. They have a ladder-like nervous system.
• Flatworms have a remarkable ability to regenerate lost body parts, and in some cases, a complete individual can regrow from a small fragment. 
• Examples are Tapeworms, Turbellaria, Flukes, Monogenea, etc.

Phylum Aschelminthes

Phylum Aschelminthes shows many similarities with the Platyhelminthes. This phylum has the characteristic feature of having pseudocoelom. They possess following characteristics:

• Usually, Aschelminthes are free-living organisms. In the human intestine, Ascaris lives as an endoparasite.
• They have a fluid-filled body cavity called a pseudocoelom that surrounds their internal organs. 
• They are triploblastic, bilaterally symmetrical, unsegmented, slender, long, and tapers at the end. 
• They have a mouth and a separate anus, with a complete digestive system.
• Aschelminthes does not have metameric segmentation.
• They have a simple nervous system with a nerve ring around the pharynx and longitudinal nerve cords.
• They have an organ-system level of organization. 
• The reproduction mode is sexual and the fertilization is internal. Development can be direct or indirect through the larval stage. 
• Examples are Human pinworm, Heartworm, Threadworm, Human whipworm, etc.

Phylum Annelida

Phylum Annelida organisms are usually known as segmented worms or ringed worms. They show the following characteristics:

• Annelids have a segmented body, with each segment typically being repeated along the length of the animal.These are triploblastic and coelomates. 
• Annelids have a true coelom, a fluid-filled body cavity surrounded by mesodermal tissue.
• Respiration is done through the body surface and for excretion they have nephridia as excretory organs. 
• Annelids have a closed circulatory system, with blood contained within vessels.
• They possess a complete digestive system with a mouth and an anus, allowing for efficient food processing.
• A common process in annelids is regeneration. 
• Most annelids have setae, which are bristle-like structures on their body segments. These aid in locomotion and anchor the worms.
• Annelids can reproduce both sexually and asexually. Some reproduce by budding or fragmentation.
• They have hemoglobin due to which they have a red color. 
• Examples are Earthworms, leeches, Bobbit worms, and Polychaete.

Phylum Arthropoda

Phylum Arthropoda is the largest and most diverse phylum in the animal kingdom, containing a wide variety of invertebrate animals. Their name itself represents jointed appendages. They show the following characteristics:

• Arthropods have an external skeleton made of a tough, flexible material called chitin, which provides protection and support. They have a segmented body. 
• Arthropods have jointed appendages, including legs and antennae, that enable them to move and interact with their environment.
• They are bilaterally symmetrical. 
• Arthropods have a true coelom (body cavity) surrounded by mesoderm.
• Arthropods have various respiratory structures, including gills, tracheae, book lungs, and cutaneous respiration, depending on the group.
• Arthropods undergo molting (shedding their old exoskeleton to have a new one) to keep growing. 
• They have approx. 10 million species. 
• Examples are Spider, Scorpion, crustaceans, Arachnid, etc.

Phylum Mollusca

Phylum Mollusca is the Second largest phylum in the animal kingdom. It has approx. 85000 species in this phylum. The organisms exhibit following characteristics:

• Mollusks are characterized by soft, unsegmented bodies that are often protected by a hard shell made of calcium carbonate.
• They have three main body parts: a muscular foot for locomotion, a visceral mass containing internal organs, and a head with sensory organs. 
• Mollusks respire using gills (aquatic species) or lungs (terrestrial species), and some rely on cutaneous respiration.
• Mollusks reproduce sexually, with separate sexes, and fertilization can be external or internal.
• Mollusks are coelomate and the main cavity is hemocoel by which blood circulates as it has an open circulatory system.
• For excretion, they have kidney-like organs. In ancient times, it was a good source of food for humans and it was a good source of luxurious goods like pearls, sea silk, Tyrian purple dye, etc. 
• Examples are Octopus, Scallops, Bivalvia, Gastropods, etc.

Phylum Echinodermata

Phylum Echinodermata includes species like starfish, brittle stars, sea cucumber, sand dollars, etc. They consists of marine animals with spiny, often calcareous endoskeletons. These organisms show following characteristics:

• They are radially symmetrical. 
• Echinoderms are usually found at the bottom of the sea. 
• They possess a unique water vascular system, a network of water-filled canals and tube feet that are used for locomotion, respiration, and feeding.
• Reproduction is done asexually and they regenerate tissues and organs. Some exceptions are there where they regenerate from a single limb. 
• This phylum has valuable species due to its ossified skeleton which is the main contributor to limestone formations. 
• They are multicellular organisms, colored with unique shapes.
• Echinoderms respire through their tube feet, exchanging gases with seawater.
• Echinoderms are diverse in their feeding habits, with some species being predators, while others are detritivores (feeding on decaying matter) or filter feeders.
• Echinoderms are triploblastic and coelomates. 
• Examples are Starfish, Sea cucumber, Sea urchins, Brittle stars, Cystoidea, etc.

Phylum Hemichordata

Phylum Hemichordata is usually known as the sister group of phyla Echinodermata as they also include marine and deuterostome species. It is the smallest phylum with only 100 species. These organisms show following characteristics:

• They exhibit bilateral symmetry, and their body consists of three main parts: a proboscis, a collar, and a trunk.
• Reproduction is mostly sexual and development is indirect but some exceptions are there which have direct development. 
• They have some similar features to chordates but they are not properly chordates. They are bilaterally symmetrical, triploblastic, and have a true coelom. 
• Earlier, they were a part of phylum chordate but after the research, it is concluded that they don’t have notochord which is the main feature of chordates.
• Hemichordates possess pharyngeal gill slits, which are a characteristic feature of chordates.
• Hemichordates have ciliated tentacles around their mouths that help in the capture and transport of food particles.
• Respiration primarily occurs through the gills located in the collar region.
• Hemichordates typically use ciliary action for locomotion, helping them move through the water. 
• Some organisms are solitary while some are colonial while all have a worm-like appearance. 
• Examples are Balanoglossus, Graptolite, Graptoloidea, etc. 

Phylum Chordata

Chordates are known for their distinguishing feature i.e., the presence of the notochord. These organisms show following characteristics:

• They possess metameric segmentation and are coelomate. 
• They are bilaterally symmetrical organisms. 
• The other characteristics feature besides notochords that are present in chordates are pharyngeal slits, dorsal nerve cord, and a post-anal tail. 
• The notochord is a flexible, rod-like structure that provides support and serves as a developmental precursor to the vertebral column in vertebrates.
• They have an organ-system level of organization. Species vary in size and length from inches to meters. 
• Most chordates have pharyngeal gill slits, which may function in respiration, filter-feeding, or as a developmental feature.
• Sexes are separate but in tunicates it shows hermaphroditism i.e., having both male and female reproductive organs.
• Fertilization is external in the cases of aquatic species. Both types of reproduction are seen in this phylum i.e., asexual is seen in tunicates whereas sexual is seen in vertebrates.
• Muscular movements are there for locomotion. 
• Examples are Vertebrates, Mammals, Tunicates, Amphibians, etc.

Each phylum contains a variety of species with diverse adaptations and characteristics. The classification of animals is based on various criteria, including body structure, embryological development, and genetic relationships. It helps scientists understand the evolutionary relationships among different animal groups and provides a systematic system for studying the vast diversity of animal life.

FAQs on Animal Kingdom

1. What are the 7 Classification of Animals?

The scientific classification system is categorised into seven major groups, (1) kingdom, (2) phylum or division, (3) class, (4) order, (5) family, (6) genus, and (7) species. The kingdom forms the largest group and a species forms the smallest.

2. What is the Animal Kingdom?

Animals are eukaryotic, multicellular, species belonging to the Kingdom Animalia. They show heterotrophic mode of nutrition that is they obtain their energy either by feeding on plants or on other animals. 

3. What is the Difference Between Vertebrates and Invertebrates?

Vertebrates are animals with a vertebral column or backbone and they belong to the subphylum Vertebrata within the phylum Chordata. Invertebrates, on the other hand, do not have a backbone and constitute the majority of animal species.

4. What are Some Examples of Animal Phyla?

Common animal phyla include Chordata (vertebrates and related species), Arthropoda (insects, spiders, crustaceans), Mollusca (snails, clams, octopuses), and Echinodermata (starfish, sea urchins), among others.

5. What is the Evolutionary History of the Animal Kingdom?

There is evidence of the first animals appearing over 600 million years ago. The animal kingdom has evoloved over time, leading to the wide variety of species that we see today.

6. What is the Most Diverse and Abundant Group within the Animal Kingdom?

Arthropods, including insects, are the most diverse and abundant group within the animal kingdom, with over a million described species.