Animal Kingdom- Definition, Characteristics, Phyla, Examples

A high-level taxonomic classification called an animal phylum refers to a collection of species that share a phylogenetic relationship based on common ancestry. The body plan of an animal phylum, which is made up of a variety of distinctive morphological characteristics, is another way that the phyla of animals are usually classified.

Animal Kingdom
Animal Kingdom

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Characteristics of Animal Kingdom

Early stages of animal development, such as gastrulation, are the foundational characteristics that characterize animal phyla. In evolution, these initial processes seldom alter and frequently pave the way for subsequent developmental processes.

  • Embryonic tissue layer- Three layers of embryonic tissue formed during gastrulation later differentiate to generate all the different types of cells in a mature animal. This kind of triploblastic growth is evident in all animals except the outliers: the phylum Porifera and phylum Cnidaria which are diploblastic.
  • Symmetry- Bilaterally symmetrical creatures have their fronts and back distinctly different, while left and right sides are roughly mirrored images of one another. Example: Humans are bilaterally symmetrical. Radial symmetry is the architectural body plan where species are more or less spherical and have no left or right sides, and can move in either direction with equal ease. These include Cnidarians. Likewise, sponges don’t have any specific symmetry i.e., asymmetry.
  • Cephalization- Cephalization is the state of presence of the head in organisms. It is lacking in cnidarians such as jellyfish. Most of our sensory organs, including the brains of humans and many other animals, are housed inside our heads. All bilaterally symmetrical animals experience cephalization.
  • Body cavity- The digestive system and other internal organs develop in the bodily cavity.  Two huge cavities in our body include the thoracic cavity(which houses the lungs) and the abdominal cavity (which houses the digestive system). Animals are divided into three cavities based on the presence or absence of internal organs:
    • Coelomate: Many internal organs develop inside a cavity (the coelom) completely walled with mesodermal tissue in the body. Arthropods and chordates are coelomates.
    • Pseudocoelomate: The internal organs develop inside the coelom, partially walled with mesodermal tissue. Worms are pseudocoelomates.
    • Acoelomate: These organisms lack cavities and have a simple body plan.
  • Segmentation- The bodies of many animals are segmented, which is evident in phylum Annelida, phylum Arthropoda, and phylum Chordata. Conversely, phyla Cnidaria are unsegmented.
  • Digestive tract- All animal phyla- aside from sponges, have a gut or digestive system. Flatworms and cnidaria have digestive tracts with only one opening. Organisms with a complete digestive tract initiate from the mouth and end in the anus.
  • Circulatory system- Some organisms have a closed system, while others have an open circulatory system.
  • Exoskeleton- Arthropods have a hard exterior exoskeleton to defend their bodies, and through the process of molting, they shed their exoskeleton, which is termed ecdysis. Similarly, nematodes molt to grow, but they are more flexible exoskeletons, i.e., cuticles.

Different phyla of Animal Kingdom

The Animal Kingdom comprises around 40 phyla, albeit several include few or poorly understood organisms. The most well-studied, abundant, and major animal phyla are nine in number, and their features are described below:

1. Porifera

  • These are the simplest multicellular organisms that possess a cellular level of body organization,
  • They are all aquatic, found mostly in marine habitats, and a few are freshwater forms.
  • These are all sessile, some are solitary, and some are colonial.
  • They possess radial symmetry or asymmetry in their body structure.
  • These are pore-bearing animals where a single large opening (osculum) permits the exit of water, and two or more pores (Ostia) facilitate the ingression of water into the body.
  • Examples: Sycon, Spongilla, Euspongia, etc.

2. Cnidaria

  • These are mostly marine entities, while few, such as hydra, are found in freshwater.
  • These are diploblastic and depict a tissue grade of organization.
  • The members of this phyla exhibit radial symmetry except for sea anemones which exhibit biradial symmetry.
  • The body layer comprises the outer epidermis and inner gastrodermis, with the gelatinous mesoglea in between the outer and inner epidermis.
  • There are two different forms of cnidarians-Polyp and Medusa. The polyp is sessile, hydroid form with mouth-up orientation. Likewise, the medusa is an umbrella or bell-shaped with a mouth-down alignment.
  • Cnidocytes or stinging cells are found in the body wall, which is useful for protecting and catching prey.
  • Gastrovascular cavity or coelenterons are blind sac-like central cavity that exits the mouth and is encircled by tentacles. These aid in digestion and blood flow.
  • The exchange of breathing gases and elimination of excretory wastes happens through the body wall diffusion mechanism.
  • Examples: Hydra, Aurelia, etc.

3. Platyhelminthes

  • They are bilaterally symmetrical, triploblastic, and acoelomate.
  • They resemble leaves because of their dorsoventral flattening and lack of segmentation.
  • They could be parasites or free-living beings.
  • They are organized at the level of organs.
  • Their digestive system is absent.
  • They just diffuse oxygen across the surface of their bodies to breathe.
  • They have a mouth but no anus or circulatory system.
  • The body has a delicate coating that may or may not have cilia.
  • Given that they are hermaphrodites, their bodies include male and female organs.
  • They procreate sexually by the fusion of gametes and asexually through fission and regeneration internally.
  • There is an underdeveloped neurological system.
  • The flame cells aid in excretion. Protonephridia with the flame are in the excretory system.
  • There is only one aperture that opens into the spacious gastrovascular cavity, and there is no anus.
  • Examples: Liverfluke, Planaria, Schistosoma, etc.

4. Nematoda

  • Their body is triploblastic and bilaterally symmetrical.
  • They demonstrate organization at the tissue level.
  • They lack both the respiratory and circulatory systems.
  • They can live freely or as parasites.
  • They exhibit sexual dimorphism.
  • They have a head and a tail, although it can be difficult to distinguish between the two.
  • A layer of skin secretes the cuticle, a tough outer layer that helps with movement, supports and protects the worm, and helps keep the internal pressure that gives the worm its recognizable round shape.
  • The mouth and anus are located at the anterior(head), and posterior(tail) ends, respectively, of the roundworm’s digestive tract.
  • Examples: Hookworm, Roundworm, etc.

5. Annelida

  • These organisms ate coelomate and triploblastic.
  • Their gut and circulatory systems are well-developed.
  • Organization at the level of the organ system is present.
  • The excretory organs are called nephridia.
  • They are known for their propensity for regeneration.
  • They breathe via the surface of their bodies.
  • Setae assist their movement.
  • Because of their particular segmentation, known as metamerism, their bodies are composed of sequentially repeating structures called metamers.
  • The majority of annelids are hermaphrodites, which means that both male and female organs are found within the same body. They can reproduce asexually or sexually.
  • Examples: Earthworm, Leech, etc.

6. Arthropoda

  • They display the level of organization found in an organ system.
  • The head, thorax, and abdomen are the three major body parts.
  • The circulatory system is of open type.
  • The body is segmented, triploblastic and symmetrically arranged on both sides.
  • Blood flows throughout the coelomic cavity.
  • Chitin constitutes the exoskeleton.
  • They are monosexual, and they can be fertilized either internally or externally.
  • Bodies have jointed appendages that aid with the movement.
  • The trachea, or general body surface, serves as their respiratory passage.
  • Aquatic arachnids use coaxial glands or green glands for expulsion, while terrestrial arachnids use Malpighian tubules.
  • Examples: Crab, Spider, Housefly, Millepede, etc.

7. Mollusca

  • Their natural habitats are primarily freshwater and marine.  Very few are terrestrial and can be found in wet soil.
  • They display the level of organization found in an organ system. 
  • The skull, muscular foot, visceral mass, and mantle are the different parts of the body.
  • Tentacles and compound eyes make up the head.
  • Locomotion is aided by the muscular foot.
  • The open circulatory system allows for the movement of blood.
  • A calcium-rich shell protects the body.
  • The radula serves as the feeding organ, and these also have a well-developed digestive system.
  • Two metanephridia are important in elimination.
  • The sensory organs are the eyes, osphradium, statocysts, and tentacles.
  • They often develop indirectly and are oviparous.
  • Examples: Octopus, Helix, Limax, Unio, etc.

8. Echinodermata

  • They are exclusively marine.
  • They display organ system-level organization. 
  • Their circulatory system is open.
  • They have a coelomic cavity and are triploblastic.
  • The excretory system is lacking, and they respire through the gills or the cloacal respiratory tree.
  • Movement is made easier by the tube feet.
  • Spiny skin covers the creatures.
  • There is no definite head, and they have an unsegmented body. On the ventral side, the mouth is located, and on the dorsal side, the anus.
  • Calcium carbonate makes up the skeleton.
  • They possess the capacity for rejuvenation. 
  • Their sensory organs are not very well developed. Chemoreceptors, tactile, organs, terminal tentacles, etc. are a few of these.
  • Examples: Sea Urchin, Sea Cucumber, Starfish, etc.

9. Chordata

  • Notochord-  These are flexible, rod-shaped mesodermal structures in all chordate embryos and the adult stages of some chordate species. It is situated between the gastrointestinal tract and the spinal cord, which offers flexible axial muscle attachment as well as stiff skeletal support. During the embryonic development of vertebrates, the notochord appears, which promotes the growth of neural tubes and acts as a support for the growing embryo. 
  • Dorsal nerve cord- The source of the dorsal hollow nerve cord is the ectoderm which develops into a hollow tube. It is situated dorsally to the notochord in chordates. Contrarily, the neurological system of protostome animal phyla is distinguished by solid nerve cords either ventrally or lateral to the gut. The brain and spinal cord, which make up the central nervous system in vertebrates, rise from the neural tube.
  • Pharyngeal Slits- The pharynx, or the area right behind the mouth, has openings called pharyngeal slits that reach out into the surrounding space. They allow water sucked into the mouth while feeding to be expelled in aquatic species. The pharyngeal slits are adapted into gill supports in fish with vertebrates and jaw supports in fish with jaws. These slits in tetrapods have undergone significant modification to become parts of the ear, thymus glands, and tonsils.
  • Post-anal Tail- A posterior extension of the body that extends past the anus is known as the post-anal tail. The tails serve as a source of propulsion in aquatic species like fish. The tail aids balance, courtship, and warning of impending danger in several terrestrial species. In humans, the post-anal tail has been reduced to a vestigial coccyx that aids in sitting balance.

Animal Kingdom References


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Prakriti Karki

Prakriti Karki completed her B.Sc. in the field of Microbiology. She is interested in working in the interface of immunology, microbiology, synthetic biology, bioinformatics, and open science. She has worked as a project lead at Media Lab Nepal, as a research associate in the BMSIS program, and as an awareness community member at the iGEM WiSTEM initiative.

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