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Animal Kingdom – Definition, Types, Characteristics, Examples

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  • Last Updated : 24 Jun, 2022

According, to the classification done by R.H. Whittaker. All the animals have included in the biggest kingdom called an animal kingdom. All eukaryotes which are multicellular and heterotrophs and in which cell wall is absent fall under this kingdom. All the animals have their own special features which make them unique. In this kingdom, the sub-division which are made is known as a phylum. There are eleven phyla in this kingdom which are classified according to the presence and absence of notochord. From phylum Porifera to hemichordate comes under non-chordates and the phylum Chordata comes under chordates. Let’s begin by explaining all the phylum is one by one.
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. This phylum has more than 5000 species in it. These are the first multicellular organism that has pores on their body. The poriferans have loosely organized cells. Mostly, are found in marine water. Very few are found in freshwater. 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, it was put under the animal kingdom. Poriferans doesn’t have any organ in their body. Moreover, spongioblast cells are secreted by spongin fibers whereas spicules are secreted by sideroblasts. Reproduction mode in this phylum is asexual i.e., budding and fragmentation. Porifera has the power of regenerating the lost parts. Development in this phylum is indirect and it has holoblastic cleavage. For e.g., Sycon, Hylonema, Spongilla, Euplectella.

Phylum Coelenterata

The simplest form of tissue organization is found in this phylum which has only two layers of cells with radial symmetry. They do not have an organized circulatory system. Circulation happens through the diffusion between the layers of the tissues. These are mostly found in marine water. To capture most of the planktonic prey they have sensory tentacles which are surrounded around the single opening present in the coelenterates which are known as hypostomes. The gastrovascular cavity is the cavity that is surrounded by tentacles. Coelenterates have both modes of digestion i.e., intracellular and extracellular whereas respiration and excretion are done by simple diffusion. Zooids are the cnidarians that show polymorphism i.e., in colony different types of individuals are present in a colony for performing different functions. The main mode of reproduction in this phylum is asexual reproduction i.e., through budding but some groups also show sexual reproduction. For e.g., Moon jelly, Lion’s mane jellyfish, Barrel jellyfish, and Portuguese man o’ war.

Phylum Ctenophora

The largest animals to swim with cilia are ctenophores. Only found in seawater called comb jellies. These organisms have two layers of cells which are thicker at the outer part and their bodies consist of jelly mass. Different types of body forms are found in this phylum like egg-shaped cydippids with retractable tentacles, flat, large-mouthed beroids, etc. Mostly ctenophores are predators but some also survive as parasites living on salps. There was a hypothesis that claims that ctenophores are the second earliest branching animal lineage with sponge being the sister group. The distinguishing feature of ctenophores is that they have colloblasts i.e., sticky and adhere to the prey but some species lack these colloblasts. Mesoglea is the jelly-like material present between the two main layers of cells that the ctenophores have in them. Hence, they are diploblastic. In some books, they are known as triploblastic because they have complex muscles which arise from middle cell layers. For e.g., Nuda, Tentaculata, Bathocyroe, Dryodoridae etc.

Phylum Platyhelminthes

A phylum that consists of unsegmented, bilaterians and soft-bodied invertebrates in it and is also known as flatworms. They lack a cavity so they are acoelomates and they don’t have any specialized organs for respiration and circulation. They have only one opening for ingestion and excretion. Mostly Platyhelminthes are free-living or parasitic in nature. 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 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. For e.g., Tapeworm, Turbellaria, Flukes, Monogenea, etc.

Phylum Aschelminthes

This phylum shows many similarities with the Platyhelminthes. This phylum has the characteristic feature of having pseudocoelom. Usually, aschelminthes are free-living organisms. In the human intestine, Ascaris lives as an endoparasite. They are triploblastic, bilaterally symmetrical, unsegmented, slender, long, and tapers at the end. Aschelminthes does not have metameric segmentation. In this phylum generally, males are longer than females so are dioecious. They have an organ-system level of organization. They have a complete alimentary canal with a muscular pharynx for digestion. They don’t have a skeleton system, the pseudocoelomic fluid acts as a hydroskeleton. They lack a respiratory system and through the body’s surface exchange of gases takes place. Excretory products are urea and ammonia. The excretory system is made of canals. The reproduction mode is sexual and the fertilization is internal. Development can be direct or indirect through the larval stage. For e.g., Human pinworm, Heartworm, Threadworm, Human whipworm, etc.

Phylum Annelida

Usually known as segmented worms or ringed worms. These are extinct species because of adaptation to various ecologies. These are triploblastic and coelomates. Respiration is done through the body surface and for excretion they have nephridia as excretory organs. Digestion and circulatory systems are well-developed. A common process in annelids is regeneration. They have setae which help in movement. They have hemoglobin due to which they have a red color. Annelida are bilaterally symmetrical invertebrates. As if they don’t have any distinguishing features. But they have a combination of distinctive features. Cuticle covers the body which doesn’t have any cells but is secreted by the cells of the skin and is made tough. For e.g., Earthworm, Leech, Bobbit worm, and Polychaete.

Phylum Arthropoda

The largest phylum has a maximum number of species in it. Their name itself represents jointed appendages. They have an exoskeleton and segmented body. Their distinguishing feature is having jointed limbs and cuticle which is made of chitin. They are bilaterally symmetrical. Arthropods undergo moulting (shedding their old exoskeleton to have a new one) to keep growing. They have approx. 10 million species. They have an open circulatory system which includes an internal cavity known as hemocoel and hemolymph (alternate of blood that circulates in its body). The nervous system is ladder-like and has paired ventral nerve code in all segments forming ganglia in every segment. The digestive and excretory system depends on the environment they are living in. For vision, they have compound eyes and pigment-pit ocelli. They have different types of chemical and mechanical sensors in them which is due to the modifications of the setae. Reproduction and developmental methods also vary in this phylum. Like, in terrestrial species usually the fertilization is internal but the process can be indirect (transferring sperm through appendages on the ground rather than direct injection). Aquatic species can have either form of fertilization i.e., internal or external. For e.g., Spider, Scorpion, crustaceans, Arachnid, etc.

Phylum Mollusca

Second largest phylum in the animal kingdom. It has approx. 85000 species in this phylum. It has 23% species that are named marine organisms so it is the largest marine phylum. Three distinguishing features of this phylum are the presence of a significant cavity mantle used for breathing and excretion along with the presence of radula and the last is the structure of the nervous system. Mollusces are coelomate and the main cavity is hemocoel by which blood circulates as it has an open circulatory system. Reproduction is simplest when the fertilization is external. 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. Some of the species are really dangerous and even lethal like the bite of blue-ringed octopus can be fatal and stings from a few species can even cause death as they secrete venom. For e.g., Octopus, Scallops, Bivalvia, Gastropods, etc.

Phylum Echinodermata

This phylum includes species like starfish, brittle stars, sea cucumber, sand dollars, etc. They are radially symmetrical. Echinoderms are usually found at the bottom of the sea. These are ecologically and geologically important species. 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. For respiration, they have a vascular system. Echinoderms are triploblastic and coelomates. The open circulatory system is present in this phylum. The nervous system is simply radial and it lacks an excretory system. Unsegmented body in which dorsal side has anus and mouth is present at the ventral side. Sensory organs are poorly developed like chemoreceptors, tactile organs, etc. For e.g., Starfish, Sea cucumber, Sea urchins, Brittle stars, Cystoidea, etc.

Phylum Hemichordata

This phylum is usually known as the sister group of phyla Echinodermata as they also include marine and deuterostome species. Smallest phylum with only 100 species. Some organisms are solitary while some are colonial while all have a worm-like appearance. 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. So, they were separated and given a new phylum. They have some similar features to chordates but they are not properly chordates. They are bilaterally symmetrical, triploblastic, and have a true coelom. The body is divided into three parts- proboscis, collar, and trunk. They have an open circulatory system. The digestive system is complete along with the anus. Gills are there for respiration. External fertilization is common in hemichordates. Reproduction is mostly sexual and development is indirect but some exceptions are there which have direct development. For e.g., Balanoglossus, Graptolite, Graptoloidea, etc. 

Phylum Chordata

Chordates are known for their distinguishing feature i.e., the presence of the notochord. They possess metameric segmentation and are coelomate. They are bilaterally symmetrical organisms. The other characteristics feature besides notochord which are present in chordates are pharyngeal slits, dorsal nerve cord, and a post-anal tail. They have an organ-system level of organization. Species vary in size and length from inches to meters. 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 seen in tunicates whereas sexual seen in vertebrates. Muscular movements are there for locomotion. For e.g., Vertebrates, Mammals, Tunicates, Amphibians, etc.

Conceptual Questions

Question 1: How do sponges filter water?

Answer 1: 

Sponges absorb debris, uneaten food, decaying plants, etc. from water which makes water clean.

Question 2: What is the common name of coelenterates and why are they called so? 

Answer 2:

Their common name is sea stick and they are called so because they are always attached to the bottom of the sea.

Question 3: What is the function of flame cells?

Answer 3: 

Flames cells are used for excretion in the phylum Platyhelminthes.

Question 4: Why Ascaris is known to have pseudocoloem? 

Answer 4:

 The germ layer arrangement in Ascaris is like that they have patches of mesoderm in between endoderm and ectoderm. That’s why they are known as pseudocoelomates.

Question 5: What are the differences seen in the circulatory system of fish and that of crustaceans.  


Single circulatory system is seen in fishes where the gills get the blood for re-oxygenation and then the oxygenated blood circulates in the body whereas crustaceans have an open circulatory system where blood is drawn to the heart through one hole called Ostia and the circulates through tissues.

Question 6: What is the role of radula?


The main role of radula is feeding. Sometimes it is compared to the tongue. It is used for cutting the food before entering the esophagus.

Question 7: What do you understand by hermaphrodites and in which phylum such species are seen?


 Hermaphrodites mean the presence of both reproductive organs (male and female) in one organism. Such species are seen in chordates. For e.g., Flukes, Snails, Slugs, etc.

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