Animal Kingdom

Animal kingdom, also known as the kingdom Animalia, is a diverse group of organisms that includes all animals on Earth. Animal Kingdom is one of the five kingdoms in the biological classification system, along with the Monera Kingdom, Protista Kingdom Fungi Kingdom, and Plant Kingdom.

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. Animals range in size from tiny microscopic creatures to massive whales and elephants. 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. 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. 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 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 do not have an organized circulatory system. Circulation happens through 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. 
• The main mode of reproduction in this phylum is asexual reproduction i.e., through budding but some groups also show sexual reproduction. 
• Examples are Moon jelly, Lion’s mane jellyfish, Barrel jellyfish, and Portuguese.

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. 
• They are diploblastic. In some books, 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 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 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. 
• 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.

• 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.
• They have an organ-system level of organization. 
• They have a complete alimentary canal with a muscular pharynx for digestion.
• 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. 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. 
• Examples are Earthworms, leeches, Bobbit worms, and Polychaete.

Phylum Arthropoda

The largest phylum has a maximum number of species in phylum Arthropoda. Their name itself represents jointed appendages.

• They have an exoskeleton and a segmented body. 
• Their distinguishing feature is having jointed limbs and cuticles which are made of chitin. 
• They are bilaterally symmetrical. 
• 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. 

• 23% of species 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. \
• Mollusks are coelomate and the main cavity is hemocoel by which blood circulates as it has an open circulatory system.
• Reproduction is simplest when 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. 
• Examples are Octopus, Scallops, Bivalvia, Gastropods, etc.

Phylum Echinodermata

Phylum Echinodermata 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. 
• 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. 
• 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. 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. 
• They have some similar features to chordates but they are not properly chordates. They are bilaterally symmetrical, triploblastic, and have a true coelom. 
• Reproduction is mostly sexual and development is indirect but some exceptions are there which have direct development. 
• Examples are 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 notochords that 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 is seen in tunicates whereas sexual is seen in vertebrates. Muscular movements are there for locomotion. 
• Examples are Vertebrates, Mammals, Tunicates, Amphibians, etc.

FAQs on Animal Kingdom

Q1:Which one is the biggest class of the animal kingdom?

Answer:

The Insecta is the biggest class of the animal kingdom. Insecta class comes under Invertebrates.

Q2: How many species are included in the animal kingdom? 

Answer:

Almost 1.7 million living organisms are present on earth of which 1.2 million are classified under the animal kingdom.

Q3: What is the function of flame cells?

Answer: 

Flames cells are used for excretion in the phylum Platyhelminthes.

Q4: Which is the Second largest phylum of the animal kingdom?

Answer: 

Mollusca is the second largest phylum. Mollusca is the largest phylum in marine ecosystems almost 23% of marine organisms.