Animal Kingdom Facts in Brief and Interesting Points

If we look around, we can see a wide variety of living organisms.   In earlier times it was quite cumbersome to remember all the characteristics of each organism separately.

Thus all the organisms were classified into groups and subgroups depending on common anatomical and morphological characteristics, phylogenetic relationships, reproductive systems, etc.

animal kingdom facts

All living organisms are divided into five groups called kingdoms – Monera, Protestants,  Fungi, Plantae, and Animalia.

Animal Kingdom Facts

The kingdom Animalia consists of multi-cellular organisms with no cell wall. This broad classification is further divided into groups and subgroups.

The next level of grouping after the kingdom is phylum.

Phylum Porifera

They have a cellular level of organization.

The sexes are not separate.

Fertilization is internal, indirect development (morphologically distinct larvae) occurs in poriferans.

Water enters through minute pores called Ostia into the body into the spongocoel (central cavity) and goes out through the osculum. This internal water transport is responsible for the ingestion of food, excretion, and respiratory exchange.

The body has a skeleton made of spicules or spongin fibers.

Phylum Coelenterata

These organisms are radially symmetrical, diploblastic, and have tissue level of organization.

Cnidoblasts (stinging cells)  present on the tentacles and body, used for defense, anchorage, etc.

They have a gastric cavity with a mouth or hypostome.

They can have two body forms polyp (sessile, cylindrical)  and medusa (motile, umbrella-shaped).

In some cnidarians, both forms occur in alternate generations. Polyps produce medusae asexually. While the medusae produce the polyps sexually, this is called metagenesis.

Phylum Ctenophora (sea walnuts, comb jellies)

Marine, radially symmetrical, diploblastic

The body has 8 external rows of ciliated comb plates which help in locomotion.

Male and female forms are not separate.

Only sexual reproduction is possible.

In these organisms, external fertilization and indirect development occur.

Phylum Platyhelminthes

These organisms are bilaterally symmetrical, triploblastic, and have an organ level of organization.

They are acoelomate (the body is dorsoventrally flattened, flatworms)

They possess flame cells that help in osmoregulation and excretion

Internal fertilization  occurs and has larval stages

Some forms are parasitic and they have hooks and suckers

Phylum Aschelminthes

They are called roundworms because they are circular in cross-section

They have an organ-system level of organization. They are triploblastic, pseudocoelomate, and bilaterally symmetrical.

The alimentary canal is developed and the body waste is removed through the excretory tube and pore. 

Sexes are separate (dioecious) in this phylum.

Internal fertilization occurs and the development may be direct or indirect.

Phylum Annelida (Latin, annulus: little ring)

They are coelomate, have an organ-system level of body organization, and are bilaterally symmetrical.

The body is divided into distinct segments or metameres.

They have longitudinal and circular muscles that help in movement.

They possess a closed circulatory system

They have nephridia which help in osmoregulation and excretion. 

They also have paired ganglia connected by nerves and a ventral nerve cord help which in neural functions.

Reproduction is sexual and could be either monoecious or dioecious

Phylum Arthropoda

This is the largest phylum.

They have an organ-system level of organization and are bilaterally symmetrical, triploblastic, and coelomate.

The body is divided into head, thorax, and abdomen and covered by a chitinous layer.

They have jointed appendages(legs).

They possess an open circulatory system and

respiratory organs like gills, book lungs, etc..

Excretion occurs through malpighian tubules.

Sensory organs like antennae, eyes balancing organs are present.

Arthropods are mostly oviparous and lay eggs.

Phylum Mollusca

This is the second-largest animal phylum.

They have an organ-system level of organization, are bilaterally symmetrical, triploblastic, and coelomate.

Mollusks may be terrestrial or aquatic.

A hard calcareous shell covers the body.

The body is divided into a prominent head,

muscular foot and visceral hump.

A mantle is present over the visceral hump. It is a soft and spongy layer of skin and protects the body from friction from the hard shell.

The gap between the hump and the mantle is called the mantle cavity where feathery gills are present which help in respiration.

Sensory tentacles are present on the head.

The mouth contains a file-like rasping organ which is meant for feeding, called a radula.

Sexes are not separate.

They are oviparous and undergo indirect development.

Phylum Echinodermata (spiny bodied)

They have a calcareous endoskeleton.

They have an organ-system level of organization, are coelomate and triploblastic.

Adult echinoderms are radially symmetrical while larval are bilaterally symmetrical.

The mouth is present on the ventral side and anus on the dorsal side(complete digestive system present ).

They have a hydraulic system (water vascular system) which helps in locomotion, respiration, etc.

No excretory system is present.

The sexes are separate.

Reproduction is sexual and indirect development occurs.

Phylum Hemichordata

Hemichordates have a primitive notochord like structure called stomochord.

They are bilaterally symmetrical, triploblastic, and coelomate.

This phylum includes a small group of worm-like marine animals. They have an organ-system level of organization. They have a cylindrical body.

The body is divided into an anterior proboscis, a collar, and a long trunk.

They have an open circulatory system.

Gills are present for respiratory functions.

Their excretory organ is the proboscis gland.

Sexes are separate (monoecious).

External fertilization and indirect development occurs

This group was previously classified under phylum Chordata but later it was given its own separate group.

Phylum Chordata

They are bilaterally symmetrical, triploblastic, coelomate, and have an organ-system level of organization.

They have a closed circulatory system.

The main features of this phylum are the presence of the notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits, dorsal heart(if present), and a post-anal tail.

Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata, and Vertebrata.

Urochordata and Cephalochordata are all marine organisms. They are called protochordate.

Of them, Urochordata has notochord only in the larval tail.

In Cephalochordates, the notochord is present from head to the tail region and is persistent throughout their life.

All vertebrates are chordates but all chordates are not vertebrates.

Organisms of the Vertebrata subphyla develop the notochord at the embryonic stage. In later stages, this notochord develops into a cartilaginous or bony vertebral column that persists throughout their life. Vertebrates have well developed muscular heart located ventrally with two, three, or four


For locomotion, all vertebrates have paired limbs or fins. All vertebrates are motile.

Vertebrates can be divided into Agnatha (lacks jaw)  and Gnathostomata (has a jaw).

Class Cyclostomata falls under Agnatha.

Class – Cyclostomata

They have 6-15 pairs of gill slits. Cyclostomes have a sucking and circular mouth and no jaws. They don’t have any scales or paired fins. They exist as ectoparasites. The closed circulatory system is present. Their skeleton is mostly cartilaginous.

Cyclostomes are marine but travel to freshwater to lay eggs. After spawning, they die. Their larvae, after development return to the ocean.

The following classes fall under Gnathostomata.

Class Chondrichthyes

They are marine animals with a streamlined body and a cartilaginous skeleton.

The gill slits are separate and the operculum is absent.

They have powerful jaws, backwardly directed teeth, placoid scales, two-chambered heart, and are cold-blooded.

As they don’t have an air bladder, they have to swim so constantly so that they don’t sink.

Sexes are separate and internal fertilization occurs.

Class – Osteichthyes

They have a bony endoskeleton, streamlined body, cycloid scales, two-chambered heart, air bladder, and 4 pairs of gills covered by an operculum.

They are cold-blooded animals  with a two-chambered heart,

Sexes are separate.

Most of them oviparous and development is direct.

Class – Amphibia

They are named so they can survive both in water and on land.

They have moist skin, tympanum in the ear,  three-chambered heart.

They are cold-blooded and the body is divided into head and trunk.

The digestive canal, urinary and reproductive tracts open into a common chamber called the cloaca.

Respiration is by gills, lungs, or through the skin. Sexes are separate and they are oviparous.

External fertilization and indirect development occur.

Class-Reptilia (creeping/crawling motion)

They have dry and cornified skin with scales.

They are cold-blooded.

The tympanum is present as the ear.

The heart has only three chambers, but only in crocodile, it is four-chambered.

Sexes are separate and they are oviparous.

Internal fertilization and direct development occur.

Class- Aves

They have special features like feathers, beak, oil gland at the base of tail,  pneumatic but ossified bones.

The forelimbs are modified into wings while hindlimbs are modified for walking, swimming or clasping.

Air sacs are connected to the lungs.

They are warm-blooded.

Most birds can fly (exceptions include ostrich, kiwi, etc).

They have a four-chambered heart.

The digestive tract of birds has additional parts- crop and gizzard.

Sexes are separate and they are oviparous.

Internal fertilization and direct development occur.


They have special features like the presence of mammary glands, body hair, external ears, or pinna.

They have two pairs of limbs, a four-chambered heart.

Respiration occurs through the lungs.

They are warm-blooded.

Sexes are separate and they are viviparous.

Internal fertilization occurs.

Humans belong to this class.

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