Overview of Kingdom Animalia

The Kingdom Animalia is a large group of multicellular eukaryotic organisms that are heterotrophic in their nature. They get their food from outside sources. While they cannot create their own food and this is among the most characteristic traits of plant cells, animal cells do not have a cell wall like those in plants.

Apart from some animals most animals are mobile that allows them to react to stimuli and search for food, for example. The majority of animals can be classified into two major groups, namely vertebrates (animals with backbones) as well as invertebrates (animals without backbones). However, they can also be divided into various phylas that will be discussed in more detail.

Some examples of animals are:

• Human beings 
• Cows
• Fish
• Birds
• Sponges – Invertebrate
• Lobster – Invertebrate
• Spiders – Invertebrate
• Clams – Invertebrate

Classification of Kingdom Animalia 

As stated, all animals fall into two major categories or groups that are vertebrates and invertebrates.

1. Vertebrates

Vertebrates comprise all animals classified in the subphylum Vertebrata. They are part of the class Chordata and have a backbone/vertebrae (where the spinal cord in). They also have the skeletal structure inside which muscles are placed.

Classification of Vertebrates

Vertebrate animals are classified in seven classes which comprise:

a. Mammalia

A majority of the species identified by epidermal hair, and females have babies and suckle the infant. However, the spiny and duck-billed anteaters lay eggs.

Aves/Birds – Aves/Birds are animals that are distinguished by their wings, feathers and a beak. Reproduction is the process of laying eggs of different sizes dependent on the species of bird. Some examples of birds are Emus, kiwi woodpecker, and emu among others. Certain birds have wings, but can’t fly. However certain animals, such as bats, have wings , but are considered mammals. So it is not true that all creatures that fly are considered to be birds.

b. Reptilia

Reptiles (cold-blooded) are four-legged creatures and are distinguished in the form of tails and dermal scales. In the majority of species eggs can be fertilized internal to the animal which results in the direct growth of the animal. Reptile examples include snakes, turtles, and crocodiles to name a few.

c. Amphibia

Amphibians are cold-blooded animals who spend a significant portion all their time in the water. They can also absorb oxygen through their skins in humid environments certain species (or at some point of their lives) have gills that allow them to breathe the water. They also breathe through their lungs on the land. They lay eggs that resemble jelly in moist areas or in water. They include salamanders, frogs, etc.

d. Agnatha

This category includes jaw-less creatures that resemble fish. Certain species such as Lampreys are distinguished by the size of their heads as well as a notochord and sensory system.

e. Osteichthyes

Osteichthyes are fish that are real and are commonly described as bony fish. They are distinguished by the bony tissue (rather rather than cartilage). Other characteristics that belong to this group are tooth-fins that are fused lobed fins, a skull, and skulls. The members of this group include clownfish, ray-finned fish and other species.

f. Chondrichthyes

Chondrichthyes is a group of organisms that have a cartilaginous skeleton , and therefore are referred to as fish that have cartilaginous skeletons. Although some species have been found in freshwater habitats however, the majority of species can be found in the ocean.

As vertebrate mammals are one phylum, they comprise only five percent of all animals known.

2. Invertebrates

Vertebrates make up an individual phylum invertebrates comprise all the other phylas of the kingdom of Animalia. The name suggests that invertebrates do not have a backbone or internal skeletons. Certain species have the external structure of the skeletal system called the exoskeleton that gives structural support. Invertebrates are currently thought to comprise more than 97 % of all species within the Animal kingdom.

Being such a diverse group, invertebrates are classified based on a number of characteristics including morphology/structure, symmetry and life cycle, etc.

While some organisms have three body layers, other organisms (primitive invertebrates) have only two layers of body. Invertebrate species include insects (like ticks and spiders) as well as snails, sea stars sponges, and hydra and so on.

Classification of Kingdom Animalia based on Levels of Organization

Although all animals in the kingdom of Animalia are multicellular, they have different types of cell structure. This is why they are classified on the cell organization.

Classification

They are divided into three major levels that include:

a. Cellular level 

• Animal cells at this stage of organization create loose aggregates. The cell types of animals (e.g. sponges) are designed in a way that permits them to fulfill different functions the cells that have similar functions are not grouped together to create tissue.
• This is why the bodies of these living things are loose aggregations composed of various kinds of cells. Although the cells of these organisms aren’t necessarily organised into tissues however, it is important to note that some cells (e.g. pinacocytes) create layers and have certain purposes.

b. Tissue level of organization

• Animals with this degree of structure have cells that perform similar tasks and are put together to create tissues. For the majority of these species the body is composed of two major layers that separate by a middle layer referred to as mesogloea (the third layer, which is not cellular).
• The the diploblastic (consist of two layer of cells) The organism is comprised from an outer layer referred to as the ectoderm which makes the epidermis. Then there is the inner layer which lines the interior of the body. This is known by the name of endoderm (making the gastrodermis).
• The body is also composed of an internal body cavity.

c. Organ system level of organization

• For organisms that have this kind of organization, tissues are put together to make organs.
• Although all cells within the organ do not perform the same function but they all have a similar function and serve the function that the organ serves.
• A few of the organisms that have this degree of organization are simpler organisms, such as Platyhelminthes as well as other organisms belonging to lower phyla (human beings and other living beings).
• Organs are linked in the organ system and perform their functions together.
• An excellent example could be an organ made up of the bladder, kidneys and the ureter that form the urinary system that is functional. Although each organ performs a specific role, they are all involved in the elimination of waste out of the human body.

Classification of Kingdom Animalia based on Symmetry

• Alongside the degree of organization animals are classified also based on bodies proportions (arrangement of the various parts of the body in relation to the centre point).
• Some of the organisms are asymmetrical and therefore cannot divide into two identical or comparable portions in the central plane. some are symmetrical and the two halves are similar or equal in the event that the organism is split in two parts across the plane of centrality.
• The majority of organisms at the level of organization at the cellular level are unsymmetrical, while those at the organ or tissue level are either bilaterally or radially and symmetrical.
• For animals with bilateral symmetry (e.g. butterflies, human beings and dogs, for example) by dividing the species into two halves starting from the head to the tail in the central portion results in equal parts. Each half will mirror the image of the opposite.
• Many animals exhibit the radial symmetry, and thus, have multiple patterns of symmetry. Because they have multiple parts that are arranged around the center point of the animal, they may be divided along different lines and have adjacent splits that result in halves that mirror the opposite half.
• A few of the organisms that have the radial symmetry include corals sea anemones and jelly fish, to name a few.
• In contrast to animals distinguished by bilateral symmetry many of species (with circular and radial symmetry) don’t have distinct bodies with distinctive sides (head areas) or ends. Instead, their bodies are characterized by the top and bottom sides with various organs of importance placed on the sides.
• The mouth of a starfish is situated at the center of the bottom or lower surface. They are usually located in water environments where they consume the food particles that are pumped out by the water’s currents.
• Although a large portion of animals, especially those that have an organ or tissue level in organization distinguished by a bilateral or radial pattern of symmetry, certain animals, particularly those with a cell degree of structure (e.g. sponges) tend to be asymmetrical, and thus can’t be split into equal portions. A majority of them are simple and have a basic body plan since the cells aren’t divided into organs or tissue.
• Diagrammatic representation of a spongy sponge If you take a look at this picture it’s clear that it isn’t able to split into equal parts. Therefore, it’s unsymmetrical.

Classification of Kingdom Animalia based on Body Cavity/Coelom

Animals are classified also according to the body cavities (coelom). The coelom is formed in the embryo’s development. Contrary to it, the gut is made up from the endoderm and the coelom will be covered by mesoderm.

Because of this, many animals that have coeloms possess mesoderm. It’s situated between the body’s wall of an organism as well as the alimentary canal. The most prominent instances of these cavities inside the body are the peritoneal space and the lung space.

Functions of the cavity

The main purposes of this cavity are:

• Insuring organs are protected from mechanical shock
• Defines the form of some organisms and contributes to movement
• involved in the transportation of gases, nutrients and waste materials
• It provides a space to allow for the development and functioning of the organs involved.

Classification

Based on the cavity of their bodies The body cavity is the basis for classification of animals into these groups:

a. Acoelomata

• The name implies acoelomates are species which do not have a body cavity.
• The majority of them are simple, including species belonging to the family Platyhelminthes.
• Since they do not have a body cavity, mesodermal tissue plays a crucial function in holding certain tissues or organs in position. Acoelomates encompass a wide range of animals, including animals that have bilateral and radial Asymmetry, as well as asymmetric animals.

b. Pseudocoelomates (animals that have a false coelom)

• The organisms that belong to this group possess bodies with cavities however, it is not made up of mesoderm. Because of this, the cavity is commonly described as the false coelom. The most frequent pseudocoelomates include nematodes and rotifers.
• This body cavity animals is called a pseudocoel (and therefore the name pseudocoelomates) and includes animals that have bilateral similarity.
• Alongside a few mesenchyme tissues, this cavity could also be made up of gelatinous material or fluid.
• Some of the primary roles of pseudocoel is related to digestion, reproduction, and the distribution of nutrients throughout the body.
• Other organisms that are classified as Pseudocoelomates comprise members of the group Loricifera Gastrotricha, Loricifera, and Entoprocta.

c. Coelomates

• Coelomates comprise animals that have the true coelom. Most of these species are distinguished by a specific organ or tissue system of organization. They also have bilateral symmetry.
• In contrast to Pseudocoelomates and coelomates, coelomates possess an elom that is completely or totally derived from the mesoderm.
• As a space located between the body’s wall as well as the stomach of all vertebrates, it’s surrounded by two types of cells (cells which are under the body wall as well as those which are around that of the gut).
• Coelomate invertebrates however are triploblastic, and are distinguished by three cellular body layers which comprise mesoderm and ectoderm and the endoderm.
• Although it’s situated between the body’s wall and the stomach, it’s important to note that the cavity isn’t directly in contact with the gut or body wall. Instead, it is separate from both via the peritoneum (peritoneal epithelium).

This is a diagram model of Mollusks (phylum Mollusca) showing location of the coelom.

When the coelom expands in size, the amount of contact between the body’s wall and the peritoneum is increased.

Different Phylum under the Kingdom Animalia

A Phylum can be described as a taxonomic level that is beneath Kingdom but above the class. It’s a significant category that classifies organisms according to a number of features that set the phylum apart from all other animals.

Within the Kingdom Animalia the animals are separated into 11 (11) Phyla that include:

1. Kingdom Animalia: Phylum Chordata

• The term Chordata is derived in the Greek terms “Chorde” which means string or cord and “ata” meaning bearing. So, the term “chordates” (members from Chordata) (members of the Phylum Chordata) are animals with an unnotochord or cord at some time in their lives.
• Although the majority of animals belonging to the Phylum Chordata are vertebrates (the majority of animals higher) however, the phylum comprises protochordates (e.g. amphioxus and squirts) that closely resemble vertebrates. Although they do not have the backbone, which is common in all vertebrates, these animals are equipped with a dorsal neural cord as well as a notochord, and therefore are classified as part of the Phylum Chordata.
• There are currently about 65,000 chordate species. Although there aren’t many in comparison to others, the group does display huge diversity and can be found in a wide range of habitats, including terrestrial and aquatic environments throughout the globe.

Examples of Phylum Chordata

Examples of Phylum chordates include:

• Birds
• Fishes
• Mammals
• Reptiles

Main Characteristics of Chordates

The majority of these species possess an anterior end which could be comprised of a head, or cephalic region, and a posterior one comprising the tail in the majority of animals.

• Bilateral symmetry
• The majority of animals are triploblastic and therefore have the 3 germinal layer (ectoderm endoderm, ectoderm, and mesoderm).
• Have a coelom
• It is characterized by an organ-system level of organization

2. Kingdom Animalia: Phylum Porifera

• The Phylum Porifera consists of some of the oldest known species of animals. They are typically found in water ecosystems (particularly within marine ecosystems) where they consume food by filtering water to capture and capture organic matter. Organisms such as Sponges serve as filters, and have been found to be able of capturing around 70 percent of suspended organic matter found in their surroundings.
• The majority of the members within this Phylum are basal animals that are characterized by a cellular organization. They do not have the appearance of tissue because cells aren’t organized into tissues.
• Since they do not have the actual tissues and organ systems that are organ systems, the those belonging to the Phylum are typically defined by an aquiferous structure which is comprised of canal networks and chambers that permit water to flow into and out.

Examples of Phylum Porifera

A few organisms belonging to the Phylum Porifera include:

• Demosponge
• Calcareous sponge
• Siliceous sponge
• Hexactinellid sponge

3. Kingdom Animalia: Phylum Platyhelminthes

• The Phylum of Platyhelminthes includes organisms that are called flatworms. Flatworms are dorsoventrally shaped and therefore do not have the coelom. The body organs’ space is thus covered with mesenchyme. While they do have a tissue or organ body level of structure (e.g. have the reproductive system) however, they do not have an circulatory system, a respiratory system, or an authentic anus.
• As with others, the species are triploblastic. This means that they comprise three embryonic cells (the endoderm, ectoderm as well as mesoderm). They also have bilaterally symmetrical and could, thus, be split into two equal parts from head to the tail.
• While certain species (e.g. Turbellaria) belong to groups of organisms. Turbellaria) are able to survive under extreme conditions (in rocks, sand, etc.) However, the majority flatworms are found in humid and aquatic environments.

Examples of Phylum Platyhelminthes

• Turbellaria (e.g. Gyratrix hermaphroditus, Pseudoceros dimidiatus, Girardia tigrina) 
• Trematoda (e.g. Clonorchis sinensis, Schistosoma japonicum, and Metagonimus yokogawai etc) 
• Cestoda (e.g. pork tapeworm, fish tapeworm, and Taenia taeniaeformis etc).

4. Kingdom Animalia: Phylum Cnidaria

• Also called Coelenterata Also known as Coelenterata, the Phylum Cnidaria is a collection of organisms that typically reside on marine habitats. However, some species, such as Hydra are located in freshwater environments.
• There are many species within the Phylum Cnidaria and Cnidaria, they all share a common body structure made up of two cell layers (endoderm as well as the ectoderm). These two layers can be separated through the mesoglea that is a gelatinous, non-cellular layer.
• While they’re simple organisms however, they do have real tissues, including epithelial tissue as well as muscular tissue and connective tissue, among others. But, they don’t have an organ-level of organization and therefore classified under the tissue class. Due to their general shape and morphology they exhibit radial symmetry and are therefore able to be classified in a variety of planes.

Examples of Phylum Cnidaria

A few species that are part of the Phylum Cnidaria comprise:

• Hydroids
• Corals
• Jelly-fishes
• Sea anemones
• Polyps

5. Kingdom Animalia: Phylum Annelida

• Aspects of Phylum Annelida also commonly referred to as annulids, are segmented worms that are found in a variety of habitats in terrestrial and aquatic habitats. While they display a significant variety regarding the body structure, research has shown most species coelomates and therefore have an internal body cavity.
• They also feature various body segments, with the primary segment (prostomium) comprising the brain of most of the species. The human body may be separated into three major areas, including the prostomium (the head portion of our body) as well as the trunk that is comprised of several segments, and the pygidium that is the post-segmental part. 
• They also have a variety of internal organs which make up the digestive system as well as the circulatory system , etc. Annelids also show bilateral symmetry.

Examples of Phylum Annelida

Some species belonging to the Phylum Annelida are:

• Earthworm
• Sludge worm
• Medicinal leech
• Lugworm
• Salinera

6. Kingdom Animalia: Phylum Mollusca

• The members of Phylum Mollusca (mollusks) are coelomate animals with bilateral symmetry. However, some species show asymmetrical characteristics later in their lives. They generally have a soft body which can have a protective calcareous shell , depending on the species. The ventral portion that is the inner part of the body for mollusks is composed of a muscle system that is used to move.
• The majority of these species are aquatic and may be located in freshwater and marine environments (a small number of species can be found on freshwater). However, a few species are also found in terrestrial habitats.

Characteristics of Phylum Mollusca

Other characteristics that are associated with mollusks are:

• Are triploblastic. They contain three primary layers
• Organ system level of organization present.
• The body cavity (the real coelom) is restricted to the pericardial cavity and the an os gonads’ lumen
• Have an open circulatory system
• Organs that are distinctive to mollusks include the rodula (a food arrangement) along with the mantle (a dorsal layer)

 Examples of Phylum Mollusca

Some species belonging to Phylum Mollusca are:

• Giant clam 
• Pacific oysters
• Hard clam 
• Garden snail 
• Freshwater pearl mussel

7. Kingdom Animalia: Phylum Arthropoda

• Aspects of Phylum Arthropoda are referred to as arthropods. While they are invertebrates arthropods have an exoskeleton , which offers structural support for the organism. Arthropods are typically characterized by an elongated body (consisting of three parts: a head, thorax and abdomen) as well as joined appendages.
• At present, it is estimated that there are more than One (1) million different species in the Phylum Arthropoda which is one of the biggest species in all of Animal kingdom. The majority of creatures on Earth are arthropods. Arthropods all have a coelom which is tiny in dimensions.
• They also display an organ system-level of organization that includes organs that form diverse organ systems, including digestive systems, the circulatory system, nervous system, among others. They also display bilateral symmetry, where the body is separated into 2 equal parts.

Examples of Phylum Arthropoda

A few of the organisms belonging to the Phylum Arthropoda are:

• Scorpions
• Crabs
• Spiders
• Millipedes
• Termites
• Fly
• Ants

8. Kingdom Animalia: Phylum Hemichordata

• A part from the Phylum Hemichordata (Hemichordates) are insects that look like worms and are encountered in marine environments. Their bodies are split into three main parts : the proboscis the collar (mesosome) as well as the trunk (posterior part of the body).
• They show bilateral symmetry and , consequently, the body can split into 2 equal parts. Since they share a number of features with chordates as well as individuals belonging to Phylum Echinodermata, Hemichordates are often thought to be the connection between vertebrate and non-vertebrate animals.
• As with other species Members of this group (especially adults) are coeloms. However, they do not have organ systems that are as developed similar to those of higher animals.

Examples of Phylum Hemichordata

Some of the animals that fall as belonging to the Phylum Hemichordata are:

• Planctosphaera pelagica
• Ptychodera flava
• Spartobranchus tenuis
• Glossobalanus minutu

9. Kingdom Animalia: Phylum Echinodermata

• The term Echinodermata comes of the Greek words “echinos” which is a reference to spiny, in the sense of spiny and “Dermos” which refers to skin. So, as the name implies the organisms are distinguished by a spiny , scaly skin.
• The members of this group (echinoderms) are typically found in marine environments in which they eat seaweed. They are also referred to as coelomates of invertebrates since they have a large coelom in between their guts and the body wall.
• They have an organ system-level of organization. They have a variety of crucial structures like mouths as well as a complete digestive system. While they reproduce sexually, they’re also capable of regenerating.

Examples of Phylum Echinodermata

The species that fall as part of this phylum include:

• Common starfish 
• Green sea urchin
• Echinus esculentus 
• Blue sea star 
• Arkarua
• Protoreaster nodosus

10. Kingdom Animalia: Phylum Ctenophora

• The members from the Phylum Ctenophora are generally known as Ctenophores. They are invertebrate species that are commonly found in marine waters around the world. Ctenophores are jelly-like species with a soft body that displays biradial symmetry.
• As Platyhelminthes and other members from the Phylum Cnidaria, Ctenophores are anacoelomates and do not have an actual body cavity. Ctenophores are also distinguished by rows of combs (eight in all) that are used to move.

Examples of Phylum Ctenophora

A few of the organisms belonging to this phylum are:

• Beroe ovata
• Pleurobrachia bachei
• Beroe cucumis
• Xanioascus
• Ctenorhabdotus capulus

11. Kingdom Animalia: Phylum Aschelminthes

• Aschelminthes are marine organisms which are bi-symmetrical and display organ-system levels of organization. They are triploblastic which means that they possess three layers of germinative cells (ectoderm as well as endoderm as well as mesoderm).
• Though they possess an elongated, worm-like body, research has shown that they are pseudocoelomate (which is derived out of the blastocoel) and don’t have a real body cavity. They do not have the respiratory or circulatory system that is common in other animals but do have digestive tracts.

Examples of Phylum Aschelminthes

The species that are part of this Phylum are:

• Horsehair worms
• Trichinella spiralis
• Loricifera
• Loa loa