Animal Kingdom: Basis Classification

Kingdom Animalia or the Animal kingdom is the largest of the five kingdoms. Animals are multicellular eukaryotes, unlike plants they lack a cell wall and chlorophyll. All Animals follow a heterotrophic mode of nutrition. In addition to these commonalities, they also relate to their coelom, level of organisation, body symmetry, presence or absence of notochord, and cell organisation. The animal kingdom has been divided into 11 separate phyla based on these characteristics. Following is a description of the various animal classification criteria.

Basis Of Classification

Levels Of Organisation

Every single animal in the animal kingdom is multicellular. However, the degree of cell organisation varies among different animals. Animals can range from basic cell aggregations to complex cell arrangements that give rise to tissues and organs.

• Cellular Level: Animals that have their cells organised loosely belong in this category. Each animal cell in this scenario is in charge of every aspect of the animal's life, including breathing, nutrition, excretion, etc. Eg: Proferans like sponges.

• Tissue Level: Group of cells that perform similar functions is called a tissue. In the case of a tissue-level organisation, a certain metabolic activity is carried out by a specific cell group also known as a tissue. There may be a tissue in charge of digestion, another in charge of respiration, etc. Eg: Coelenterates and Ctenophores.

• Organ Level: Group of tissues that perform a similar function are called organs. Some specialised organs are present for specific purposes when it comes to organ-level organisation. One organ may be in charge of respiration, another of digesting, and a third of excretion. Eg: Platyhelminthes and Aschelminthes.

• Organ System Level: Many animals exhibit this kind of arrangement more frequently. For instance, Arthropods, Echinoderms, Chordates, etc. illustrate that organs form a group to conduct a functional system, with each system (group of organs) designated to execute a certain physiological function.

Symmetry

If a figure can be split into two equal halves, it is said to be symmetrical. Animals can have radial symmetry, bilateral symmetry, or asymmetry.

• Asymmetry: Some of the animals have nearly asymmetrical shapes. No plane can divide its body into two equal parts. Eg: Sponges.

• Bilateral Symmetry: Bilateral symmetry means that just one plane may be used to split the body into two identical halves. Annelids, arthropods, molluscs, echinoderms, hemichordates, and chordates all exhibit bilateral symmetry. It's crucial to keep in mind that echinoderms can exhibit bilateral or radial symmetry depending on their stage of development.

• Radial Symmetry: In the case of radial symmetry, the body is split into two identical halves by any plane that passes through the central axis. In this instance, whether viewed from the top or bottom, the animal's body appears circular. In coelenterates, ctenophores and echinoderms radial symmetry is seen.

Germinal Layers

The layers of cells in an embryo are called germinal layers. These layers are known as the germinal layers because they are where many organs originate or germinate. Organs can develop from endoderm, mesoderm, or ectoderm, depending on the kind.

• Diploblastic: A diploblastic animal is one whose cells are organised by two embryonic layers. Ectoderm and endoderm are the two layers. Between the ectoderm and the mesoderm lies a layer that is not differentiated. Mesoglea is the name of this layer.

Eg: Coelenterates and Ctenophores.

• Triploblastic: A triploblastic animal is one whose cells are organised into three embryonic layers. Ectoderm, mesoderm, and endoderm are the three layers.

Eg: Platyhelminthes, Annelids, Arthropods, Chordates, etc.

Coelom

Different animals may or may not have a coelom or body cavity. Mesoderm lines the coelom. Animals are divided into three groups based on this feature.

• Coelomates: Animals having a body cavity/coelom are called coelomates.

Eg: Molluscs, Arthropods, Chordates, etc.

• Pseudocoelomates: These animals have a false cavity, also known as a pseudo coelom, which means they lack a true cavity. The cavity is not lined by mesoderm, but they do have patches of mesoderm in them.

Eg: Aschelminthes.

• Acoelomates: These animals lack coelom/body cavity.

Eg: Platyhelminthes.

Segmentation

In certain species, the body is segmented both internally and externally, with at least some organs repeated serially. For instance, the body of an earthworm has a pattern known as metameric segmentation.

Eg: Annelids

Notochord

On the dorsal side of the animal is a rod-shaped structure called a notochord. The notochord originates from the embryo's mesoderm and is mesodermally generated. In most animals, the notochord forms during embryonic development and vanishes later. Even in later phases in some species, it could still be rudimentary. Animals are classified as chordates if a notochord is present. Non-chordate refers to an animal lacking a notochord, such as echinoderms and Porifera. The notochord is a structure seen in hemichordates and chordates.

Different Phyla of Animal Kingdom

Kingdom Animalia is the name for the classification of animals. It is further divided into many phyla. Phyla are basically classifications that group together animals that share similar traits. In addition to the basic characteristics of the Animal Kingdom (eukaryotic, multicellular, without a cell wall, and heterotrophic), each phylum contains species with common characteristics. They progress from the most basic to the most complex in the chart classifying animals. The animal kingdom is classified into 11 different phyla namely: Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata.

1. Porifera

Poriferans are the most basic multicellular animals, and they are mostly found in aquatic environments. The bodies of these animals are covered with minute pores called Ostia. They have a canal system that aids in the circulation of oxygen, food particles, and water. There is little tissue differentiation or division seen in the body design. Their body is supported by spicules. Often referred to as "sponges," notable examples are Spongilla and Sycon.

2. Coelenterata

The phylum Coelenterata is also known as Cnidaria. These animals have more body differentiation. They are aquatic animals. A single ingestion and egestion opening is located in the body's sac-like cavity. These animals are known as diploblastic because they contain two germ layers and show metagenesis. These animals can be seen residing either alone or in colonies. Jellyfish, sea anemones, and hydra are a few examples.

3. Ctenophora

Ctenophores are marine animals which are radially symmetric and diploblastic. These are also known as comb jellies or sea walnuts. These animals show bioluminescence. They have 8 rows of comb plates which are ciliated and used for locomotion. Eg: Pleurobrachia and Ctenoplana.

4. Platyhelminthes

These are frequently referred to as flatworms. Dorsoventrally, their bodies are flattened. They are the earliest triploblastic animals, having three germ layers. These animals are acoelomates. The left and right portions of the body have identical designs, making the body bilaterally symmetrical. Flames cells are present in the animals of this phylum which helps in excretion and osmoregulation. Flatworms come in two different forms: parasitic and free-living. Planaria, liver fluke, and tapeworm are a few examples.

5. Aschelminthes

These animals show exhibit triploblastic and bilateral symmetry. But rather than being flattened, the body is more cylindrical. True coeloms do not exist in the body cavity. The term "pseudo coelom" was coined for this reason. Organs are not present, only tissues are present. These animals exhibit a complete, fully developed alimentary canal. The majority of the creatures in this phylum are parasitic worms that spread illness. Ascaris and Wucheria are two examples.

6. Annelida

Annelids may be found in a variety of environments, including on land, in freshwater, and even in the ocean. They contain three germ layers (Triploblastic) and a body that is bilaterally symmetrical. They have a true body cavity, hence these are called coelomates which makes them distinct. The body is divided into segments, and organ differentiation is obvious. Earthworms and leeches are two examples.

7. Arthropoda

Arthropoda is the largest phylum of the animal kingdom. The majority of insects belong to this phylum. Jointed legs are referred to as "Arthropoda." These creatures' bodies are comprised of the head, thorax, and abdomen. They have two compound eyes in addition to their joint legs. These animals have an open circulatory system, which is another distinguishing trait. They have Malpighian tubules which help in excretion, and chitinous exoskeleton and statocysts are used for balancing. Examples include crabs, spiders, mosquitoes, and houseflies.

8. Mollusca

The second-largest animal phylum is the Mollusca. They might be aquatic or terrestrial. Organ-system level organisation may be seen in them. They are coelomate, triploblastic, bilaterally symmetrical animals. They have kidney-like excretory organs and an open circulatory system. The radula, a rasping organ for eating that resembles a file, is located in the mouth. Unsegmented body with a distinct head, muscular foot, and visceral hump, protected by a calcareous shell in present in them. The visceral hump is covered by a covering of soft, spongy skin. Octopus, snails, and mussels are a few examples.

9. Echinodermata

These animals are known as Echinodermata (spiny skinned organisms) because of their calcium carbonate endoskeleton, or calcareous ossicles. They are just free-living marine animals that are organised at the organ-system level. These animals are triploblastic with a coelomic cavity. Echinoderm larvae are bilaterally symmetrical but adult echinoderms are radially symmetrical. Water-driven tube systems, also known as water vascular systems, are utilised for breathing, food capture, and locomotion. The development of a free-swimming larva is indirect. Examples include brittle stars, sea urchins, sea lilies, starfish, and sea urchins.

10. Hemichordata

Hemichordata was once thought to be a subphylum of the phylum Chordata. However, it is presently classified under non-Chordata as a distinct phylum. A small collection of marine animals that resemble worms and are organised at the organ-system level makes up this phylum. They are triploblastic, cylindrical coelomates that are bilaterally symmetrical. The circulatory system of the body is an open system. Gills are used for breathing. The sexes are distinct. External fertilisation occurs. Indirect development occurs. Balanoglossus and Saccoglossus are two examples.

11. Chordata

A notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits are the three main characteristics of animals that belong to the phylum Chordata. They are coelomate with the organ-system level of organisation, bilaterally symmetrical, and triploblastic. They have a closed circulatory system and a post-anal tail. The three subphyla of the phylum Chordata are Vertebrata, Cephalochordata, and Urochordata.

Protochordate is the term used to describe the only marine subphyla Urochordata and Cephalochordata. While the notochord is only present in the larval tail of the Urochordata, it is permanent throughout the life of the Cephalochordata and extends from the head to the tail region. Examples include Amphioxus or the Lancelet in the Cephalochordata and Ascidia, Salpa, and Doliolum in the Urochordata. Vertebrates are further divided into 5 different classes namely: Pisces, amphibians, reptiles, aves and mammals.