Animal Kingdom Animalia Subphylum - An Overview

Organisms are formed of groups of complex cells in which organs carry out life processes in an orderly fashion. Robert Harding Whittaker, an American ecologist, prepared a classification of organisms. He divided the organisms into five kingdoms, namely Monera, Protista, Fungi, Plantae, and Animalia. Monera, as the name suggests, consists of unicellular bacteria and similar organisms. Protista is composed of eukaryotic unicellular organisms like protozoans. All fungi, yeasts, etc., are included under the kingdom Fungi. Kingdom Plantae included all plant species, from small shrubs to tall trees, while Animalia includes all the multicellular organisms other than plants (autotrophs). These animals lack a cell wall and are heterotrophic.

Kingdom Animalia And Its Classification

Kingdom Animalia consists of multicellular, heterotrophic, and eukaryotic organisms. They obtain food from autotrophs (plants) and other animals and nutrition by ingesting plant or animal material. These ingested materials are decomposed to produce energy for the metabolic activities of the organism. Depending on their structure and composition, the animals of this kingdom are divided into ten phyla. It consists of sponges, insects, worms, fishes, reptiles, birds, amphibians, and mammals. The phyla are:

• Porifera

• Cnidaria

• Platyhelminthes

• Nematoda

• Annelida

• Arthropoda

• Mollusca

• Echinodermata

• Hemichordata

• Chordata

Phylum Porifera

As the name suggests, these are organisms with pores. Most commonly referred to as Sponges, they have a porous body with a hard outer skeleton. They are non-motile and multicellular. Their body is undifferentiated into their head and tail. There are no developed organ systems, but the pores create a canal-like system to transport and circulate materials within the body. These are abundant in marine habitats like Sycon and Spongilla.

Phylum Cnidaria

They are also called Coelenterata after the Greek word 'kilos,' meaning hollow-bellied, because of their hollow body cavity. All aquatic animals come under cnidaria. The body of these animals can be differentiated and has outer and inner linings. They usually live in colonies, like the corals, or individually as in sea anemones. Hydra and jellyfish are the most common coelenterates.

Phylum Platyhelminthes

Commonly known as flatworms, these organisms have a flattened body structure oriented in the dorso-ventral direction (extending from back to belly). They have bilateral symmetry and have either free-living or parasitic natures. They do not have an internal cavity, but the tissues are differentiated into three layers to form a triploblastic structure. Tapeworms and Planaria are the best examples.

Phylum Nematoda

They have a cylindrical body with a pseudo (false) cavity. They possess bilateral symmetry and are triploblastic. They are parasitic and cause diseases like ascariasis (caused by Ascaris). Wuchereria is another example.

Phylum Annelida

These are the ringworms that have a segmented and cylindrical body structure. The body is differentiated into the head and tail and has a true body cavity (coelom). They also have a triploblastic structure and possess bilateral symmetry. Annelids are found in the land, freshwater, and marine habitats. Earthworms and leeches belong to this group.

Phylum Arthropoda

The word arthropod indicates the presence of jointed legs and is a characteristic feature of arthropods. Jointed appendages, a segmented body, and the presence of an exoskeleton are possessed by Arthropoda, which is the largest phylum of the kingdom Animalia. They have bilateral symmetry and a well-differentiated organ and organ system. Their circulatory system is open, but there are no blood vessels. Mosquitoes, spiders, and butterflies are the most common examples.

Phylum Mollusca

The body of these organisms are less segmented and have bilateral symmetry. They are triploblastic with well-developed organs and organ systems. There are limbs present, and the circulatory system is open. Snails and octopuses are examples of the phylum Mollusca.

Phylum Echinodermata

The name comes from the Greek words 'chinos,' meaning hedgehog and 'derma,' meaning skin. They possess radial symmetry and are triploblastic, with a true cavity (coelom). They are free-living marine animals having hard skeletal structures made of calcium carbonate. Sea urchins, starfish, etc., are some of the Echinodermata.

Phylum Hemichordata

They have a soft and fragile body with a single-layered epidermis. With an open circulatory system, these animals have an organised organ system. Since they are marine animals, they breathe through their gills. There are separate sexes, and fertilisation happens externally.

Phylum Chordata

These animals have bilateral symmetry and a closed circulatory system with well-defined blood vessels. They have a notochord (backbone) at some point or throughout their life. They are triploblastic, having an organ system classification. Chordates are divided into Urochordata, Cephalochordata, and Vertebrata, depending on the stage of life when notochord is present. Urochordates have notochords in the tails of the larvae only. Cephalochordates have notochords throughout their body. Vertebrates possess notochord during the embryo stage and later get replaced by a vertebral column.

General Characteristics Of The Kingdom Animalia

• Among the five kingdoms listed by R H Whittaker, Animalia is the largest, with more than 8.7 million species.

• These are multicellular, heterotrophic organisms, with sexual reproduction as their multiplication mode.

• They are composed of cells lacking the outer cell wall present in plants.

• They are sensitive to external stimuli.

• Most of these animals are diploid (two copies of each gene).

• In some animals, the early stage of life is called the larva stage and later undergoes morphological modifications to become an adult organism. This process of transformation is called metamorphosis. For example, the tadpole undergoes metamorphosis to form a frog.

• All the animals, except those belonging to Porifera, have radial or bilateral symmetry. Cnidaria has radial symmetry, while all vertebrates possess bilateral symmetry.

• Depending on the position and formation of the mouth and anus, there are two groups of animals - protostomes, where the mouth first forms from the cellular mass called blastopore and deuterostomes, where the anus forms first from the blastopore. Annelida, Arthropoda, Mollusca, and other simpler organisms belong to protostomes, while others like Echinodermata and Chordata belong to deuterostomes.