Animal Kingdom

We see different animals with distinct forms and structures when we look around us. Over a million animal species have been discovered; therefore, there is a great need for animal kingdom classification. The animal kingdom study material will help assign a systematic position to the newly described species. 

Kingdom Animalia

Animals seem to be multicellular, eukaryotic life forms that are characterised by the heterotrophic mode of nutrition. They inhabit complete types of ecosystems, including marine, freshwater and terrestrial. They further have been segregated into two groups based on notochord availability – chordates and invertebrates. Chordates only comprise 5 percent of the animal kingdom; however, invertebrates claim the majority of the animal kingdom. Vertebrates are a subphylum under chordates and are characterised by the availability of vertebral columns or backbones. Concerning sponges, all animal cells are organised into tissue that forms well-defined organs and organ systems to carry out essential bodily functions. Most animals are also diploid and reproduce sexually through the formation of gametes.

Below are the major species that comprise the animal kingdom and their salient features, which help us distinguish them.

Phylum Porifera

• Sponges are multicellular, primitive animals that comprise the cellular grade organisation, exemplifying the phylum Porifera
• They are free-living, aquatic life forms with no fixed body shape or plane of symmetry. A new sponge can be regenerated even from the detached cells from pre-existing sponges
• They usually thrive in marine ecosystems and are characterised by a body wall, which is formed by two layers of loosely arranged cells having mesenchyme between them

Example: Spongilla (freshwater sponge)

 Phylum Cnidaria

• The members of phylum cnidaria, the coelenterates, are radially symmetrical and feature the tissue-level organisation
• The animal bodies usually have two forms, i.e. medusa and polyp. The former is fixed and has a cylindrical structure; however, medusa is freely swimming and comprises an umbrella-shaped structure

• The form of polymorphism can be different among the members of the species and can even comprise multiple varieties

Example: Adams (Sea anemone), Meandrina (Brain coral)

 Phylum Platyhelminthes

• Members of Phylum Platyhelminthes are bilaterally symmetrical animals having the organ-system level of organisation
• The animal doesn’t possess an anus. However, it has a mouth
• The mode of reproduction is internal. Excessive precautions need to be taken to ensure minimum incidences of self-fertilisation because animals are hermaphroditic

Example: Fasciola (liver fluke) 

Phylum Aschelminthes

• They are also recognised as triploblastic animals, which comprises pseudocolours placed between the body wall and gut. The mesodermal epithelium does not line the cavity of the animal, and therefore, members of Phylum Aschelminthes are classified as animals that lack a true body cavity
• The body wall of the animal is formed of a hard, resistant cuticle, epidemics and a muscle layer
• The opposite reproductive organs are available in various animals; the process of fertilisation is internal, and the mode of reproduction is sexual
• While referring to the different members of species, nematodes are of great significance as they can live in both terrestrial and aquatic environments and are parasitic

Example: Wuchereria bancrofti (filarial worm)

Phylum Annelida

• The members of Phylum Annelida are triploblastic, which indicates that they can develop from the three germ layer organisation and seem to have bilaterally symmetrical bodies
• The body is elongated, cylindrical or flattened in shape and with ring-like grooves, which are known as annuli
• The animals of this species have a proper closed circulation

Example: Hirudinaria (leech), Pheretima (earthworm)

Phylum Arthropoda

• Arthropods exhibit organ system level of organisation along with bilaterally symmetrical and triploblastic bodies. They also possess a true body cavity
• Each member of the species has been characterised by the availability of an exoskeleton composed of chitinous cuticles
• Arthropod bodies are subdivided into thorax, heads, jointed appendages and abdomen
• Arthropods have an open circulatory system, which helps to connect to the body cavity known as haemocoel. The excretory and respiratory organs seem to be different for the animals

Example: Apis (honey bee), Periplaneta americana (cockroach)

 Phylum Mollusca

• Mollusca is the second largest phylum under the animal kingdom that comprises common aquatic animals such as slugs, snails, oysters, and octopuses
• They exhibit organ-level organisation in their system
• Bodies are bilaterally symmetrical, triploblastic and coelomate. The bodies are covered with calcareous shells and comprise a distinct head portion, muscular feet and a visceral hump
• Nephridium will be acting as an excretory organ for the removal of wastes from the body. All molluscs have an open circulation except cephalopods

Example: Pila (apple snail), Dentalium (tusk shell)

 Phylum Echinodermata

• The triploblastic animals belonging to the phylum Echinodermata are marine and inhabit the ocean floor. Adult animals of the specified group are radially symmetrical; however, during the larval stage, they show bilateral symmetry
• The most salient feature of Phylum Echinodermata is the availability of a water vascular system in their bodies
• The animals can swim freely, and along with such tubes, feet are contractile appendages that serve the function of respiration, locomotion and capturing food
• The mode of replication in Phylum Echinodermata is sexual, and fertilisation takes place externally

Example: Echinoidea (sea urchin), Antedon

Phylum Protochordata

• Members of Phylum Protochordate are also known as hemichordates. These are exclusively marine animals that possess organ system level of organisation
• The animals’ bodies are bilaterally symmetrical, triploblastic and include a true body cavity. The body of the animals are cylindrical and have an anterior proboscis, a long trunk, and a collar
• The mode of reproduction in Phylum Protochodata is sexual, and fertilisation occurs externally

Example: Ptychodera, Balanoglossus (tongue worm)

 Phylum Chordata

• Animals that fall under the category of Phylum Chordata are triploblastic, showing bilateral symmetry and organ system level of organisation. The most important feature is the availability of dorsal, notochord and paired pharyngeal slits
• The subphylum Vertebrata comes under this species, and animals belonging to Phylum Chordata possess a vertical column or backbone from which the name is derived
• This species includes reptiles, birds, fishes, amphibians and mammals

 Example: Betta (fighter fish), Bufo corvus (crow), (toad), Draco (flying lizard), Panthera leo (lion), Scoliodon (dogfish), Homo sapiens (Human).

Conclusion

We have provided the animal kingdom study material to understand how animals are classified into different groups and subgroups based on similarities and differences.