Animal Kingdom Overview and Biological Classification

The diverse world around us, with millions of biotic and abiotic components that make up our ecosystem, is nothing less of a wonder. Of them, the biotic or the living community can be further subdivided into two broad groups- plants, and animals. In this discussion, we explore the kingdom animalia and the various phyla that it houses, along with their distinctive features.

Kingdom Animalia

Animals are multicellular, eukaryotic life forms, characterized by heterotrophic mode of nutrition and inhabit all types of ecosystems including terrestrial, freshwater and marine. They can be further segregated into two groups based on the presence of notochord- chordates and invertebrates. Chordates constitute only 5% of the animal kingdom while the invertebrates claim the remaining majority. Vertebrates form a subphylum under chordates and are characterized by the presence of a vertebral column or backbone. With an exception of sponges, all animal cells are organized into tissues and in majority, the tissues organize to form well-defined organs and organ systems for carrying out essential bodily functions. Most animals are also diploid in nature and reproduce sexually by the formation of gametes.

The following are the major phyla that constitute the animal kingdom and their respective features that distinguish them from one another.

Phylum Porifera

• Sponges, a multicellular, primitive animal with cellular grade organization, exemplify 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 from even a few detached cells from a pre-existing sponge. 

• They mostly thrive in marine ecosystems and are characterized by a body wall formed by two layers of loosely arranged cells with a mesenchyme between them. Inside the body wall, a large cavity (spongocoel) is present, containing mostly a number of small canals to supply oxygen and food, and to eliminate toxic waste matters. 

Example: Spongilla (freshwater sponge)

Phylum Cnidaria

• The members of phylum cnidaria, the coelenterates, are radially symmetrical and possess tissue level organization (their tissues do not form organs). 

• The organisms’ bodies generally have two main forms, polyp, and medusa. The former is fixed and is cylindrical in structure, while medusa is freely swimming and umbrella-shaped. This form of polymorphism can differ among the members of this phylum and can have multiple varieties. Usually, when both the forms are found in the same organism, they alternate in the life cycle, a phenomenon termed as alternation of generation. 

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

Phylum Platyhelminthes

• The members of phylum Platyhelminthes are bilaterally symmetrical animals with organ-system level of organization.

• Flatworms, the members of phylum Platyhelminthes, lack body cavities (acoelomate) and are devoid of proper circulatory and respiratory organs. Parenchyma, a type of connective tissue, packs the organs of the flatworms’ bodies.

• The animals do not possess anus but have a mouth. 

• The mode of reproduction is internal. Extensive precautions are also taken to ensure minimum incidences of self-fertilization as the animals are hermaphroditic in nature. 

Example: Fasciola (liver fluke) 

Phylum Aschelminthes

• They are also triploblastic organisms with a pseudocoelom located between the gut and the body wall. The cavity is not lined by mesodermal epithelium, and thus, despite the presence, the members of aschelminthes are classified as pseudocoelomates or organisms lacking true body cavity. 

• The body wall is formed of a hard, resistant cuticle, a muscle layer, and epidermis. The latter can be both cellular and syncytial.

• Opposite reproductive organs are present on different organisms and fertilization is internal. The mode of reproduction is sexual.

• Among the various members of the phylum, nematodes are of most importance. They can inhabit both aquatic and terrestrial and are parasitic in nature. 

Example: Wucheraria bancrofti (filarial worm)

Phylum Annelida

• The members of phylum Annelida are triploblastic, i.e. they develop from the three germ layer organization and have bilaterally symmetrical bodies. They are also in possession of a true coelom or body cavity, which is lined by a layer of mesodermal epithelium.

• The body is elongated, flattened or cylindrical in shape and ring-like grooves, called annuli (from which the name of the phylum is derived) divides the body into segments.

• The animals of this phylum also have a proper closed circulation.

Example: Hirudinaria (leech), Pheretima (earthworm)

Phylum Arthropoda

• With the largest phylum under animal kingdom, the arthropods exhibit organ-system level of organization with bilaterally symmetrical, triploblastic bodies. They also possess true body cavity or coelom (coelomates).

• All members of this phylum are characterized by the presence of an exoskeleton composed of chitinous cuticles. To keep up with the growth of the organism, the cuticle sheds from time to time and is replaced by a new one.

• The body of arthropods are subdivided into head, thorax, abdomen and jointed appendages. 

• They have an open circulatory system, which connects to the body cavity called haemocoel. The respiratory and the excretory organs vary from organisms to organisms.

Example: Apis (honey bee), Periplanata (cockroach)

Phylum Mollusca

• The second largest phylum under the animal kingdom, Mollusca includes common aquatic animals like snails, slugs, octopuses, oysters etc.

•  They possess organ-level organization in their systems and their body is bilaterally symmetrical, triploblastic (made up of three germ layers) and coelomate (containing true body cavity, lined by mesodermal epithelium). The body is covered by calcareous shells and consists of a distinct head portion, a visceral hump, and muscular feet.

• Nephridium acts as an excretory organ for the removal of wastes from the body. With the exception of cephalopods, all other mollusks have open circulation. 

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

Phylum Echinodermata

• Triploblastic, coelomate animals belonging to phylum Echinodermata are marine and inhabit the bottom of the sea (banthoic). The adult organisms are radially symmetrical while at the larval stage they exhibit bilateral symmetry.

• The most distinctive feature of the phylum Echinodermata is the presence of a water vascular system in their bodies. They can swim freely and the tube feet are contractile appendages serving the functions of locomotion, respiration and capturing of food, besides keeping the body attached to the substratum.

• The mode of replication is sexual and fertilization occurs externally.

Example: Echinoidea (sea urchin), Antedon

Phylum Protochodata

• Protochordates, also called hemichordates, are exclusively marine animals with organ system level of organization. 

• Their bodies are bilaterally symmetrical, triploblastic and consist of a true body cavity or coelom. The body is cylindrical in shape and consists of an anterior proboscis, a long trunk, and a collar. The notochord is hollow and originates from the endodermal germ layer.

• The mode of reproduction is sexual and fertilization takes place externally. The circulatory system is open.

Example: Ptychodera, Balanoglossus (tongue worm)

Phylum Chordata

• Animals belonging to phylum Chordata are triploblastic, exhibit bilateral symmetry and organ system level of organization, along with a true coelom. The most significant feature of this phylum is the presence of notochord, a dorsal, hollow nerve cord, and paired pharyngeal slits. 

• The subphylum Vertebrata falls under this phylum; the animals belonging to this group possess a backbone or vertebral column from which the name is derived. The other two subphylas of Chordata are urochordata and cephalochordata.

• This phylum includes birds (Aves), reptiles (Reptilia), amphibians (Amphibia), fishes, and mammals (Mammalia).

 Example: Scoliodon (dog fish), Betta (fighter fish), Bufo (toad), Draco

 (flying    lizard), Corvus (crow), Panthera leo (lion), Homo sapiens (Human).