The Animal Kingdom includes a vast diversity of species, each adapted to different environments. The classification of animals is essential to organize and systematically understand this diversity. In this chapter, we explore the basis of classification, including cell organization, body symmetry, coelom development, and digestive systems, among other features.
Did You Know?
There are over a million species of animals described till now, making classification an essential tool to categorize and understand this immense diversity.
Although all animals are multicellular, the organization of cells can vary:
NEET Tip:
Focus on how different animal phyla are categorized based on the level of organization, as this is frequently tested in NEET.
Animals exhibit different types of body symmetry:
Common Misconception:
Students often confuse radial symmetry with bilateral symmetry. Radial symmetry allows for multiple planes of division, while bilateral symmetry has only one plane.
The presence of a body cavity, or coelom, is a key factor in classification:
Visual Aid Suggestion:
A diagram showing the differences between acoelomate, pseudocoelomate, and coelomate animals can help clarify this concept.
Sponges are the simplest multicellular animals, exhibiting a cellular level of organization. They possess a canal system through which water flows, aiding in food collection and gas exchange. Sponges reproduce both sexually and asexually.
Examples: Sycon, Spongilla, Euspongia.
Cnidarians are aquatic animals that exhibit radial symmetry and have specialized cells called cnidoblasts for capturing prey and defense. They have a gastrovascular cavity for digestion, and they may exist in two forms—polyp (sessile) and medusa (free-swimming).
Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Aurelia (Jellyfish).
Visual Aid Suggestion:
Diagrams of polyp and medusa forms can help visualize the alternation of generations seen in cnidarians.
Flatworms are acoelomate, bilaterally symmetrical animals. Many are parasitic, like tapeworms and liver flukes, and possess specialized structures like hooks and suckers to attach to hosts. These organisms exhibit organ-level organization.
Examples: Taenia (Tapeworm), Fasciola (Liver fluke).
Annelids are coelomate, bilaterally symmetrical, and segmented animals. The segments (metameres) are internally and externally repeated, contributing to movement and organization. Annelids have a closed circulatory system, and respiration occurs through the skin or gills.
Examples: Pheretima (Earthworm), Hirudinaria (Leech).
The largest phylum in the Animal Kingdom, arthropods are characterized by jointed appendages, a chitinous exoskeleton, and segmented bodies. They have an open circulatory system and specialized sensory organs.
Examples: Apis (Honeybee), Bombyx (Silkworm), Locusta (Locust).
Real-life Application:
Arthropods play important roles in ecosystems as pollinators, decomposers, and vectors of disease. They also have significant economic importance in industries like apiculture and sericulture.
Mollusks are coelomate animals with a soft body covered by a calcareous shell. They have a muscular foot for locomotion, a visceral hump, and a mantle. Respiration occurs through gills called ctenidia.
Examples: Pila (Apple snail), Octopus (Devilfish).
Chordates are characterized by the presence of a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits at some stage of development. They are bilaterally symmetrical, triploblastic, and coelomate.
NEET Tip:
Remember the key characteristics that distinguish chordates from non-chordates: notochord, dorsal nerve cord, and gill slits.
Chordata is divided into three subphyla:
Examples: Ascidia (Urochordata), Branchiostoma (Cephalochordata), and all vertebrates like fish, amphibians, reptiles, birds, and mammals.
Visual Aid Suggestion:
A diagram showing the basic chordate body plan can help clarify the distinction between chordates and non-chordates.