Animal Kingdom - Comprehensive NEET Biology Notes

1. Introduction to Animal Kingdom

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.

2. Basis of Classification

2.1 Levels of Organization

Although all animals are multicellular, the organization of cells can vary:

  • Cellular Level of Organization: Seen in sponges, where cells function independently.
  • Tissue Level of Organization: Seen in coelenterates (Cnidaria), where similar cells aggregate to form tissues.
  • Organ and Organ-System Level of Organization: Seen in animals like flatworms, annelids, and chordates, where tissues form organs, and organs form organ systems.

NEET Tip:

Focus on how different animal phyla are categorized based on the level of organization, as this is frequently tested in NEET.

2.2 Body Symmetry

Animals exhibit different types of body symmetry:

  • Radial Symmetry: Body parts are arranged around a central axis. Seen in coelenterates, ctenophores, and echinoderms.
  • Bilateral Symmetry: Body is divided into identical left and right halves in one plane, as seen in annelids, arthropods, and chordates.

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.

2.3 Coelom

The presence of a body cavity, or coelom, is a key factor in classification:

  • Acoelomates: No body cavity (e.g., flatworms).
  • Pseudocoelomates: Incomplete body cavity (e.g., roundworms).
  • Coelomates: True body cavity lined with mesoderm (e.g., annelids, mollusks, chordates).

Visual Aid Suggestion:

A diagram showing the differences between acoelomate, pseudocoelomate, and coelomate animals can help clarify this concept.

Quick Recap

  • Animals are classified based on levels of organization, symmetry, and the presence of a coelom.
  • Bilateral symmetry and the development of a true coelom are key features in more complex animals.

3. Classification of Animals

3.1 Phylum Porifera

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.

3.2 Phylum Cnidaria (Coelenterata)

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.

3.3 Phylum Platyhelminthes

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).

Quick Recap

  • Sponges have a cellular level of organization, while cnidarians exhibit tissue-level organization.
  • Flatworms are bilaterally symmetrical and often parasitic, with organ-level organization.

4. Higher Animal Phyla

4.1 Phylum Annelida

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).

4.2 Phylum Arthropoda

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.

4.3 Phylum Mollusca

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).

Quick Recap

  • Annelids show segmentation, with a closed circulatory system, while arthropods have jointed appendages and an exoskeleton.
  • Mollusks are soft-bodied animals with a calcareous shell and gills for respiration.

5. Phylum Chordata

5.1 Characteristics of Chordates

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.

5.2 Subphyla of Chordates

Chordata is divided into three subphyla:

  1. Urochordata (Tunicata): Notochord only in larval tails.
  2. Cephalochordata: Notochord extends from head to tail throughout life.
  3. Vertebrata: Notochord is replaced by a vertebral column in adults.

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.

Quick Recap

  • Chordates are defined by the presence of a notochord, dorsal nerve cord, and pharyngeal slits.
  • The subphylum Vertebrata includes animals with a vertebral column replacing the notochord in adults.

NEET Exam Strategy

  • Focus on understanding the differences between animal phyla, especially the unique features of chordates and non-chordates.
  • Practice diagram-based questions on the structure of animals like earthworms, arthropods, and chordates.
  • Be clear on the various levels of organization, body symmetry, and coelom development.

Practice Questions

  1. Which of the following is a feature of chordates?
    Solution: b) Notochord
    • a) Ventral nerve cord
    • b) Notochord
    • c) Open circulatory system
    • d) Jointed appendages
  3. Which phylum includes organisms with a water vascular system?
    Solution: c) Echinodermata
    • a) Porifera
    • b) Arthropoda
    • c) Echinodermata
    • d) Chordata
  5. What is the role of cnidoblasts in Cnidaria?
    Solution: b) Defense and prey capture
    • a) Respiration
    • b) Defense and prey capture
    • c) Reproduction
    • d) Digestion
  7. Which of the following has a pseudocoelom?
    Solution: b) Roundworms
    • a) Flatworms
    • b) Roundworms
    • c) Earthworms
    • d) Mollusks
  9. What is the primary distinguishing feature of vertebrates?
    Solution: c) Vertebral column
    • a) Open circulatory system
    • b) Exoskeleton
    • c) Vertebral column
    • d) Radial symmetry

Glossary

  • Notochord: A flexible, rod-shaped structure that supports the body in all chordates during some stage of development.
  • Coelom: A fluid-filled body cavity entirely lined by mesoderm.
  • Radial Symmetry: Symmetry around a central axis, as seen in starfish and jellyfish.
  • Bilateral Symmetry: A body plan in which an organism can be divided into identical left and right halves.