Lecture Notes: Basis of Classification of Animal Kingdom

Levels of Organization

• Cellular Level: E.g., Porifera
• Tissue Level: E.g., Cnidaria (Coelenterata), Ctenophora
• Organ Level: E.g., Platyhelminthes
• Organ System Level: Higher animal phyla (from Nematoda onwards)

Body Symmetry

Types of Symmetry

• Spherical Symmetry: Division into equal parts like a sphere (e.g., some sponges)
• Radial Symmetry: Division into equal halves through a central axis (e.g., Cnidaria, adult Echinodermata)
• Bilateral Symmetry: Division into two equal halves along one plane (most animal phyla from Platyhelminthes onwards)
• Biradial Symmetry: Combination of radial and bilateral features (e.g., some Ctenophores)

Examples

• Asymmetry: Some members of Porifera
• Radial Symmetry: Adults of Echinodermata, Cnidaria, some members of Ctenophora
• Bilateral Symmetry: Most animal phyla starting from Platyhelminthes

Body Plans

Types of Body Plan

1. Cell Aggregate Plan: Seen in Porifera
2. Blind Sack Plan: Seen in Cnidaria and Ctenophora
3. Tube within a Tube Plan: Seen from Nematoda onwards (human example: gastrointestinal tract within the body)

Germ Layers

Definition

• Germ layers are layers formed during embryonic development from which different tissues of the body originate.
• Types: Ectoderm (outer), Mesoderm (middle), Endoderm (inner)

Classification

• Diploblastic: Animals with two germ layers (Ectoderm and Endoderm); e.g., Cnidaria and Ctenophora
• Triploblastic: Animals with three germ layers (Ectoderm, Mesoderm, and Endoderm); e.g., from Platyhelminthes onwards
• No Germ Layers: Seen in Porifera

Coelom (Body Cavity)

Concept

• Coelom is seen only during embryonic development and not in adult stages.
• Types: Acoelomates, Pseudocoelomates, Coelomates

Examples

• Acoelomates: Porifera till Platyhelminthes (lack a body cavity)
• Pseudocoelomates: Nematoda(Ascehelminthes) (body cavity not fully lined with mesoderm)
• Coelomates: Annelida onwards (true coelom fully lined with mesoderm)

Importance

• Coelomate animals generally have better-organized systems and structures.

Embryonic Stages

• Single cell (zygote) divides and forms multiple layers.
• In organisms with tissues, organs, and organ systems, the germ layers develop into different tissues/organs.
• E.g., Ectoderm forms neural tissues, Endoderm forms epithelial linings, Mesoderm forms connective and muscular tissues.

Notes and Correction

• Some members of Porifera show asymmetry, majority show radial symmetry.
• Major focus on understanding differences in body plan and germ layers among different phyla.

Important Reminders

• Academic success is just a step towards your overall goals; continuous effort and refinement are crucial.

Next Class Preview

• Deeper dive into the coelom, its formation, and significance during the embryonic stages.
• Discussion on which animal phyla do not show the coelom during embryonic stages.