Lecture Notes on Biological Classification

Introduction to Biological Classification

• Millions of species are classified based on:
  • Feeding habits: Herbivores, carnivores, omnivores
  • Cellular organization: Cellular, tissue, organ level
  • Body symmetry: Asymmetrical, radial, bilateral
  • Body cavity: Presence or absence of coelom

Levels of Organization

1. Cellular Level

   • Cells combine to form tissues.
   • Example: Sponges

2. Tissue Level

   • Tissues perform specific functions.
   • Example: Cnidarians

3. Organ Level

   • Organs perform specific functions.
   • Example: Platyhelminthes (flatworms)

4. Organ System Level

   • Systems of organs work together.
   • Example: Annelids (segmented worms)

Body Symmetry

• Asymmetrical: No symmetry
• Radial Symmetry: Symmetry around a central axis (e.g., Cnidarians like jellyfish)
• Bilateral Symmetry: Divided into two equal halves (e.g., most animals)

Body Cavity Types

• No Body Cavity: Examples: Platyhelminthes
• Pseudocoelom: False body cavity present
• Coelomate: True body cavity present (e.g., Annelids)

Germ Layers and Development

• Diblastic: Two germ layers (ectoderm & endoderm) - Example: Cnidarians
• Triblastic: Three germ layers (ectoderm, mesoderm, endoderm) - Example: Most animals

Circulatory Systems

• Open Circulatory System: Blood is not always contained in vessels.

  • Example: Arthropods

• Closed Circulatory System: Blood is contained in vessels.

  • Example: Earthworms (Annelids)

Segmentation

• Segmented body structure seen in Annelids (e.g., Earthworm).
• Metamerism: Division into segments.

Phylum Porifera (Sponges)

• Simplest multicellular organisms.
• Asymmetrical with cellular level of organization.
• Water enters through Ostria and exits through Osculum.
• Digestion is intracellular.
• Reproduction: Asexual (fragmentation) and sexual (internal fertilization).

Examples: Sycon, Spongia

Phylum Cnidaria (Jellyfish, Corals)

• Radially symmetrical, diploblastic.
• Two forms: Polyp (attached) and Medusa (free-swimming).
• Stinging cells called Nematocysts for defense and capturing prey.
• Digestion: Extracellular and intracellular.

Examples: Hydra, Aurelia (jellyfish), corals.

Phylum Platyhelminthes (Flatworms)

• Dorsoventrally flattened, bilaterally symmetrical.
• Mostly parasitic (e.g. liver fluke).
• Hooks and suckers for attachment.

Examples: Planaria, Fasciola (liver fluke).

Phylum Nematoda (Roundworms)

• Round in cross-section, found in various habitats.
• Sexual dimorphism: distinct male and female forms.
• Complete digestive system from mouth to anus.

Examples: Ascaris, Wuchereria (filarial worm).

Phylum Annelida (Segmented Worms)

• Divided into segments; possess well-defined organs.
• Closed circulatory system.

Examples: Earthworms, leeches.

Phylum Arthropoda (Insects, Crustaceans)

• Largest phylum; jointed appendages, segmented body.
• Exoskeleton made of chitin.
• Open circulatory system; respiratory organs include tracheae.

Examples: Insects (butterflies, beetles), arachnids (spiders).

Phylum Mollusca (Soft-bodied Animals)

• Bilateral symmetry, usually covered by a shell.
• Organ system level of organization.

Examples: Octopus, snail, cuttlefish.

Phylum Echinodermata (Spiny-skinned Animals)

• Radial symmetry in adults; bilaterally symmetrical in larvae.
• Unique water vascular system for locomotion and feeding.

Examples: Starfish, sea cucumbers.

Phylum Chordata (Vertebrates)

• Presence of notochord, dorsal hollow nerve cord, and pharyngeal gill slits.

Subphylums:

• Urochordata: Notocord only in larval stage.
• Cephalochordata: Notochord throughout life.
• Vertebrata: Notocord replaced by vertebral column in adulthood.

Conclusion

• Organisms are classified into various kingdoms and phyla based on distinct characteristics such as levels of organization, symmetry, body cavity, and specific adaptations.