Lecture Notes: Animal Phyla and Body Cavity Evolution

Overview of Animal Phyla

1. Previously Covered:

   • Porifera: Sponges
   • Cnidaria: Anemones, corals, jellies, Portuguese man-of-war, Hydra
   • Platyhelminthes: Flatworms
   • Nematodes: Roundworms

2. Key Learning Points:

   • Phylum Name
   • Included Animals
   • Advancements over Previous Groups
   • Shared Characteristics

Importance of Body Cavities

• Body Cavity (Coelom): Crucial for animal complexity and size.

• Flatworms (Acoelomates):

  • Three tissue layers: endoderm, ectoderm, mesoderm.
  • No body cavity (acelomate).
  • Inability to develop complex organs due to lack of space.

• Nematodes (Pseudocoelomates):

  • Have a pseudocoelom (partial body cavity not entirely lined with mesoderm).
  • Allows for some complexity, e.g., hydrostatic skeleton.

• True Coelomates:

  • Examples: Mollusca, Annelida, Arthropoda.
  • Coelom is completely surrounded by mesoderm.
  • Supports complex organs and systems (digestive, excretory, circulatory, reproductive).

Advantages of a Coelom

1. Organ Protection: Muscles contract without crushing organs.
2. Space for Complex Organs: Allows development of complex systems (digestive, excretory, etc.).
3. Efficient Transport: Better nutrient, gas, hormone, and waste transportation.
4. Larger Size: Supports larger organisms due to complex internal systems.

Review of Embryonic Tissue Layers

• Ectoderm: Epidermis, nervous system, teeth.
• Mesoderm: Skeletal, muscular, circulatory systems.
• Endoderm: Lining of digestive, respiratory, excretory tracts.

Phylum Mollusca (Mollusks)

• Characteristics:
  • Second largest animal group by species.
  • Includes chitons, bivalves (oysters, clams), cephalopods (squid, octopus), gastropods (snails, slugs).
  • Basic body plan: Shell, foot, mantle, and mantle cavity.
  • True coelomate, allowing size and complexity.

Phylum Annelida (Segmented Worms)

• Characteristics:
  • Includes earthworms, marine worms, leeches.
  • True coelomate with segmentation.
  • Closed circulatory system.
  • Leeches produce anticoagulant (hirudin).

Phylum Arthropoda

• Characteristics:
  • Most numerous animal group.
  • Includes insects, arachnids (spiders), crustaceans (crabs, lobsters), myriapods (centipedes).
  • Jointed appendages with diverse functions: locomotion, feeding, sensory.
  • Exoskeleton made of chitin.
  • Specialized body segments (head, thorax, abdomen).

Phylum Echinodermata

• Characteristics:
  • Includes sea stars, sea urchins, sand dollars, sea cucumbers.
  • Five-part body plan, radial symmetry.
  • Water vascular system for locomotion.
  • Deuterostomes: Developmental pattern aligns with chordates.

Protostomes vs. Deuterostomes

• Protostomes (Mollusca, Annelida, Arthropoda):
  • Spiral cleavage, blastopore becomes mouth.
• Deuterostomes (Echinodermata, Chordata):
  • Radial cleavage, blastopore becomes anus.
  • More closely related to humans.

Next Steps

• Upcoming Topic: Phylum Chordata, including vertebrates.
• Lecture Video 3: Will cover chordates and their features.