Lecture 3-4: Invertebrates - Mollusks, Annelids, and Arthropods

Phylum Mollusca

Class Cephalopoda

• Includes nautiluses, cuttlefish, squid, and octopi.
• Body Plan Modifications:
  • Foot modified into arms/tentacles (may have suckers).
  • Shell can be external (nautilus) or internal (squid's pen, cuttlefish's cuttlebone).
  • Jet propulsion for movement: Water expelled to propel body.
• Circulatory System: Closed, unlike other mollusks.

Class Scaphopoda

• Commonly known as tusk or tooth shells.
• Conical shell open at both ends.

Phylum Annelida

• Known as segmented worms; includes earthworms, leeches.
• Body Plan: Segmented body with coelom in each segment.
• Circulatory System: Closed, with segment-specific coelom fluid circulation.
• Key Structures:
  • Setae: Bristles aiding in locomotion.
  • Parapodia: In aquatic worms, aiding in swimming and respiration.

Class Polychaeta

• Aquatic bristle worms.

Class Cletellata

• Includes earthworms and leeches.
• Clitellum: Reproductive structure.

Superphylum Ecdysozoa

• Key Feature: Exoskeleton that requires molting (ecdysis) for growth.

Phylum Nematoda

• Known as roundworms; microscopic or very small.
• Body Plan: Pseudocoelomates with hydrostatic skeleton.
• Notable Species: C. elegans, a model organism for genetic research.

Phylum Arthropoda

• Largest animal phylum with 1.1 million species.
• Body Features:
  • Jointed appendages (legs, antennae, wings).
  • Segmented body into tagmata (e.g., head, thorax, abdomen).
  • Chitinized exoskeleton.
  • Molting (Ecdysis): Necessary for growth.

Subphylum Chelicerata

• Includes spiders, scorpions, ticks, and horseshoe crabs.
• Key Structures:
  • Chelicerae: Mouthparts, often with venom fangs.
  • Book Lungs: Unique respiratory organs in spiders.

Subphylum Myriapoda

• Includes centipedes and millipedes.
• Characteristics: Numerous body segments with legs.

Subphylum Crustacea

• Includes crabs, shrimp, lobsters, barnacles, pill bugs (terrestrial), krill.
• Body Plan: Typically two tagmata (cephalothorax and abdomen).
• Exoskeleton: Often hardened with calcium carbonate.

Note: The lecture ended with an introduction to Subphylum Crustacea, to be continued in the next lecture.