Reviewer: Ecdysozoa, Nematoda, and
Arthropoda
Ecdysozoa
• Ecdysozoans are characterized by a multi-layered cuticle composed of organic material.
• This cuticle is periodically shed or molted in a process called ecdysis.
• The name Ecdysozoa comes from this molting process.
• Includes phyla such as Nematoda and Arthropoda.
Phylum Nematoda (Roundworms)
• Pseudocoelom (false body cavity) located between endoderm and mesoderm.
• Fluid-filled cavity for diffusion of O2, CO2, and nutrients.
• Covered by a tough cuticle that undergoes ecdysis.
• Cylindrical body with tapered ends.
• No circulatory system; nutrients distributed via hemocoel fluid.
• Complete digestive system: mouth and anus.
• Free-living in soil, water, and host tissues; parasitic forms exist.
• Sensory structures include amphids (anterior) and phasmids (posterior).
• Beneficial in pest control, but also harmful by damaging plant roots.
Ascaris Life Cycle
• Eggs hatch in the stomach, releasing immature larvae.
• Larvae migrate to the lungs, are coughed up, swallowed, and return to intestines.
• Mature into adults in intestines and lay eggs, which exit via feces.
Representative Nematode Parasites
• Roundworms (Ascaris)
• Hookworms (Ancylostoma)
• Pinworms (Enterobius)
• Trichina Worms
• Filarial Worms
• Whipworms (Trichuris)
2022 Nematoda Classes (M. Hodda)
• Class Enoplea
• Class Dorylaimida
• Class Chromadorea
Phylum Arthropoda
• Largest animal phylum: 900,000 species (~75% of known species).
• Includes insects, spiders, crustaceans, millipedes, ticks, scorpions.
• 'Arthropod' means 'jointed foot'.
Reasons for Arthropod Success
1. Versatile exoskeleton for protection, support, and sensory input.
2. Efficient locomotion through jointed appendages and tagmatization.
3. Tracheal system pipes air directly to cells.
4. Advanced sensory organs (sight, touch, smell, balance).
5. Complex innate and learned behavior.
6. Metamorphosis reduces intraspecific competition.
Characteristics of Arthropods
• Bilateral, triploblastic, schizocoelous body.
• Segmented body with tagmata (head, thorax, abdomen).
• Open circulatory system, dorsal heart, hemocoel.
• Exoskeleton of chitin and proteins, often with calcium carbonate.
• Respiratory structures: gills, tracheae, or book lungs.
Arthropod Subphyla
1. Trilobita – extinct, flattened three-lobed body.
2. Chelicerata – spiders, ticks, scorpions; have chelicerae, pedipalps.
3. Myriapoda – centipedes and millipedes; uniramous legs.
4. Crustacea – lobsters, shrimp; mostly marine with biramous appendages.
5. Hexapoda – insects with three body parts and six legs.
Metamorphosis in Insects
• Holometabolous: complete (larva → pupa → adult).
• Hemimetabolous: incomplete (nymph → adult with gradual change).
Class Insecta
• Most diverse group of arthropods.
• Mouthparts outside the head capsule, mandibles with two articulations.
• Compound eyes and separate sexes.
• Life cycle includes metamorphosis; major cavity is the hemocoel.