Echinoderms & Arthropods: Morphology, Features, and Biology

Echinoderms

Phylum: Echinodermata

• Sea stars (starfish)
• Sea urchins
• Sand dollars
• Sea cucumbers
• Brittle stars

Key Features:

• Symmetry: Larvae: Bilateral; Adults: Pentaradial
• Body Covering: Spiny skin, endoskeleton made of calcium carbonate
• Movement: Tube feet powered by water vascular system
• Nervous System: No brain; nerve ring with radial nerves
• Circulation: Water vascular system (no true circulatory system)
• Respiration: Diffusion via tube feet or skin gills
• Reproduction: Mostly external fertilization; can regenerate lost parts
• Habitat: Marine only (no freshwater/terrestrial species)

Water Vascular System

A hydraulic system for movement, feeding, and respiration.

Frequently Asked Questions

1. What type of symmetry do adult echinoderms have? Pentaradial symmetry
2. Which unique system do echinoderms use for movement and gas exchange? Water vascular system
3. Name three classes of echinoderms. Asteroidea (sea stars), Echinoidea (sea urchins, sand dollars), Holothuroidea (sea cucumbers)
4. What is the function of tube feet in echinoderms? Movement, feeding, and respiration
5. True or False: Echinoderms have an exoskeleton. False – they have an endoskeleton made of calcium carbonate
6. How do echinoderms reproduce? Mostly through external fertilization; many can regenerate lost parts.

Arthropods

Phylum: Arthropoda (largest animal phylum)

Examples:

• Insects (bees, ants)
• Arachnids (spiders, scorpions)
• Crustaceans (crabs, lobsters)
• Myriapods (centipedes, millipedes)

Key Features:

• Symmetry: Bilateral
• Body Covering: Exoskeleton made of chitin (must molt to grow – ecdysis)
• Body Plan: Segmented body (head, thorax, abdomen), jointed appendages
• Movement: Legs and wings (in some insects), jointed limbs
• Nervous System: Brain with paired ventral nerve cords
• Circulation: Open circulatory system (hemolymph)
• Respiration: Insects: Tracheal tubes; Arachnids: Book lungs; Crustaceans: Gills
• Reproduction: Mostly sexual; internal or external fertilization
• Habitat: Marine, freshwater, and terrestrial

Special Feature:

Tagmatization – specialization of body segments (e.g., head, thorax, abdomen)

Frequently Asked Questions

1. What is the exoskeleton of arthropods made of? Chitin
2. What is ecdysis? Molting – shedding of the exoskeleton for growth
3. Name two respiratory structures found in arthropods. Tracheal tubes (insects), book lungs (arachnids), gills (crustaceans)
4. What body segments are found in insects? Head, thorax, abdomen
5. Which group of arthropods includes crabs and lobsters? Crustaceans
6. How are arthropods different from echinoderms in terms of nervous system? Arthropods have a brain and ventral nerve cords; echinoderms lack a central brain