Biol120 - Lecture 8: Mollusc Echinoderm

Learning Objectives

  • Understanding the key characteristics and diversity of Mollusca.

  • Understanding the key characteristics and differences between different Mollusca classes

  • Understanding the key characteristics and diversity of Echinodermata.

  • Understanding the key characteristics and differences between different Echinodermata classes

  • Understanding the significance of pentaradial symmetry and the water vascular system in Echinodermata.

Phylum Mollusca

• Name means “soft”; >8500085000 extant spp.
• Body plan:
– Bilateral, triploblastic, coelomate, non-segmented
– Muscular foot (→ tentacles in cephalopods)
– Mantle → secretes protein-CaCO3_3 shell
– Mantle cavity: respiration, feeding, locomotion
– Radula (lost in bivalves)

Class Polyplacophora (chitons)

• 88 dorsal shell plates + girdle → flexibility
• Marine, sedentary grazers on algae

Class Gastropoda (snails, slugs, nudibranchs)

• All habitats; herbivores or carnivores
• Torsion: visceral mass rotates 180∘180^{\circ} (anus over head)
• Shell portable retreat; lost in many lineages

Class Bivalvia (oysters, clams, scallops, shipworms)

• Two lateral valves; enlarged mantle cavity
• Suspension feeders—water over ciliated gills; radula absent
• Shipworms: wood-borers; digestion via symbiotic bacteria

Class Cephalopoda (octopus, squid, cuttlefish, nautilus)

• Active marine predators; foot → ≥\ge88 tentacles around beak
• Mantle cavity used for jet propulsion; ink, chromatophores for colour change
• Shell reduced/lost (present in nautiloids, extinct ammonites)
• Largest invertebrates; high intelligence

Phylum Echinodermata

• “Spiny-skinned” marine deuterostomes
• Triploblastic, coelomate endoskeleton (ossicles/test)
• Water vascular system (madreporite → tube feet) for locomotion & feeding
• Larvae bilateral → metamorphose to pentaradial adults (holothurians 2° bilateral)

Class Asteroidea (sea stars)

• Arms from central disc; tube feet on oral side
• Predators/scavengers; high regenerative ability

Class Ophiuroidea (brittle & basket stars)

• Distinct disc + long flexible arms; most speciose (>20002000 spp.)
• Diverse feeding; common in deep sea/crevices

Class Echinoidea

• Regular urchins: rigid spherical test, long spines
• Irregular urchins/sand dollars: flattened; burrow in sand
• Larvae can clone (predator evasion)

Class Crinoidea (sea lilies, feather stars)

• Long branching arms with pinnules; tube feet lack suckers
• Sea lilies stalked; feather stars free-moving

Class Holothuroidea (sea cucumbers)

• Elongated, skeleton reduced to micro-ossicles
• Mouth with tentacles; respiratory tree from cloaca
• Sediment or filter feeders; evisceration & Cuvierian tubules as defence

Core Takeaways

• Molluscs: unified by foot, mantle, mantle cavity, radula, shell—yet show wide morphological divergence.
• Echinoderms: unique pentaradial symmetry & hydraulic water vascular system distinguish them from other coelomates.
• Holothurians exhibit secondary bilateral traits within an otherwise pentaradial phylum.