Chapter 3: Mollusks and Echinoderms

Two of the most ecologically and morphologically diverse invertebrate phyla are Mollusca and Echinodermata. Together they span habitats from tidal pools to abyssal trenches, and range in complexity from simple filter-feeding clams to the highly intelligent cephalopods.

Phylum Mollusca

Mollusca is the second-largest animal phylum with over 100,000 living species. Despite enormous variety, all mollusks share a common body plan: a visceral mass (containing organs), a mantle (tissue that secretes the shell), and a foot (muscular locomotion organ). Most also have a radula — a ribbon-like rasping tongue used to scrape food.

Major Molluscan Classes

Class	Shell	Foot	Examples
Gastropoda	Single, coiled (or absent)	Broad, flat	Snails (Helix), slugs, nudibranchs
Bivalvia	Two hinged valves	Wedge-shaped	Clams, oysters, mussels (Mytilus)
Cephalopoda	Reduced/internal or absent	Modified to tentacles	Octopus, squid, nautilus
Polyplacophora	8 overlapping plates	Flat	Chitons

Cephalopod Intelligence

Cephalopods (Octopus, Sepia, Loligo) are remarkable among invertebrates for their complex nervous systems. Octopus vulgaris has approximately 500 million neurons — comparable to a dog. Their abilities include:

• Problem-solving: opening jars, navigating mazes
• Camouflage: chromatophores allow instant color and texture change
• Tool use: Amphioctopus marginatus carries coconut shells for shelter
• Short-term memory and limited learning transfer

Phylum Echinodermata

Echinoderms (“spiny skin”) are exclusively marine and display pentaradial symmetry as adults (larvae are bilaterally symmetric, hinting at their shared ancestry with chordates). Key features include an internal calcite endoskeleton and the unique water vascular system.

Water Vascular System

This hydraulic network of canals functions in:

• Locomotion: tube feet extend by hydraulic pressure
• Gas exchange: diffusion across thin tube foot walls
• Feeding: sea stars evert their stomach into prey (e.g., bivalves)

Major Echinoderm Classes

Class	Body Form	Examples
Asteroidea	5+ arms, central disk	Sea stars (Asterias)
Echinoidea	Globular, no arms	Sea urchins, sand dollars
Holothuroidea	Elongated, leathery	Sea cucumbers
Ophiuroidea	Slender arms, distinct disk	Brittle stars
Crinoidea	Cup-shaped, filter-feeding arms	Feather stars, sea lilies

Sea stars are famous for arm regeneration. Linckia can regenerate an entire body from a single severed arm — a capability rooted in totipotent coelomocytes.

Key Checklist

• I can identify the three major molluscan classes and their distinguishing shell and foot characteristics
• I can explain at least three cognitive abilities observed in cephalopods with specific examples
• I can describe how the water vascular system functions in sea star locomotion and feeding