π Phylum Mollusca β The Soft-Bodied Animals
Exploring the Life Processes, Behavior, and Adaptations of Mollusks
Definition: Mollusca is a phylum of protostomic invertebrate animals with soft bodies, often protected by a hard shell. With approximately 86,600 recognized living species, it is the second-largest animal phylum after Arthropoda (insects, spiders, crustaceans).
π Diversity & Classification
Mollusks live almost everywhere β roughly 55,000 species in the ocean, up to 30,000 on land, and up to 7,000 in freshwater. Scientists classify them into several classes, but the three you need to know are:
π Gastropoda
Snails & Slugs
- β Largest class (~80% of mollusks)
- β Single coiled shell (or no shell in slugs)
- β Move on a muscular foot
- β Have a radula for scraping food
π¦ͺ Bivalvia
Clams, Oysters & Mussels
- β Two hinged shells ("bi" = two, "valve" = shell)
- β No head and no radula
- β Filter feeders
- β Use a hatchet-shaped foot to burrow
π Cephalopoda
Octopuses, Squid & Cuttlefish
- β "Head-foot" animals β tentacles around mouth
- β Most have no external shell
- β Jet propulsion movement
- β Most intelligent invertebrates
π¬ Basic Body Plan
All mollusks share a basic body plan with three main parts:
| Body Part | Function |
|---|---|
| Head | Contains the mouth, sensory organs (eyes, tentacles), and the brain/nerve ganglia |
| Visceral Mass | Contains the internal organs β heart, digestive system, reproductive organs, nephridia |
| Muscular Foot | Used for locomotion, burrowing, or grasping (modified into tentacles in cephalopods) |
| Mantle | A thin tissue layer that covers the visceral mass and secretes the shell (if present) |
Interactive visualization β available in the full lesson