Phylum Mollusca — Core Principles

Core Principles

Phylum Mollusca is the second largest animal phylum, characterized by soft, unsegmented bodies. They are triploblastic, bilaterally symmetrical (though often modified), and coelomate, with a reduced coelom.

Key distinguishing features include a muscular foot for locomotion, a dorsal mantle that secretes a calcareous shell (present externally, internally, or absent), and encloses a mantle cavity housing gills (ctenidia) or lungs.

Most molluscs also possess a radula, a chitinous feeding organ with teeth, absent in filter-feeding bivalves. Their body is typically divided into a head, visceral mass, and foot. The circulatory system is generally open, except in cephalopods where it is closed.

Respiration occurs via gills or lungs. Excretion is handled by metanephridia. Reproduction is usually sexual, with separate sexes, and development often involves trochophore and veliger larval stages in aquatic forms.

Major classes include Gastropoda (snails, slugs), Bivalvia (clams, oysters), and Cephalopoda (squids, octopuses), each with unique adaptations in shell, foot, and feeding mechanisms.

Important Differences

vs Phylum Annelida

Aspect	This Topic	Phylum Annelida
Body Segmentation	Unsegmented (soft body)	Segmented (metamerically segmented)
Coelom	True coelom, often reduced (e.g., pericardial cavity)	True coelom, well-developed and spacious
Appendages	Muscular foot, sometimes modified into arms/tentacles	Parapodia or setae (bristles)
Protective Covering	Calcareous shell (external, internal, or absent)	Cuticle (no shell)
Unique Feeding Organ	Radula (present in most)	No radula
Larval Stage	Trochophore and Veliger larvae (marine forms)	Trochophore larva (marine forms)
Circulatory System	Mostly open (closed in cephalopods)	Closed

While both Mollusca and Annelida are protostome coelomates, they exhibit fundamental differences in body organization. Molluscs are characterized by their soft, unsegmented bodies, often protected by a shell, and possess unique structures like the mantle, foot, and radula. Annelids, in contrast, are distinctly metamerically segmented, lack a shell, and typically use setae or parapodia for locomotion. Their coelom is typically more spacious and serves as a hydrostatic skeleton. The presence of a veliger larva is unique to molluscs, distinguishing their developmental patterns from annelids, which share only the trochophore stage.