Mollusk and Annelid Characteristics & Classes
Mollusks: Diverse Invertebrates
Mollusks are a diverse group of invertebrates, including snails, clams, squids, and octopuses. They have soft bodies, often protected by a shell, and exhibit a wide range of adaptations.
Mollusk Body Plan
- Head-foot: Includes the sensory organs and movement structures.
- Visceral mass: Contains internal organs like the heart, digestive system, and reproductive organs.
- Mantle: Secretes the shell and forms the mantle cavity, housing gills or lungs.
- Radula: A specialized feeding organ with rows of tiny teeth (absent in bivalves).
Circulatory System
- Open circulation in most (e.g., snails, clams).
- Closed circulation in cephalopods (e.g., squids, octopuses) for higher efficiency.
Reproduction
- Most are dioecious (separate sexes), but some are hermaphrodites.
- Some undergo external fertilization (e.g., bivalves), others internal fertilization (e.g., cephalopods, gastropods).
- Larval stages: Trochophore and sometimes Veliger.
Respiration & Excretion
- Aquatic species use gills; terrestrial species use a modified mantle cavity (lung-like structure).
- Nephridia filter waste from the blood and excrete it.
Major Classes of Mollusks
Class Gastropoda (Snails, Slugs, Limpets)
- Largest group.
- Undergo torsion (twisting of the body during development).
- Mostly have a single, coiled shell (except slugs).
Class Bivalvia (Clams, Oysters, Mussels, Scallops)
- Two hinged shells.
- No radula; filter feeders using gills.
- Typically sessile or burrowing.
Class Cephalopoda (Squids, Octopuses, Cuttlefish, Nautiluses)
- Highly intelligent, with complex eyes and a closed circulatory system.
- Use jet propulsion for movement.
- Possess tentacles with suction cups.
- Some (e.g., octopuses) lack a shell, while others (e.g., nautiluses) have one.
Class Polyplacophora (Chitons)
- Have eight overlapping plates forming a shell.
- Marine; attach to rocks using a strong foot.
Annelids: Segmented Worms
Annelids are segmented worms, including earthworms, leeches, and marine polychaetes.
Key Annelid Characteristics
Segmented Body (Metamerism)
- Internally and externally divided into repeating segments.
- Each segment often has its own muscles, nerves, and excretory structures.
Circulatory System
- Closed circulatory system (blood remains within vessels).
- Multiple “hearts” (aortic arches) pump blood.
Nervous System
- Ventral nerve cord with paired ganglia in each segment.
- Cephalization (concentration of nervous tissue at the head end) is more pronounced in some classes like Polychaeta.
Excretion
- Nephridia in each segment filter waste from coelomic fluid.
Reproduction
- Earthworms and leeches are hermaphrodites, exchanging sperm during mating.
- Polychaetes typically have separate sexes and reproduce sexually.
- Some species can regenerate lost segments.
Locomotion
- Use circular and longitudinal muscles for movement.
- Setae (tiny bristles) help with traction (except in leeches).
Major Classes of Annelids
Class Polychaeta (Marine Worms)
- Possess parapodia (fleshy appendages) bearing setae.
- Mostly marine; some are free-swimming, others burrow or live in tubes.
Class Oligochaeta (Earthworms)
- Few setae per segment.
- Important for soil aeration and decomposition.
Class Hirudinea (Leeches)
- No setae; flattened bodies.
- Possess anterior and posterior suckers for attachment and movement.
- Some are parasitic bloodsuckers with anticoagulant saliva; others are predators.