Chapter 4Phylum Mollusca: the Molluscs

Characteristics

• body with 2 parts:

• head-foot - responsible for locomotion

• visceral mass - contains the body’s organs

• mantle secretes the shell

• Mantle Cavity functions in:

• excretion

• gas exchange

• elimination of waste

• release of reproductive products

• Bilateral symmetry

• Open circulatory system

• except for Cephalopods

• Radula functions in gathering food

4.1 Class Gastropoda - Molluscs with One Shell

• Over 35,000 living species (largest molluscan class)

• Marine, freshwater or terrestrial

• Slugs, snails, limpets

• Respiration

• rely on diffusion or a single gill

• terrestrial gastropods must be moist

• gastropods that burrow extend a siphon to take in water

• Circulation

• open circulatory system

• heart keeps blood moving around the organs

• blood collects in sinuses

• Feeding

• most feed by scraping algae

• some are herbivores

• some are predators

• http://www.hhmi.org/biointeractive/cone-snails-versatile-hunters

• Nervous Function

• ganglia control nervous system

• sense organs

• eyes (simple and complex)

• statocysts (sense gravity)

• osphradia (sense chemicals)

• Reproduction

• monoecious or dioecious

• fertilization can be internal or external

• hatch as larva

• torsion - twisting of the body to place gill and body openings at the opening of the shell

• operculum - covering for the entrance of the shell when the snail is retracted

Different types of Gastropods

• Subclass Prosobranchia

• largest group of gastropods

• sea snails and land snails

• approximately 20,000 species

• Subclass Opisthobranchia

• approx. 2,000 species

• sea slugs and sea hares

• feed on cnidarians and use the nematocysts for their own protection

• Subclass Pulmonata

• approx. 17,000 species

• snails with mantle cavity modified to function as a lung