image from Mr. Martin's powerpoint
General Characteristics
- Body of two parts: head-foot and visceral mass
- Mantle that secretes a calcareous shell and covers the visceral mass
- Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, and release of reproductive products
- Bilateral symmetry
- Trochophore larvae, spiral cleavage, and schizocoelous coelom formation
- Coelom reduced to cavities surrounding the heart, nephridia, and gonads
- Open Circulatory system in all but one class (Cephalopoda)
- Radula usually present and used in scraping food
Body Structures
- The head-foot is elongated with an anterior head that contains the mouth and certain nervous structures. It also has an elongated foot for attachment and locomotion.
- The visceral mass contains the organs of digestion, circulation, reproduction, and is excretion; located on the dorsal side of the head-foot.
- The mantle is attached to the visceral mass, enfolds in on the body, and my secrete shell that overlies the mantle.
- Periostracum outer layer of the shell.
- Prismatic layer is the middle layer of the shell.
- The inner layer of the shell is the nacreous layer formed by a thin layer of calcium carbonate and organic matter.
- The mantle cavity is the cavity between the mantle and the foot.
- The radula is a rasping structure contained in the mouth, consisting of a chitinous belt and rows of posteriorly curved teeth
- The odontophore is a cartilaginous tongue-like structure
Classes
- Bivalvia- body enclosed in a shell consisting of two valves, hinged dorsally; no head/radula; wedge=shaped foot; marine/freshwater
- Gastropoda- Shell, when present, usually coiled; body symmetry distorted by torsion; some monoecious species; marine/freshwater/terrestrial
- Cephalopoda- Foot modded into a circle of tentacles and a siphon; shell reduced or absent; head in line with the elongate visceral mass; marine
Other Classes
- Caudofoveata-wormlike molluscs; marine burrowers
- Aplaophora- shell, mantle, lack foot; wormlike; poor cephalization; marine
- Polyplacophora- elongate, dorsoventrally flattened; reduced head size; marine
- Monoplacophora- single arched shell; broad/flat foot
- Scaphopoda- body enclosed in a tubular shell that is open at both ends; tentacles used for deposit feeding; no head