The Molluscs

• "Soft-bodied"
• Over 50,000 species described
• Marine (most diverse), freshwater, terrestrial
• Variety of forms - chitons, snails, clams, squids
• Same basic body plan - head, foot, mantle, shell
• Widely used as food by humans
• Shells are well preserved in fossil record

Advances in Molluscs

1. Digestive System

• Special radula - a belt-like chitonous sheath bearing recurved teeth
• Stomach, digestive gland branches to digestive tract

2. Circulatory and Excretory Systems

• Heart, arterial & venous system, pericardial sinus, open sinuses or closed capillary system (cephalopods)
• Hemocyanin respiratory pigment in several groups
• Gills well developed, except for pulmonate snails which have a specialized vascular "lung" (a highly modified mantle cavity)
• Metanephridal system: ultrafiltration from heart to pericardial cavity, reabsorption along kidney tubule to nephridipore

3. Nervous System

• Increased cephalization, especially from chitons to cephalopods
• Chemosensory organs (gastropods, bivalves), image-forming eyes (cephalopods)

4. Skeletal and Muscular Systems

• Large foot, visceral mass and mantle, locomotion well-developed
• Calcareous shell, secreted by mantle

Class Polyplacophora

• "Many shelled", chitons
• 800 species, all intertidal, attached
• Shell is 8 calcareous dorsal plates
• Mantle partly covers shell to form a girdle
• Mantle cavity is lined with rows of gills
• Ladder-like nervous system - limited cephalization

Class Gastropoda

• "Belly foot", snails and slugs
• Most diverse class, over 40,000 species
• Several derived traits not shown by other classes

Characteristics of Gastropods

1. Well-developed head: tentacles, pigment eyes, CNS

2. Torsion and development

• unique to gastropods, torsion involves twisting of the body
• during the veliger stage, a 180^o turn and upward twist of the posterior occurs to reposition it over the head
• represents a change of shell from a shield to a retreat

3. Shell

• Asymmetric helical coiling - separate event from torsion
• Secondary loss of shell and detorsion - nudibranchs, slugs

4. Reproduction - dioecious or hermaphroditic (snails)

5. Specialized respiration in pulmonates

Class Bivalvia

• "Two-valved", or Pelecypoda ("Hatchet-foot")
• clams, mussels, oysters, scallops
• 7,000 species (mostly marine; about 2000 freshwater species)

Characteristics of Bivalves

1. Laterally compressed, right and left shell valves

2. Shell secreted by two-lobed mantle

3. Attached (byssal threads, cemented) or burrowing

4. Large flattened foot used in locomotion in burrowing forms

5. Filter-feeders, gills highly modified as food collectors

6. Water-flow by ciliary action (gills) and muscle contraction, mantle folds create incurrent and excurrent siphons

7. Nervous system much reduced, loss of cephalization

Click here for some interesting facts about our native freshwater mussel fauna

Class Cephalopoda

• "Head-foot"
• squids, octopuses, nautiluses
• 600 species (over 7000 fossil species)
• all are marine and predatory:
  • squids pelagic swimmers
  • nautiluses and octopuses are bottom dwellers

Characteristics of Cephalopods

1. Highly modified molluscs, d-v elongated, streamlined

2. Foot modified to form flexible arms and a funnel to squirt water in swimming movements

3. All have a radula and beak-like jaws

4. Circulatory system with closed capillary system (rare among invertebrates)

5. Highly centralized nervous system (brain, ganglia)

6. Image-forming eyes with lens

• Convergent features to active vertebrate predators