Molluscan Characteristics Large range in size 1 cm long to 18 m All mollusks… 2 Body Parts- head-foot and visceral mass Mantle that secretes a calcareous shell (made of calcium carbonate) and covers the visceral mass

Molluscan Characteristics Continued All mollusks… Mantle cavity functions in Excretion Gas exchange Elimination of digestive wastes Release of reproductive products Bilateral symmetry Coelom reduced to cavities surrounding the heart, nephridia, and gonads

Molluscan Characteristics Continued All mollusks… Trocophore larvae (aquatic with cilia) Open circulatory system in all but one class (Cephalopoda) Radula usually present and used in scraping food

Molluscan Vocabulary Head-foot: Anterior head- contains mouth and sensory structures Elongated foot- used for attachment and locomotion Visceral Mass: Contains organs for digestion, circulation, reproduction, and excretion Dorsal to head-foot

Molluscan Vocabulary Mantle: Attaches to visceral mass May secrete a shell Mantle Cavity: Opens to the outside Functions in gas exchange, excretion, elimination of digestive wastes, and release of reproductive products

Molluscan Vocabulary Radula: Functions in scraping food (teeth and tongue in one structure) Rows of posteriorly curved teeth

Class Gastropoda Snails, limpets, and slugs 35,000 living species Largest and most diverse molluscan class Used for food (escargot) Intermediate hosts for some human parasites Means “stomach foot” Snail Slug Limpet

Class Gastropoda Torsion: 180⁰ counterclockwise twisting of the visceral mass, mantle, and mantle cavity Allows the head to enter the shell first- protects against predators Operculum (covering on the foot)- prevents the snail from drying out

Class Gastropoda Coiled shells- assymetrical Locomotion- ciliated, flattened foot Feeding: May scrape organisms (ex. Algae) using radula May eat plants, dead organisms, parasites, or predators

Class Gastropoda Gas exchange: Occurs with the mantle Modern gastropods have one gill (compared to 2 in ancestors) Siphon- inhalant tube that is a rolled extension of the mantle Open circulatory system Hydraulic skeleton- fluid supports body structure

Class Gastropoda Reproduction: Marine snails Dioecious External fertilization Land snails Monoecious Internal fertilization Eggs hatch into free- swimming trochophore larva

Class Bivalvia 30,000 species 2 nd largest molluscan class Clams, oysters, mussels, and scallops Sheet-like mantle & shell 2 valves Edible May form pearls Filter feeders Remove bacteria from the water Conchology- study of mollusk shells NB #97

Class Bivalvia Structures: 2 convex shell halves- called valves Hinge along shell made of protein Teeth- Tongue & groove modifications that prevents the valves from twisting Umbo: Oldest part of the shell Swollen area near the shell’s anterior margin

Class Bivalvia Structures: Shell forms as a single structure (although it looks like two) Hinge- elastic due to more protein and less calcium carbonate (allows the shell to open when certain muscles relax) Adductor muscles- strong muscles that close the shell (makes it difficult to pry a shell open)

Class Bivalvia Pearls Mantle attaches to the shell around the adductor muscles If one grain of sand lodges between the shell and the mantle, the mantle secretes nacre around the irritant (which may also be a parasite) This process eventually forms a pearl Nacre is also referred to as “Mother-of-Pearl” Process takes approximately two years

Class Bivalvia Processes Gas exchange Gills form folded sheets (lamellae) One end attached to the foot and the other end attached to the mantle Cilia move water into the gills Digestion The gills also trap food particles Cilia move food particles from the gills through ciliated food grooves Considered filter feeders

Class Bivalvia Filter feeding Valuable process for the environment Average oyster filters approximately 6 liters of water an hour Pollution, harvesting, and environmental changes have reduced bivalve numbers in certain areas

Class Bivalvia Open circulatory system Nervous system 3 pairs of interconnected ganglia Some species (ex. Scallops) develop photoreceptors as complex eyes with a lens and a cornea Eyes

Class Bivalvia Reproduction Most are dioecious Few are moneocious Some are protandric (male early and then develop into a female) Usually reproduce externally

Class Bivalvia Habitat and Diversity Aquatic (either marine or freshwater) May bury themselves, attaches to substrates, or bore in materials (ex. Wood) The foot secretes strands that cement the animal to it’s location

Class Cephalopoda Cephalopod- “head-foot” Octopuses, squid, cuttlefish, and nautiluses Most complex mollusks (actually most complex invertebrate) Anterior foot modified into tentacles to capture prey

Class Cephalopoda Shell The only living cephalopod with an external shell is the nautilus As it grows, the nautilus secretes a new shell and leaves the empty shell behind An internal shell is present in the cuttlefish Has gas-filled spaces that increase buoyancy The shell (called a cuttlebone) is used to make powder for polishing or feed pet birds

Class Cephalopoda Locomotion Move with a jet- propulsion system Force water out of the mantle cavity in a funnel Muscles on the funnel control the direction of movement Squids have posterior fins for stability May swim as fast as 30 km/hr Crawl around using tentacles

Class Cephalopoda Feeding & Digestion Locate prey by sight Capture prey with tentacles Tentacles are made of protein- occasionally have a hook Beak- jaws used to tear food Cuttlefish & nautiluses- eat small invertebrates Octopuses- nocturnal & eat snails, fishes, & crustaceans (inject venom) Squid- fishes & shrimp

Class Cephalopoda Closed circulatory system Eyes similar to vertebrate eyes Ink gland located behind the anus Ink contains melanin Black or brown in color Octopus Video

Class Cephalopoda Nervous system Complex lobes 20 th Century experiments demonstrated that cephalopods may be trained to attack, kill, and feed when stimulated May navigate mazes Can remember information for up to four months

Class Cephalopoda Reproduction Dioecious Males encase sperm in spermatophores One male tentacle (hectocotylus) is modified for spermatophore transfer Male and female tentacles intertwine during copulation Male inserts hectocotylus into female mantle cavity and releases the spermatophore Females release eggs Hatchlings resemble the adults Adults do not care for the young

Other Mollusks Class Polyplacophora Chitons Shell that divides into eight valves Class Scaphopoda Tooth shells or tusk shells At least 300 species Shell is open at both ends Class Monoplacophora Thought extinct until 1952 Class Aplacophora Lack shells Chiton Tusk shell Monoplacophora Aplacophora