Phylum Mollusca

• General characteristics: soft-bodied animals, usually have an inside or outside shell, most members share a common early development involving a special larva called a Trochophore
• Body plan has four basic parts: 1. foot (soft, muscular structure used for feeding + movement, includes the mouth and other feeding structures) 2. mantle (may/may not secrete a hard shell, thin delicate tissue layer that covers most of the body which usually lines the shell) 3. shell (made by the mantle that secrete calcium carbonate, main functions are protection and support) 4. visceral mass (contains all internal organs)
• Herbivores and some carnivores: radula (snails) (flexible skin with many teeth, used to drill through the shells of animals, sccrape algae off rocks,), sharp jaws (squid) (beak)
• Filter feeders (clams): use gills to filter food from water, cilia on gills push food and mucus toward the mouth.
• food enters the mouth and travels through the esophagus to the stomach
• digestive glands secrete enzymes into the stomach
• nutrient absorption occurs across the walls of the intestine
• solid wastes exit the body through the anus
• aquatic mollusks use gills located inside the mantle cavity to do gas exchange
• terrestrial mollusks (snails) use the mantle cavity which is lined with blood vessels (similar to a lung), mantle cavity lining must remain moist to allow diffusion (must live in damp or moist environments)
• slow or sessile mollusks (snails or clams) use open circulatory system with a simple heart. blood does not always flow in blood vessels (flows through blood sinuses)
• fast moving mollusks (squid or octopi): use closed circulatory system with a simple heart, blood always flows in blood vessels
• general plan of circulatory system = single loop: blood carries nutrients and O2 to all body cells, blood removes wastes and CO2 from all body cells.
• cellular/liquid wastes are removed from body tissues or blood by nephridia, nephridia release the waste to the outside through excretory pores
• sessile mollusks (clams and other bivalves) have very simple nervous system, small ganglia near important structures which are connected by nerve cords, may have simple sense organs.
• motile/predatory mollusks (octopi and other tentacled mollusks): well developed brain (high memory capacity and intelligence), complex sensory organs (eyes, touch receptors)
• aquatic mollusks: i. bivalves (clams) can use muscles to flap their shells, can use to foot to move/crawl, ii. cephalopods (octopi and squid) can crawl or shoot water, take water into the mantle cavity, muscle contraction forces water out of the funnel in a jet motion
• terrestrial mollusks (snails): use muscular foot (ventral side) to crawl, usually very slimey, produce mucus to slide across.
• only reproduce sexually
• external fertilization (most aquatic mollusks): sexes are usually separate, adults release sperm and eggs into water, fertilization occurs by chance in the open water, fertilized egg become Trochophore
• internal fertilization (most terrestrial mollusks): if hermaphroditic, cross-fertilization occurs, if sexes are separate, fertilization occurs inside the female
• class gasstropoda (snails, slugs, etc.): gastropod = "stomach foot", usually have one-piece shell to protect their soft body except slugs
• class bivalvia (clams, oysters, and scallops): have two shells, have glands to produce a mother-of-pearl layer to keep the inside of the shells smooth and comfortable (may produce pearls)
• class cephalopoda (octopi, squid, cuttlefish, nautiluses): cephalopoda = "head foot" (head is attached to foot), foot is divided to tentacles, tentacles are usually flexible with round sucker disks, have a small internal shell (squid and cuttlefish) or no shell at all (octopi)