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Ch. 27.4 Oldest and most diverse phyla Many sizes, shapes, and forms Soft bodied with an internal or external shell Phylum: MOLLUSCA o Snails, slugs, clams, squids, and octopi Share similar development stages Aquatic mollusks have free swimming larval stage TROCOPHORE o Similar to annelids (could be more closely related) True coelom surrounded by mesoderm tissue Complex organ systems (respiration and excretion) BODY PLAN 4 parts o FOOT: can crawl, burrow, and capture o MANTLE: thin layer of tissue that covers the body o SHELL: made by glands and secrete calcium carbonate o VISCERAL MASS: beneath the mantle; contains internal organs Can be herbivores, carnivores, filter feeders, detritivores, or parasites Snails/slugs use a tongue shaped RADULA; has many teeth to scrape Some have sharp jaws to eat prey Clams/oysters are filter feeders (gills) SIPHON: tubelike structure where water flows Aquatic mollusks breathe via gills inside the mantle cavity Land mollusks have the mantle cavity lined with blood vessels for diffusion OPEN CIRCULATORY SYSTEM: blood pumped through vessels via a simple heart Works well for slow moving mollusks Fast moving mollusks have CLOSED CIRCULATORY SYSTEMS (blood is moved faster throughout the body) Release nitrogen waste via ammonia NEPHRIDIA remove the ammonia Two shelled mollusks burrow in the mud or sand o Small ganglia, a few nerve cords, simple sense organs, eyespots Octopi (and relatives) have a highly developed nervous system o Memory and may be more intelligent than other invertebrates o Well developed brains o Complex behavior (trainable) Secrete mucus by the foot Some use foot to make a rippling motion Jet propulsion Sexually via external fertilization (egg and sperm in open water) Tentacled mollusks have internal fertilization (females) Some are hermaphrodites GASTROPODS: pond snails, land slugs, sea butterflies, etc o SHELL LESS or SINGLE SHELLED that move by using a ventral foot BIVALVES: clams, oysters, mussels, scallops o TWO SHELLS held together by one or two muscles o Most stay in place for periods of time CEPHALOPODS: most active; octopus, squid, cuttlefish, etc o SOFT BODIED, HEAD ATTACHED to a single foot (foot divided into tentacles/arms) Can help keep waters clean by acting as filters Some are parasitic Also act as food for other organisms New research has shown that bivalves have been good with determining water quality
