 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