Transcript Ch. 27.4x
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