Phylum Mollusca
(Chapter 27.4)
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Mollusks
Characteristics
Soft-bodied w/ external or internal shell
Have a free swimming larval stage called
trochophore
Also appear in annelids
Form and function
Body plan
Bilaterally symmetrical
Four parts
Foot  flat structure for crawling, shovel-shaped for
burrowing or tentacles for capturing prey
Mantle  thin layer of tissue that covers the body
Shell  made by glands in the mantle that secrete calcium
carbonate
Reduced or lost in some groups
Visceral mass  internal organs
Feeding
Herbivores, carnivores, filter feeders, detritivores or parasites
Snails and slugs have a raspy, tongue shaped structure known
as a radula
Can scrape algae off rocks, drill thru shells and tear tissues
Octopi and some sea slugs use sharp jaws to eat
Some octopi use poison to subdue prey
Clams, oysters and scallops are filter feeders
Siphon  a tube-like structure that brings water in and
out of the body
Respiration
Aquatic mollusks use gills inside the mantle cavity
Terrestrial mollusks use a large mantle cavity lined with blood
vessels
Lining must be moist
Circulation
Some mollusks have an open circulatory system (ie. Snails
and clams)
Other mollusks have a closed circulatory system (ie. Octopi
and squid)
Excretion
Nephridia remove ammonia from the blood and release it
outside of the body
Response
Simple nervous system  small ganglia, few nerve cords and
eyespots
Clams, oysters
Complex nervous system  well developed brains and can
remember things for long periods
Octopi, squid, cuttlefish
Movement
Snails secrete mucus along base of foot then use rippling motion
Octopi use jet propulsion by drawing water into mantle and
expelling from siphon
Reproduction
Sexual
External fertilization  release large number of eggs and
sperm into water
Snails and two shelled mollusks
Internal fertilization
Tentacled mollusks and some snails
Some mollusks are hermaphrodites but fertilize other
organisms eggs
Classes of mollusks
Class Gastropoda  shell-less or single shelled, move by using
foot on ventral surface
Snails, slugs, nudibranches (sea slugs)
Nudibranches recycle nematocysts from cnidarians to use for
protection
Snails withdraw into shell for protection
Class Bivalvia  two shelled mollusks
Clams, oysters, mussels, scallops
Tend to stay in one place
Filter feeders
Class Cephalopoda  soft-bodied, head attached to a foot that is
divided into tentacles
Octopi, cuttlefish, squid, nautilus
Small internal shell or no shell
The only cephalopod with shell is a nautilus
Large eyes