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Chapter Six: Phylum Mollusca
The Mollusks
Who are the Mollusks?
Unlike the phyla we have talked about before, the different classes of mollusks don’t look alike.
They vary from terrestrial snails and slugs to aquatic clams, sea slugs and octopi. Some have shells
(snails and clams) and some don’t (slugs and octopus). So what characteristics do they all have in
common that links them together? Well, they all have soft bodies (even though some of them are covered
with a shell). Most of them (except the bivalves – clams) have a specialized feeding structure called a
radula. Most of them form a larval stage of development called a trochophore except for the
cephalopods (octopus and squids). They all have a true coelom which is a fluid-filled hollow cavity
inside the body that is completely lined with mesoderm tissue.
Nudibranch (gastropod)
Scallop (bivalve)
Octopus (cephalopod)
What are these specialized structures that mollusks have (or why are they advantageous?)
Let’s start with discussing the coelom. Unlike the flatworms that had no coelom (acoelomate) and
the nematodes that had a fake coelom (pseudocoelom – lined with mesoderm and endoderm), the
mollusks have a true coelom. So what. Well, the coelom is lined completely with mesoderm which
means the mollusk can move its body wall muscles without affecting the digestion of food. Basically, the
coeolom allows the animal to multitask. It can move and digest food at the same time. Nifty. It can also
move its blood without other organs getting in the way.
What is this radula thing you were talking about? Well a radula is kind of like a tongue with
sandpaper on it. It can be used for cutting leaves like snails do to your garden or it can scrape algae off of
your aquarium or, in some cases, in can be used to drill into other shells for a tasty meal. Because clams
usually live a pretty sedentary lifestyle, they don’t have radulas but filter feed food particles out of the
Close-up of the radula on an
What about the trochophore, what’s that? The trochophore is a free swimming oddly shaped
larval form of a mollusk. This form helps mollusks move from one area to another more easily. They can
be moved by water currents but they also can swim to be closer to food sources. The best swimmers in the
mollusk family are the cephalopods (squids and octopus) and they skip the trochophore stage probably
because they are more than capable of moving around as adults.
A mollusk
Shells – are they worth the trouble? If you are a soft-bodied animal, you can be a bit vulnerable to
predators. It would be nice to have a protective shell to retreat into when trouble comes your way but
having a shell also has a price. Sure shells offer protection but they also slow you down and reduce the
area available for oxygen and carbon dioxide to move in and out of your body. Perhaps there is a way to
offset the gas exchange problem by developing some other way to move gases in and out of the body like
hmmmmmm……..gills! So what is the conclusion on shells? If you are a slow animal and have the time
and nutrients to put into forming a shell, go for it – like the bivalves and most of the gastropods but if you
are fairly quick and a predator, forget the shell – like the cephalopods. Of course, this still doesn’t explain
why the slugs are naked. Go figure.
What about the circulatory system of mollusks, is it open or closed. Yes.???? The circulatory
system plan depends on which class we are looking at. Bivalves and gastropods have open systems that
does not keep the circulatory fluid (blood in us, hemolymph in mollusks) completely in vessels.
Cephalopods need to respond more quickly to the environment and therefore need closed circulatory
systems to move nutrients and gases more efficiently. When you’re in hot pursuit of dinner, you can’t
wait for the hemolymph to ooze on over and bring the oxygen to your muscles, you need that oxygen
Head (anterior)
The Cephalopods: the geniuses of the invertebrate world!
Cephalopod literally means head-foot and those two parts form the basis of the cephalopod body.
Cephalopods include squid, octopus, cuttlefish and nautilus. They are all free-swimming predators with
specialized tentacles to help them capture prey. As mentioned before, they have a closed circulatory
system that moves nutrients and gases entirely enclosed in vessels to help them have the get up and go
they need to catch prey. They need bigger brains to do this as well. Cephalopods have millions of nerve
cells to react to the environment and can even learn to do simple tasks. (One hint: do not try to store them
in jars with screw top lids and large holes to allow water to flow through. They will learn how to unscrew
the lid.) They also need to be better camouflaged than their more primitive relatives who filter feed or
graze. For this they need chromatophores which are pigment cells that can change color. They are good
mimics too and have a lot of patience. Can you spot the octopus in the photo below (or did you think it
was something else)? Their eyes are pretty well-developed too and that can easily distinguish between
objects to find prey they like.
Photo Credits:
Mollusk body plan
Nudibranch (gastropod):
Octopus mimic: