Mollusks
There are over 50,000 known species of
Mollusks, which makes them second only to
the Arthropods in invertebrate phylum size.
Characteristics of Mollusks
• Mollusks have three body regions: a head, a
visceral mass, and a "foot."
• The head contains the sense organs, while the
visceral mass contains the internal organs. The
"foot" is the muscular lower part of the body. Most
mollusks have a shell.
• Mollusks also have an extension of the body wall
called the mantle. This portion of the animal's
anatomy is responsible for secreting the shell. The
mantle encloses the mantle cavity which contains
the Ctenidia (gills), anus and excretory pores.
More characteristics
• Many mollusks have a radula, a tongue of sorts,
which is rough like sandpaper and is used to rasp
away at food.
• The radula is made of a hard material called chitin,
the same material of which Arthropod
exoskeletons are made.
• Mollusks have well developed body organs
(nervous system, circulatory system, respiratory
system, etc.) but lack body segmentation.
Different Mollusks
• There are seven classes of Mollusks:
Monoplacophora, Polyplacophora,
Aplacophora, Gastropoda, Scaphapoda,
Bivalvia and Cephalopoda.
Class Gastropoda
• The class Gastropoda (meaning "stomachfoot") contains about 70% of the Molluscan
species (around 35,000). These are the
familiar snails, limpets, slugs, nudibranchs
and abalones.
Gastropoda
Class Polyplacophora
• These creatures have eight plates, made of chitin.
• Chitons crawl along rocks and forage for food
(mostly algae) using their radulae to scrape the
ground foraging for food.
• These creatures are extremely slow moving. In a
year, a chiton may not move more than ten feet.
They can detect the presence of light with
primitive eyes embedded in the shell plates.
Examples
Class Bivalvia
• The bivalves (meaning "two-shells") are
perhaps the most well known mollusks
simply because of their history as a source
of food. Clams, mussels, oysters and
scallops are all bivalves. There are about
15,000 known species of bivalve, with
about 80% of them being marine.
Class Bivalvia
Class Cephalopoda
• This includes squids, and octopuses
• They are one of the few animals of this phylum
that are excellent swimmers.
• The "feet" (usually called arms or mistakenly
called tentacles) are attached to the part of the
body containing the eyes (the "head") while the
rest of the body is out in front of the head. Thus,
the body does not connect directly to the arms.
More characteristics…
• They are similar in internal construction. Perhaps
the most obvious difference between most
cephalopods and other mollusks is the apparent
lack of a shell.
• Octopuses have no shell and squids have an
internal shell made of chitin.
• Cephalopods, therefore, have extremely good
eyesight.
• This eyesight is well suited for finding prey.
Interesting facts
• The Blue-Ringed octopus of the South
Pacific has a salivary gland which secretes a
venom to subdue prey. The venom is so
powerful that the bite of this octopus is
almost always lethal to a human.
Blue Ringed Octopus
Class Aplacophora and
Scaphapoda
• Aplacophora and Scaphapoda are rare
and/or extremely deep-water creatures.