you think that snails, clams, mussels, squid, and octopods are very different.
Yet, they are all in the same category of animals known as mollusks and
are structurally similar. Mollusks are some of the most well known of invertebrate
sea creatures (there are over 50,000 species). Some are very rare and are
only found in very deep-water.
Mollusks have three body regions.
1. The head contains
the "brain" and the sense organs.
Mollusks usually, but not always, have a shell, which is
secreted by a body wall called the mantle. Many mollusks have a tongue
of sorts, called a radula. The radula is rough like sandpaper. Mollusks
have well developed body organs that are used in the respiratory, circulatory
and nervous systems.
2. The "visceral mass" contains
the internal organs.
3. The "foot" is the muscular
part of the body.
stomach-foots (class Gastropoda) contains about 70% of all mollusk species.
The stomach-foots include snails, limpets and abalones, which have shells.
Slugs and nudibranchs are also stomach-foots, but do not have shells. A
few stomach-foots are found on land.
The Many-Plated or Chitons
The many-plated or chitons (class Polyplacophora) have
eight plates and look like pill-bugs (but pill-bugs are not chitons). Chitons
crawl along rocks looking for food (usually algae). A chiton uses its radula
(tongue) to scrape algae off rocks. It also has very hard teeth that are
also used to scrape algae of rocks. These teeth are hard enough to etch
glass! Embedded within their shells are primitive "eyes" that can detect
Chitons are very, very slow
moving. During a year, a chiton may move only ten feet!
bivalves (class Bivalvia) are very well known. They include clams, mussels,
oysters and scallops. All bivalves have two shells (the name means "two
shells"), and there are about 15,000 species. Most bivalves are marine,
but about 20% are found in fresh waters. Most bivalves do not have radula
because they eat by filtering water through their gills to obtain organic
particles. Most bivalves attach themselves to something or burrow underground.
Some scallops, however, do not attach themselves to anything and are able
to swim by squirting water through their mantle.
How Are Pearls Made?
Pearls are made by most bivalves (not just oysters) and
even some snails (such at the conch). When an irritant, such as a grain
of sand, becomes embedded in the mantle of a bivalve, the creatures coats
the irritant with the same material used to produce the lining of its shell.
This makes the irritant smooth and less painful to bivalve. Over time,
the irritant gets covered with more of the material, making a pearl!
Squids & Octopuses
class cephalopoda, meaning "head-footed," includes squids, octopods, cuttlefish
and nautiluses. The feet, or arms, of these creatures are connected to
their heads, not their bodies. The rest of the body is in front of the
head. That's why they're called, "head-footed." The "feet" of cephalopoda
are called "arms," not tentacles.
The cephalopods appear to be very different from other
mollusks, but physiologically they are similar. Cephalopods, like most
mollusks, have a mantle, a mantle cavity, a radula, and a U-shaped digestive
tract. Cephalopods have two kidneys and three hearts, which pump blue blood.
They are carnivores that feed on fish, shrimp, crabs and other cephalopods.
The most obvious difference between most cephalopods and
other mollusks is the apparent lack of a shell. Octopuses do not have shells
at all, while squids have a small internal shell. (Nautiluses, which are
found in the South Pacific and Indian oceans, are the only cephalopods
with an external shell. They are also the only cephalopods with four gills
instead of two.)
|Did You Know?
|A bite from a Blue-Ringed octopus, found in the South
Pacific, is almost always fatal to humans. However, it takes a lot of provocation
to get any octopus to bite a person.
Cephalopods have a more developed nervous system than
other mollusks. They also have very well developed eyesight that is used
in finding prey. Once prey is found, it is grasped firmly and eaten with
a mouth located at the base of the arms. Cephalopods also have a parrot-like
beak which is used in biting into prey.
What is the Difference Between
an Octopus and a Squid?
An octopus has eight arms. A squid has ten: eight of
the same length and two extra long ones used in grabbing prey. These extra
two arms are called tentacles. The arms of both squids and octopuses have
suckers on them used to latch onto prey.
What is the Plural Form of "Octopus"
Many people believe that the answer is "octopi." They
are wrongthe correct answer is "octopuses." However, if you say "octopuses"
many people will think that you are wrong and try to correct you. Try using
"octopods," which is the proper plural form of "octopoda," the order of