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Diagnostic Characteristics
The phyla Mollusk (derived from the Latin mulluscus), meaning soft) contains more than
150,000 known species of invertebrates and can be divided into eight classes; the four
most prevalent being Polyplacophora (chitons), Gastropoda (limpets, snails, whelks),
Bivalvia (clams, oysters, scallops), and Cephalopoda (octopuses, squid).
Although mollusks generally appear to be different, most mollusks exhibit bilateral
symmetry and have similar body plans. This includes a muscular foot for movement,
visceral mass, which houses most of the internal organs, and mantle, a tissue that folds
over the visceral mass and secretes a shell (usually constructed of calcium carbonateCaCO3). In general, Mollusks are soft-bodied animals protected by a shell.
Cephalopods such as squid have an internal mantle, while most octopi lack a mantle
altogether.
Acquiring and Digesting Food
In general, mollusks have a strap-like rasping organ called a radula that scrapes up food,
which then passes directly into the esophagus, the beginning of the alimentary canal (tube
extending between a mouth and an anus). This food then travels to crops for storage and
to the stomach where extracellular digestion takes place. The stomach is lined with ducts
that lead to digestive glands called cecae, which release digestive enzymes and break
down food. After being processed in the stomach, the food enters the intestine, where
nutrients are absorbed into the blood (hemolymph). Once nutrient uptake has been
completed, the food then passes through the anus and exits the organism.
Most of mollusks in the class Bivalvia (clams, oysters) are suspension feeders, the
absorption of small food particles from the water. Most bivalves have gills that are coated
in mucus that trap these food particles, which are then conveyed to the mouth by cilia
(sweeping finger-like structures).
Mollusks in the class Polyplacophora (chitons) use their radulas to cut and ingest algae.
Similarly, mollusks in the class Gastropoda use their radula to graze on plants; however,
some gastropods act as predators capable of consuming other mollusks or fish, and their
radulas can form poison darts.
Most Cephalopods, such as the squid or octopus, have a mainly carnivorous diet. Using
their long tentacles and beaklike jaws, cephalopods are adept at capturing and consuming
prey. Some cephalopods are also capable of injecting a toxin to immobilize their prey.
Sensing the Environment
As shown in the diagram to the right, the complexity of eye structure and function varies
greatly between the types of mollusks: from clusters of photoreceptors in limpets, to
camera-like eyes of cephalopods. This difference can largely be accredited to differences
between lifestyles and evolutionary needs of the different organisms. Limpets are
perfectly capable of being successful with a simple eye structure due to their less
intensive lifestyle; however, cephalopods, such as a squid or octopus, require complex
eye structures due to their predacious lifestyles and need to capture fast-moving prey.
Locomotion
In general, mollusks are capable of moving by swimming, digging and crawling, while
others are sessile and are anchored to a certain spot. Most mollusks have a muscular foot
used for movement. Chitons (in class Polyplacophora) have a foot that acts as a suction
cup, allowing them to attach to and creep along rock surfaces in the intertidal zone.
Gastropods move via the rippling motion of their elongated foot. Most mollusks of the
class Bivalvia contain a hatchet-shaped foot that is extended when the bivalve opens and
is used for digging or anchoring. Cephalopods are more skillful swimmers. For example,
the squid moves by drawing water into it's mantle cavity and then fires a jet of water
acting as propulsion.
Respiration
Mollusks in aquatic environments generally use gills located in the mantle cavity as the
primary method for gas exchange. These gills are surrounded by cilia, which create water
currents for more efficient oxygen uptake of the counter-current exchange system. In
terrestrial mollusks such as land snails, the mantle cavity functions as a lung allowing for
gas exchange with the air.
Circulation
Most mollusks (mainly in the Gastropoda, Polyplacophora, and Bivalvia classes) have an
open circulatory system in which blood bathes the organs directly. Because there is no
barrier between interstitial fluid and the blood, body fluid of mollusks is called
hemolymph. The heart then pumps hemolymph through vessels and out into sinuses,
which are spaces surrounding organs.
The class Cephalopoda, however, contains a closed circulatory system in which blood is
confined to vessels and is separated from the interstitial fluid. The heart then pumps
blood into vessels that branch into smaller ones that surround organs.
Osmotic Balance
Because mollusks can be terrestrial, marine, or freshwater organisms, there are different
strategies for maintaining an osmotic balance. Terrestrial mollusks have waxy cuticles
that help prevent water loss, such as the shell of land snails. Marine mollusks, like other
marine organisms, constantly face the threat of dehydration. Therefore, mollusks drink a
lot of seawater, excrete excess salts by active transport out of the gills, and produce very
little urine. Freshwater mollusks face the opposite problem of absorbing too much water
and losing salts, therefore they regain lost salts via active uptake (and from food) and
excrete very large amounts of dilute urine.
Metabolic Waste Removal
Mollusks contain tubular metanephridia, which function as kidneys. Excretory pores open
into the mantle cavity. Aquatic mollusks excrete ammonia, while terrestrial mollusks
excrete uric acid. Waste from digestion is expelled through the anus.
Self Protection
Most mollusks are soft-bodied creatures protected by a shell formed by the mantle. If a
mollusk feels threatened, the animal can simply retreat into this shell. Mollusks in the
class Bivalvia are made up of a shell divided into two parts that are hinged along the middorsal line. Strong adductor muscles shut the two halves together, forming a defensive
fortress for the mollusk.
Cephalopods are famous for their ability to create a "smoke screen" out of ink to allow
for a quick escape. Another interesting self-defense maneuver is cryptic coloration, or
camouflage, which is caused by pigmented cells called chromatophores, allowing for the
cephalopod to blend in with the environment.
Temperature Balance
Aquatic invertebrates are thermoconformers and have little control of their body
temperature.