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Mollusca
The Soft-bodied animals
The mollusks constitute one of the largest phyla of animals,
both in numbers of living species (at least 47,000, and
perhaps many more) and in numbers of individuals.
A significant characteristic of mollusks is their possession
of a coelom, a fluid-filled cavity that develops within the
mesoderm. The coelom not only functions as a hydrostatic
skeleton but also provides space within which the internal
organs can be suspended by the mesenteries.
All mollusks have a soft body (their name is derived
from the Latin word mollus, meaning "soft"), which is
generally protected by a hard, calciumcontaining shell. In some forms however, the shell has been
lost in the course of evolution, as in slugs and octopuses, or
greatly reduced in size and internalized, as in squids.
Body plan
Structurally, mollusks are quite distinct from all other
animals. However, all modern mollusks have the same
fundamental body plan.
There are three distinct body zones: a head-foot, which
contains both the sensory and motor organs; a visceral mass,
which contains the well-developed organs of digestion,
excretion, and reproduction; and a mantle, a specialized tissue
formed from folds of the dorsal body wall, that hangs over and
enfolds the visceral mass and that secretes the shell.
The mantle cavity, a space between the mantle and the
visceral mass, houses the gills; the digestive, excretory, and
reproductive systems discharge into it.
Radula
Mollusks are also characterized by a
toothed tongue, the radula, composed
primarily of chitin. The radula serves both
to scrape off algae and other food
materials and also to convey them
backward to the digestive tract. In some
species, it is also used in combat.
Digestion and Excretion
The digestive tract is complete and ciliated, with a
mouth, anus and complex stomach. The pattern of the
stomach varies according to the mollusks diet. Food is
taken up by cells lining the digestive glands arising from
the stomach, and then is passed into the
blood. Undigested materials are compressed and
packaged, then discharged through the anus into the
mantle cavity and are carried away from the animals in
the water currents. This packaging of wastes in solid
form prevents fouling of the water passing over the gills.
Excretory functions are carried out by a pair of nephridia,
tubular structures that collect fluids from the coelom and
exchange salts and other substances with body tissues
as the fluid passes along the tubules for excretion. The
nephridia empty into the mantle cavity.
Nervous System and Sensory
Capability
Mollusks have a relatively complex nervous system,
which varies from species to species reaching the
height of complexity at the octopus. The octopus is
thought to be among the most intelligent of all
invertebrates, with a mental capacity likened to that
of a domestic cat. Sensory ability in some mollusks
(notably the cephalopods) is considerable, with a
variety of organ systems, as well as large, complex
eyes. The eyes of the giant squid are the largest in
the animal kingdom, approaching the size of dinner
plates. It has recently been demonstrated that
squid can successfully locate and capture
transparent prey in the water by means of a
specialized polarization vision.
Respiration and Circulation
Excluding cephalopods, mollusks have an open circulatory system,
meaning the blood does not circulate entirely within vessels but is
collected from the gills, pumped through the heart, and released
directly into spaces in the tissues from which it returns to the gills
and then to the heart. Such a blood-filled space is known as a
hemocoel ("blood cavity"). In the mollusks, the hemocoel has
largely replaced the coelom, which is reduced to a small area
around the heart and to the cavities of the organs of reproduction
and excretion.
Cephalopods, whose vigorous activities require that the cells be
supplied with large quantities of oxygen and food molecules, have a
closed circulatory system of continuous vessels and accessory
hearts that propel blood into the gills. The presence of discrete
respiratory and circulatory systems has led to an improved capacity
for oxygen uptake and distribution, and hence an increase in body
mass. Molluscans include the largest and most advanced of living
invertebrates with the cephalopods, including the monstrous Giant
Squid Architeuthis, which can reach a total length, including
tentacles, of 18 meters.
Locomotion
Locomotion among the molluscans varies considerably, and
is dictated by the structure of the foot. Herbivorous forms are
commonly gliders, moving on waves of muscular contraction.
However, many carnivorous forms have achieved more
advanced forms of locomotion. Cephalopods swim actively
by a type of jet propulsion, in which water is rapidly expelled
from the mantle cavity via the siphon. In the streamlined
squid this technique has enabled to animals to achieve the
fastest speeds of any aquatic invertebrate. The Cuttlefish
and the Sea Hares rely upon undulating lateral fins for highly
maneuverable locomotion. In the bivalves the foot has
developed into a tool for burrowing, which can be remarkably
rapid for example in the common Razor Shells.
Diversity among the mollusks
Mollusks exhibit a tremendous diversity of
form and behavior. The three major
classes range from largely sedentary or
sessile filter-feeding animals, such as
clams and oyster (class Bivalvia), through
aquatic and terrestrial snails and slugs
(class Gastropoda), to the predatory
cuttlefish, squids, and octopuses (class
Cephalopoda).
The Mollusks
The molluscs rival the arthropods in their
diversity of body forms and sizes, as well as
their ecological success. The phylum also
provides some of the most familiar animals, such
as snails , clams , mussels , squids , and
octopus (which, like the arthropods , are well
known because they're good to eat). The phylum
Mollusca also includes lesser known forms such
as the chitons , tusk shells. Approximately
50,000 species of Molluscs have been
described, and because of the shelled forms
they have left a rich fossil record.
Class Gastropoda
Most of the approximately 40,000 living
species of gastropods have shells, however
there are quite a few groups that have either
reduced or internal shells, or no shell at all.
Shelled forms are generally called "snails "
and forms without shells are called "slugs",
however the terrestrial slugs are not closely
related to the various marine forms that are
without shells. Although most Gastropods are
marine, there are numerous forms in both
freshwater and terrestrial environments.
Class Pelecypoda
(=Bivalvia )
The bivalves include forms such as clams , mussels ,
scallops , oysters , as well as many less familiar forms.
Bivalves are laterally compressed animals, with two shell
"valves " that are hinged on the animal's dorsal surface. The
approximately 8,000 living species of bivalves are mostly
marine, but there are many freshwater species as well
(however, no terrestrial ones). Bivalves are found in just
about every marine environment, from the intertidal zone to
the deepest marine habitats. Most bivalves are suspension
feeders, filtering small organisms and organic particles from
the water (such as bacteria , phytoplankton , zooplankton ,
and nonliving organic detritus ).
Class Cephalopoda
Cephalopods , which include the familiar squids and octopus
, include species which are the largest known invertebrates
(giant squid , up to 20 meters long, including tentacles), the
most intelligent, and the fastest swimming aquatic
invertebrates (squid ). There are also forms with external
shells (nautilus ), and internal hard shells (cuttlefish ). Most of
the approximately 650 living species of Cephalopods are
active swimmers, however most species of octopus have
secondarily assumed a benthic existence.
Cephalopods have a closed circulatory system, an adaptation
to their active lifestyle, as opposed to the open circulatory
system found in other molluscs . Squid in particular are often
very abundant in pelagic marine environments, where they
are voracious predators of many organisms, especially fish.
Class Polyplacophora
The Polyplacophora , commonly known as
chitons , have seven or eight dorsal shell
plates (although they may be covered mostly
or entirely by soft tissue in some species).
The approximately 600 described species
are generally flattened and elongated
animals that are typically found in the
intertidal zone grazing on epibenthic algae .
However, a few deep sea species have been
described.
Introduction to the Scaphopoda
The Scaphopoda are a distinctive group of
molluscs commonly known as the "tusk
shells" because their shells are conical and
slightly curved to the dorsal side, making the
shells look like tiny tusks (see the photos
below). The scientific name Scaphopoda
means "shovel foot", a term that refers to the
"head" of the animal, which lacks eyes and is
used for burrowing in marine mud and
sediments. The most distinctive feature of
scaphopods is that the tubular shell is open at
both ends, not just one end as in most
molluscs.
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Scaphopods live their adult lives buried in sand or
mud, with their head-end pointed downwards. Only
the narrow posterior end of the shell sticks up into
the seawater for water exchange and waste
expulsion. Gills have been lost in the scaphopods,
so the mantle tissue not only produces the shell, but
also serves the function of gills in obtaining oxygen
from seawater. The mantle is fused into a tube that
surrounds the body of the animal, but it is open at
both ends. Water is circulated around the mantle
cavity by the action of numerous cilia. When the
dissolved oxygen runs low, the water is ejected
through the top end of the shell by contraction of the
foot.