Mollusca

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Mollusca
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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, calcium-
  containing 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.

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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.

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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.

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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.

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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.

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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.

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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.  

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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).  

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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.

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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.

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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 ).

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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.Cephalop
  ods 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.

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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.

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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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more

• 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.

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