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Mollusca

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Title: Mollusca


1
Mollusca
2
Phylum Mollusca
  • The molluscs possess a true coelom, a
    fluid-filled space within the mesoderm in which
    organs are suspended by mesodermal membranes
    called mesenteries.

3
Mollusca
  • The molluscs are a very diverse group with over
    100,000 living species including such familiar
    organisms as clams, mussels, limpets, snails,
    squids and octopuses.

4
Mollusca
  • Many molluscs possess a hard shell and as a
    result there are many fossil molluscs (more than
    30,000 species have been described).
  • Molluscs were abundant in the oceans of the
    Cambrian period and must have evolved in the
    pre-Cambrian over 570 million years ago.

5
Molluscs
  • Living molluscs range in size from small clams
    and snails to the giant and colossal squids which
    can weigh up to 1,000 lbs and measure 60 feet
    long with tentacles extended.
  • The shells of giant clams may be 1.5 m wide and
    weigh 225 kg, but most molluscs have shells less
    than 5cm across.

6
Molluscs
  • The word Mollusca is from the Latin molluscus
    meaning soft and describes the soft body, which
    is one of the key features of the group.
  • Most molluscs live in the sea and range from
    tropical to Arctic waters. Others occur in
    freshwater and on land.
  • There is a great range of life styles that
    includes bottom feeding, filter feeding, boring,
    burrowing and pelagic forms.

7
Molluscs
  • Molluscs have diversified into a great variety of
    body forms from the sessile, filter-feeding clam
    to the slow-moving grazing snail to the actively
    hunting, intelligent octopus.
  • These different forms are all derived from a
    basic molluscan body plan.

8
Molluscan body plan
  • At its simplest the molluscan body consists of a
    head-foot and a visceral mass.
  • The head-foot contains the locomotory, feeding,
    cephalic, and sensory organs.
  • The visceral mass includes the digestive,
    circulatory, respiratory and reproductive organs.

9
Molluscan body plan Head-foot
  • Many molluscs have a well developed head that
    contains a mouth, often tentacles, and some
    specialized sensory organs.
  • Many possess eyes that range from very simple
    light sensing structures to the highly developed
    eyes of cephalopods.

10
Molluscan body plan Head-foot
  • Within the mouth of most is a structure unique to
    molluscs known as the radula.
  • The radula is a tonguelike organ that can be
    protruded and acts like a rasp.
  • It is covered in small teeth and scrapes material
    off whatever it is rubbed against (e.g. algae,
    leaf material) and the scraped material is
    directed into the mouth.

11
Fig 10.3
12
Molluscan body plan Head-foot
  • The molluscan foot is usually a ventral,
    muscular, sole-like structure.
  • The foot is used for locomotion and/or adhesion
    depending on the species. In some species mucus
    is secreted as an aid to attachment or for the
    foot to glide on.
  • Modifications include the attachment disk of
    limpets and the funnel used for jet propulsion by
    cephalopods.

13
Molluscan body plan Visceral mass--mantle and
mantle cavity
  • The mantle is a layer of skin that extends from
    the top of the visceral hump.
  • It hangs over both sides of the body and both
    protects the body and creates a space between it
    and the body called the mantle cavity.
  • The mantle also secretes the shell.

14
Mantle cavity
  • The mantle cavity is a very important structure
    for molluscs.
  • Respiratory organs, either gills or lungs,
    develop from the mantle and exchange gases in the
    mantle cavity.

15
Mantle cavity
  • In addition, the reproductive, digestive, and
    excretory systems empty into the mantle cavity
    allowing wastes to be evacuated to the outside.
  • In aquatic molluscs the beating of cilia or
    muscular pumping moves water in and out of the
    mantle cavity flushing out wastes and bringing in
    fresh water.

16
10.2
17
Shell
  • A molluscs shell is secreted by the mantle and
    is lined by it.
  • There are usually three layers to the shell
  • Periostracum (outermost)
  • Prismatic layer
  • Nacreous layer

18
Shell
  • The outermost layer the Periostracum is made of a
    protein called conchiolin.
  • Protects against erosion of underlying calcareous
    areas. It is secreted by a fold of the mantle and
    new growth occurs only at the edges.
  • This layer is thick in freshwater molluscs as it
    protects against acids produced by leaf litter.
    In many marine molluscs, it is thin or absent.

19
Shell
  • The middle layer of a shell is called the
    prismatic layer and is made of densely packed
    prisms of calcium carbonate deposited within a
    protein matrix.
  • Growth of this layer also occurs only at the edge
    where the mantle overlaps it,

20
Shell
  • The inner nacreous layer of shell is made up of
    thin sheets of calcium carbonate laid over a thin
    matrix of protein.
  • As it abuts the mantle this layer is secreted
    continuously during life and grows thicker as the
    animals ages.

21
10.4
22
Shell Pearl formation.
  • In some cases a grain of sand, parasite or other
    irritating object lodges between the mantle and
    shell.
  • The clam or oyster responds by depositing many
    layers of nacre around the object eventually
    forming a pearl.

23
Shell Pearl formation.
  • Pearls can be cultured by deliberately seeding
    oysters with an irritant, usually a tiny piece of
    freshwater mussel shell.
  • This process developed by Kokichi Mikimoto of
    Japan produced the first cultured pearl in 1893.
    Most pearls commercially available are cultured
    pearls.

24
The size, color and shape of a pearl varies
depending on the mollusc that produced it.
25
Internal structure and function circulatory
system
  • Molluscs possess a circulatory system.
  • In most species it is an open circulatory system
    with a heart, blood vessels and sinuses in which
    tissues are bathed in blood directly.

26
Circulatory System
  • The typical open circulatory pattern in molluscs
    is for blood to drain from the gills into paired
    auricles in the heart, which connect to a single
    muscular ventricle.
  • The ventricle pumps blood through a single aorta
    which branches to deliver blood to sinuses that
    bathe tissues directly. Blood then drains from
    the sinuses back into the gills

27
Circulatory system
  • In cephalpods which are more active than other
    molluscs, selection has favored the evolution of
    a closed circulatory system with a network of
    blood vessels and capillaries.

28
Internal structure and function nervous system
  • The basic nervous system plan in molluscs is for
    a nerve ring around the esophagus from which
    extend nerves that innervate the foot, mantle and
    visceral mass.
  • Sense organs include tentacles, eyes and
    osphradia in the mantle cavity that monitor
    current flow.

29
Bay scallop (note the blue eyes).
30
Cuttlefish
31
Internal structure and function nervous system
  • Most molluscs do not have a well developed brain,
    but the cephalopds (octopus, squid, cuttlefish)
    are a glaring exception.
  • The brain development of cephalopods is unequaled
    among the invertebrates and they have the ability
    to learn, remember and problem solve.

32
Internal structure and function reproduction and
larvae
  • Most molluscs are dioecious, although some
    gastropods are hermaphroditic.
  • Many aquatic molluscs pass through free-swimming
    trochophore and veliger larval stages.

33
Internal structure and function reproduction and
larvae
  • Trochophores are minute and translucent with an
    obvious ring of cilia (prototroch).
  • Trochophore larvae are found in molluscs and
    annelids and are evidence of the common
    phylogenetic origin of these groups.

34
Generalized trochophore larva
Fig 10.5
35
Internal structure and function reproduction and
larvae
  • A veliger larva develops from a trochophore and
    shows the beginnings of a foot, mantle, and
    shell.

Veliger larva of a snail
36
Mollusc Classification
  • There are 7 or 8 classes depending on whether the
    Scaphopoda are split into two class or not.
  • Class Aplacophora Caudofoveata and Solenogastres
  • Class Monoplacophora Neopilina
  • Class Polyplacophora chitons
  • Class Scaphopoda tusk shells or tooth shells
  • Class Bivalvia or Pelecypoda clams, oysters,
    mussels
  • Class Gastropoda snails, slugs, limpets, whelks,
    seas slugs, conchs periwinkles, etc.
  • Class Cephalopoda Octopus, squid, nautilus.

37
Class Aplacophora
  • Class Aplacophora (sometimes split into two
    classes the Caudofoveata and Solenogastres).
  • Relatively few known species. They are wormlike
    and shell-less.
  • The Caudofoveata burrow in marine sediments and
    feed on detritus and microorganisms.
    Solenogastres live free on the ocean bottom and
    often feed on cnidarians.

38
Two aplacophoranshttp//www.ucmp.berkeley.edu/taxa
/ inverts/mollusca/aplacophora.php
39
Class Monoplacophora
  • This group was known only from fossils until
    specimens of Neopilina (from Greek Neo new and
    pilos felt cap) were dredged from the ocean
    bottom in 1952 off west coast of Costa Rica. Two
    species are known.
  • From above, the Monoplacophorans appear
    limpet-like. The shell is similar in appearance
    and size (about 2.5 cm).
  • However, from below Neopilina is unmistakable
    having serially repeated gills and internally
    serially repeated nephridia (kidneys) and gonads.

40
Neopilina
41
Class Monoplacophora
  • There is debate whether the serial repetition
    indicates relatedness of the molluscs to the
    annelids with their segmented (metameric) bodies
    and suggests that their common ancestor was
    segmented.
  • Most authors consider Neopilina exhibits only
    pseudometamerism and that the common ancestor was
    not segmented.

42
Class Polyplacophora chitons
  • Chitons are somewhat dorsoventrally flattened
    with a dorsal surface covered with a series of
    usually 8 (rarely 7) overlapping, articulated,
    calcareous plates.
  • Hence, the name Polyplacophora (bearing many
    plates).

43
http//farm4.static.flickr.com/3216/3048505966_342
51e67d3.jpg?v0
Chitons
44
10.8
Chiton
45
Class Polyplacophora chitons
  • Most chitons are small (lt5cm), the largest
    reaching 30cm. About 600 living species.
  • They are most often found on rocky surfaces in
    the intertidal zone where they graze on algae,
    which they scrape off using their radula.
  • Chitons grip their substrate firmly and are very
    hard to dislodge. If displaced, they curl into a
    ball for protection like an armadillo.

46
Class Scaphopoda tusk shells
  • Tusk shells are marine molluscs that have a long
    tusk-like shell open at both ends. Most of the
    350 or so species are found in the deep sea, but
    some occur in shallower water.
  • They burrow into sediments using their foot and
    the narrow end of the shell protrudes into the
    water above. They lack gills and gas exchange
    occurs across the mantle itself.

47
Class Scaphopoda tusk shells
  • Most food is detritus and protozoa and other
    small organisms found in the sediment.
  • Food is captured using threadlike mucus- covered
    tentacles called captacula which have adhesive
    knobs on their end.
  • Scaphopods appear to be most closely related to
    the bivalves.

48
Fig 10.10
49
Class Bivalvia
  • The Bivalvia (two valves shells) are sometimes
    referred to as the Pelecypoda (Greek pelekus,
    hatchet, pous foot).
  • These are the two-shelled molluscs and include
    the mussels, scallops, oysters, clams, and
    shipworms.

50
Class Bivalvia
  • The Bivalvia are the second largest class of
    molluscs after the Gastropoda with approximately
    15,000 species.

51
Characteristics of Class Bivalvia
  • Bivalves are laterally compressed.
  • There are two shells (valves) and these are held
    together dorsally by a hinge ligament.
  • The ligament is constructed so that when the
    shell is closed the outer or dorsal part is
    stretched and the inner part compressed.

52
Characteristics of Class Bivalvia
  • Adductor muscles oppose the natural tendency of
    the shells to gape open. When contracted they
    pull the shells together to close them.
  • When the adductor muscles relax the shells gape
    open.

53
Fig 10.22 show adductor muscles 10.23 c
adductors open and shut.
54
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55
Class Bivalvia
  • The shells in bivalves are mostly for protection,
    but in some species they have evolved to be used
    for boring into wood (as in shipworms) or rock
    (rock borers).
  • In the scallops the shells can be used to move.
    They clap the shells together, which moves the
    scallop forward in spurts.

56
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57
Class Bivalvia
  • The foot like the rest of the body is also
    laterally compressed and this helps the bivalve
    work its way into soft sediments.
  • To burrow, a bivalve extends its foot and pumps
    blood into it, which causes the foot to swell.
  • The bivalve is now anchored by the foot and
    longitudinal muscles contract to draw the animal
    forward.

58
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59
Gas Exchange and Filter Feeding
  • Most bivalves are filter feeders and they have
    evolved a series of adaptations to this feeding
    mechanism.
  • The visceral mass in bivalves is suspended from
    the dorsal midline (where the two shells join)
    and the foot is attached at the bottom/front
    anteroventrally.

60
Gas Exchange and Filter Feeding
  • The mantle is greatly enlarged and forms a large
    sheet of tissue lying beneath the valves.
  • Under the mantle is a large mantle cavity in
    which are found large gills, which function not
    only in gas exchange, but also in filter feeding.

61
Fig 10.22 show adductor muscles 10.23 c
adductors open and shut.
62
Gas Exchange and Filter Feeding
  • There is a one-way flow of water through a
    bivalve.
  • Water enters the bivalve through an incurrent
    aperture drawn in by ciliary action. The water
    flows into the mantle cavity and enters gill
    pores in the gill.
  • It then moves up to a tube called the
    suprabranchial (above the gills) chamber and
    flows out of an excurrent aperture.

63
10.27
64
Gas Exchange and Filter Feeding
  • The gills are well supplied with blood pumped
    there by the well-developed three chambered
    heart.

65
Gas Exchange and Filter Feeding
  • In some marine bivalves the incurrent and
    excurrent openings are extended into long siphons
    formed by the mantle being extended into tubes.
  • The siphons allow a bivalve to burrow beneath the
    sediment, yet still respire and feed.

66
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67
Incurrent and excurrent siphons of Northwest
ugly clam
10.26
68
Gas Exchange and Filter Feeding
  • In addition to gas exchange, the gills are
    involved in filter feeding.
  • Gland cells secrete large quantities of mucus.
    This mucus traps particles suspended in the water
    passing through the gill pores.

69
Gas Exchange and Filter Feeding
  • The particles trapped in the mucus move along
    ciliary tracts to the mouth where they are
    consumed.
  • Filter feeding in bivalves is very efficient and
    extremely small particles are captured.
  • Mussels, for example, filter out essentially all
    particles 3-5 microns in diameter and 50 of
    particles 1-2 microns in diameter.

70
Class Bivalvia shipworms
  • Not all bivalves are filter feeders. Some, the
    shipworms, make use of a surprising resource for
    marine animals wood.
  • Large quantities of wood end up in the ocean
    carried downstream from rivers and provide a
    valuable resource for organisms that can exploit
    it.

71
Class Bivalvia shipworms
  • Shipworms are superficially wormlike bivalves in
    which two anterior valves are used as rasping
    organs to burrow into wood (like all bivalves,
    shipworms, dont have a radula).

72
Shipworm (note paired valves at anterior end).
73
10.24
Shipworm in wood
74
Class Bivalvia shipworms
  • Shipworm larvae float in the ocean until they
    encounter a piece of wood and then settle.
  • They bore into the wood and live their entire
    life in the same piece.

75
Shipworms
76
Class Bivalvia shipworms
  • Wood is difficult to digest and just like
    termites on land, shipworms, use bacteria to do
    the actual digesting.
  • The bacteria also take nitrogen from the water
    and use it to make proteins. In exchange, the
    shipworm provides the bacteria with other
    nutrients.

77
Class Bivalvia shipworms
  • Shipworms are extremely destructive and they can
    do severe damage to wooden boats.
  • On Christopher Columbus 4th voyage to the
    Americas he was marooned when two of his ships
    were destroyed by shipworms. In addition, the
    Hudson River piers were destroyed largely by
    shipworms.

78
Class Bivalvia reproduction
  • In bivalves the sexes are separate and
    fertilization is usually external.
  • Marine larvae typically go through three
    free-swimming stages before settling
    trochophore, veliger and spat.

79
Class Bivalvia reproduction
  • In freshwater clams fertilization usually occurs
    internally and the larvae go through early
    development in a brood chamber.
  • The larvae develop into specialized veligers
    called glochidia.
  • These are released into the water and attach to
    passing fishs gills where they feed and
    hitchhike as parasites for several weeks before
    dropping off and settling.

80
Glochidium
81
Freshwater clam lifecycle.
82
Glochidium attached to fish gills
83
Class Bivalvia reproduction
  • Some freshwater clams have evolved behaviors and
    structures that enhance the chances of their
    larvae successfully attaching to a fish.
  • They have evolved mantle edges that look like
    worms or small fish, which the clam can move in a
    realistic manner. These lures attract hungry
    fish, which when they approach are sprayed with
    glochidia by the clam.

84
10.29
85
Class Gastropoda
  • The Gastropoda are by far the largest class of
    molluscs.
  • There are about 40,000 living species and 15,000
    known fossil species.
  • They include snails, slugs, whelks, limpets,
    conchs, sea slugs, sea hares and periwinkles.

86
Class Gastropoda
  • Most gastropods are slow moving as most have
    heavy shells and depend on using their single
    slow moving foot to get around.
  • The shell, if present, is almost always a
    one-piece shell and is often coiled.

87
Class Gastropoda
  • Some snails possess an operculum a proteinaceous
    lid that can be used to close the shells
    opening.
  • Closing the operculum reduces water loss.

88
Class Gastropoda
  • Gastropods are basically bilaterally symmetrical.
  • However, because of the process of torsion, a
    twisting process that occurs during the
    gastropods embryonic/larval development, the
    visceral mass has become asymmetrical.

89
Class Gastropoda
  • Torsion moves the mantle cavity to the front.
  • Pre-torsion the mouth is at the front and the
    anus at the rear.
  • Because of the twisting of the viscera the anus
    now opens anteriorly. As a result, there is a
    problem with fouling as waste tends to wash back
    over the gills.

90
10.11
91
Class Gastropoda
  • Given the fouling problem, there must be a
    selective advantage to torsion.
  • It has been suggested that twisting the viscera
    enlarges the space within the shell for the head
    to be withdrawn, which serves an obvious
    protective function.
  • It is also possible that moving the mantle cavity
    to the front allows sense organs (osphradia) to
    better sample the water as the animal moves
    forward.

92
Class Gastropoda
  • Coiling Most gastropods have coiled shells.
    Coiling and torsion were separate evolutionary
    developments and coiling preceded torsion.
  • Coiling appears to have evolved as a way of more
    compactly packing the gastropod into its shell.

93
Class Gastropoda
  • Early gastropod shells were bilaterally
    symmetrical with all of the whorls on a single
    plane.
  • Thus, each new whorl wrapped around the ones
    inside which took up a lot of space and required
    a larger shell.

94
Class Gastropoda
  • By coiling the shell by drawing the apex out to
    the side so that each whorl wraps to the inside
    of the previous one much less space is required.
  • Because this arrangement is unbalanced with a lot
    of weight hanging to one side, selection has
    favored the shell shifting upwards and back so
    that the shell is at an oblique angle to the
    longitudinal axis of the foot.

95
Helix aspera. http//farm1.static.flickr.com/96/2
34155433_3af927b83f.jpg
96
10.12
97
Class Gastropoda
  • As a consequence of the shell shifting in this
    way the right side of the mantle cavity became
    compressed.
  • This compression has resulted in all but a few
    gastropods losing their right gills and kidneys
    thus becoming bilaterally asymmetrical.

98
Class Gastropoda
  • The loss of the right gill also has largely
    solved the fouling problem of torsion.
  • In most aquatic gastropods there is a one way
    flow of water, which comes in the left side and
    exits the right side (where the anus and
    nephridiopore open) carrying wastes away with it.

99
Class Gastropoda
  • Some primitive gastropods such as abalones, which
    have retained two gills, solve the fouling
    problem by venting excurrent water through a
    dorsal opening in the shell above the anus.

100
10.14A
Abalone
101
Gastropoda feeding habits
  • Being such a large group it is not surprising
    that gastropods feed on a wide variety of
    different foods.
  • All of them however make some use of the radula
    in foraging.
  • Many are herbivores and feed by rasping algae or
    vegetation with their radula. Others are
    scavengers or predators and use their radular
    teeth to tear apart their prey.

102
Gastropoda feeding habits
  • Some gastropods feed by drilling holes in other
    molluscs.
  • Using its radula and a shell-dissolving secretion
    (that contains sulphuric acid), an oyster borer
    can easily drill a hole in the shell of a mollusc
    such as an oyster.
  • It then inserts an extensible proboscis and
    consumes the animal inside. Oyster borers, not
    surprisingly, are major pests of oyster farms.

103
Gastropoda feeding habits
  • Cone shells are also interesting predatory
    gastropods.
  • They are highly venomous and use their proboscis
    to kill their vertebrate prey.
  • The proboscis when extended looks like a worm and
    it lures a fish that attempts to eat it.
  • When the fish grabs the proboscis, it is stung in
    the mouth with a radular tooth that injects a
    lethal venom.

104
10.16
105
Gastropoda feeding habits
  • The cone shell engulfs the fish using its
    extensible stomach and later regurgitates the
    indigestible remains.
  • Cone shells are highly toxic and several species
    are capable of killing humans.

106
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107
Gastropoda reproduction
  • There are both dioecious and hermaphroditic
    gastropods.
  • Mating in hermaphroditic land snails is very
    interesting. During the process of mating the
    snails entwine and both insert sperm in the
    other.
  • During the process both individuals attempt to
    insert love darts into the other individual.

108
Gastropoda reproduction
  • For a long time it was thought that the love dart
    was a nuptial gift of calcium.
  • However, it appears that the love darts play a
    role is sperm competition.

109
Gastropoda reproduction
  • Donated sperm are moved to a storage duct by the
    recipient and used for later fertilization.
    However, much of the sperm is digested instead.
  • Mucus in the love dart temporarily contracts a
    part of the female reproductive system and that
    allows more sperm to reach the storage area
    safely.

110
Mating snails.
111
Gastropoda Classification
  • Major groups of gastropods. Three subclasses are
    traditionally recognized
  • Prosobranchia
  • Opisthobranchia
  • Pulmonata

112
Gastropoda Prosobranchia
  • Prosobranchs are the largest subclass of
    gastropods.
  • Most species are marine, but many freshwater
    species and a few terrestrial forms are known.
  • Prosobranchs include such familiar gastropods as
    conchs, periwinkles, whelks, cowries, oyster
    borers, and limpets.

113
10.18A
Keyhole limpet
114
Gastropoda Prosobranchia
  • Prosobranchs have an operculum (lacking in
    pulmonates), and most have a spirally coiled
    shell (in a few, the shell is cup-shaped or
    tubular).
  • The head includes eyes that are located on
    tentacles. The mantle cavity is anteriorally
    directed and near the head.

115
Cone shell
116
Volute
117
GastropodaOpisthobranchia
  • The opisthobranchs are a group of marine
    gastropods that include the sea slugs, sea
    butterflies, sea hares, and others.
  • The bodies of most opisthobranchs show evidence
    of detorsion in which the torsion in other groups
    of gastropods is reversed so that the mantle
    cavity is at the rear.

118
Sea hare
119
GastropodaOpisthobranchia
  • Opisthobranchs generally have no mantle cavity
    and generally have no operculum.
  • Many species of opisthobranch have no gills and
    instead gas exchange takes place across the skin,
    which may be folded or possess projections that
    increase the surface area.

120
Unidentified nudibranch
121
GastropodaOpisthobranchia
  • Shells are present in some but not all
    opisthobranchs and in many the shell, even if
    present, is reduced or may be internal.
  • Nudibranchs have no shell and are very
    beautifully colored being among the most striking
    of all molluscs.

122
Unidentified nudibranch (sea slug)
123
Phylladia, a nudibranch
124
Gastropoda Pulmonata
  • Pulmonates include the land snails and slugs.
    There are also a handful of marine species.
  • Most species possess a coiled shell, but
    obviously slugs do not. Terrestrial species
    usually possess two pairs of tentacles, the
    posterior pair of which have eyes.
  • Some detorsion has occurred in many species.

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10.20A
Pulmonate land snail. Note the presence of two
pairs of tentacles.
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Gastropoda Pulmonata
  • The pulmonates are so called because they possess
    lungs in the mantle cavity rather than gills.
  • Contractions of the mantle floor help to move air
    in and out to the lungs and they lungs open to
    the outside through an opening called the
    pneumostome.

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10.20b
128
Gastropoda Pulmonata
  • Pulmonates are dioecious and hermaphroditic, as
    are the prosobranchs, but there is no larval
    form. Instead pulmonates develop directly in the
    egg.

129
Escargot (Helix pomatia)
130
Banana slug
131
Cephalopods
  • The cephalopods (head foot from Greek kephale
    head and pous foot) are the most complex of the
    molluscs and are among the most complex of all
    invertebrates
  • All are marine and the group includes squids,
    octopuses, cuttlefishes and nautiluses.

132
Cuttlefish
Nautilus
Octopus
133
Cephalopods
  • All of the cephalopods have tentacles that they
    use to grasp and manipulate objects.
  • Octopuses have 8, cuttlefish and squids have 10
    including (in squids) two extended arms that
    flattened at the end and nautiluses have 38
    suckerless tentacles.

134
Cephalopods
  • Tentacles are equipped with suckers, and in
    squids many of them also have hooks on them.
  • Cephalopods possess a radula, but more
    importantly, have a horny parrotlike beak which
    they can use to bite and tear tissue. They also
    inject poison when they bite.

135
Cephalopods
  • The brain in cephalopods is well developed and
    octopuses in particular are quite intelligent.
    They can solve problems and have a memory.
  • The enlarged brain also equips them for an
    active predatory lifestyle.

136
Cephalopods
  • Squids are fast, active hunters that seize prey
    using their long arms.
  • Octopuses, in contrast, are slower moving and
    follow a sit-and-wait strategy of hunting, slowly
    searching for prey or exploring in holes and
    crevices to find prey.
  • An octopus can squeeze into extremely small
    spaces because it has few hard parts and so can
    seek prey in places that seem inaccessible to it.

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Cephalopods
  • http//www.youtube.com/watch?vL_6jQ6b-Tqg

138
Cephalopods vision
  • The cephalopods possess eyes that are comparable
    in structure to those of vertebrates, possessing
    a retina, lens, cornea, and iris. There is some
    debate as to how well they see, but in certain
    aspects their eye is better designed than that of
    humans.

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Cephalopods vision
  • In humans the retinal cells face the back of the
    eye and their nervous connections and associated
    blood vessels run between the retina and the
    lens.
  • These interfere with the flow of light and also
    require a blind spot in the eye where there are
    no retinal cells where the nerves connect to the
    optic nerve.

141
Cephalopods vision
  • In cephalopods the retinal cells are innervated
    and supplied with blood from behind the retina
    and so do not suffer from these problems that the
    vertebrate eye has.

142
Cephalopods jet propulsion
  • The cephalopods are highly active predators and
    this has driven the development of intelligence
    and eyesight in the group.
  • In addition to being intelligent, cephalopods are
    highly mobile and quick moving as they have
    developed jet propulsion.

143
Cephalopods jet propulsion
  • Most cephalopods swim by rapidly expelling water
    from the mantle cavity.
  • The mantle contains two types of muscle fibers
    radial and circular. During the inhalant phase
    the circular fibers are relaxed and the radial
    fibers contract.
  • As a result the volume of the mantle cavity
    increases and water is sucked in.

144
Cephalopods jet propulsion
  • When the mantle cavity is full, the radial fibers
    relax and the circular fibers contract.
  • This seals the edges of the mantle tightly around
    the head and simultaneously increases the water
    pressure inside the mantle cavity.
  • The water is then forcefully expelled through the
    ventral tubular funnel, which pushes the animal
    rapidly in the opposite direction.

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Cephalopods jet propulsion
  • The funnel is very mobile and can be oriented
    forward or backwards allowing the cephalopod to
    move in any direction.
  • Cephalopods have fine control of the force with
    which water is expelled, which allows them to
    swim in a very controlled manner.

147
Cephalopods jet propulsion
  • Squid and cuttlefish have the best swimming
    ability and can hover and vary their speed
    through the water with ease.
  • Squid are the fastest swimming invertebrates and
    their bodies show numerous adaptations to this.
    The body is long and tapered and possesses a pair
    of stabilizing lateral fins.

148
Cephalopods jet propulsion
  • One group of squids, the Onycoteuthidae, are
    known as the flying squids.
  • They have highly tapered bodies and very well
    developed funnels and fins and frequently leap
    from the water and glide through the air.

149
Cephalopods chromatophores
  • The skin of most cephalopods contains
    chromatophores, which are pigment-containing
    cells that can be expanded or contracted to
    change the animals color.
  • Tiny muscles attached to the periphery of the
    chromatophores contract to expand the cell into a
    plate and make the color more obvious or relax to
    concentrate the pigment into a spot and make it
    less obvious.

150
Cephalopods chromatophores
  • Chromatophores contain various different colors
    of pigment including yellow, orange, red, blue
    and black and particluar colors may be arranged
    together in layers.
  • The chromatophores effect is enhanced by a layer
    of iridocytes below the layer of chromatophores,
    which differentially reflect light.

151
Cephalopods chromatophores
  • Together the chromatophores and iridocytes enable
    a cephalopod to quickly change its color even to
    the extent of sending waves of color sweeping
    over its body as some octopuses will do when
    threatened.

152
Cephalopods chromatophores
  • Hormones and the nervous system control the color
    changes.
  • Color changes can occur when the animal is
    alarmed and putting on a defensive display or is
    displaying to a potential mate, but also provide
    spectacular camouflage enabling the animals to
    hide from potential predators and prey.

153
Cephalopods ink release
  • Another means of deterring attack that
    cephalopods (except Nautilus) possess is the
    ability to squirt ink.
  • They have a large ink sac located near the
    intestines, which opens via a duct and terminal
    ejecting ampulla into the rectum near the anus.

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Cephalopods ink release
  • The ink is brown/black and contains a high
    concentration of melanin pigment.
  • When startled, the cephalopod releases a jet of
    ink through the anus, which forms a cloud that
    distracts the predator, while alkaloids in the
    ink are thought to be objectionable and
    anesthetize chemoreceptors in fish.

156
Octopus camouflage and ink release
  • http//www.youtube.com/watch?vckP8msIgMYE

157
Reproduction in Cephalopods
  • Almost all cephalopods are dioecious.
  • Sperm are encased within a spermatophore and are
    kept until needed in a storage gland.
  • Mating involves couplation, but males do not
    possess a penis. Instead, one of the arms is
    modified as an intromittent organ or hectocotylus.

158
Reproduction in Cephalopods
  • The hectocotylus in squids has an area with
    smaller suckers that form an adhesion area for
    the spermatophore. In Octopus the tip of the arm
    has a spoon-like depression.
  • A male cephalopod usually displays (for example
    rapidly changing color or displaying its arms)
    for the female before she agrees to mate.

159
Reproduction in Cephalopods
  • Copulation involves the male transferring a
    spermatophore using his hectocotylus to the
    females mantle cavity where it attached to the
    mantle wall near where the oviducts open.
  • Eggs are fertilized as they are released from the
    oviduct.

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10.34B
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Reproduction in Cephalopods
  • There are no larval stages in cephalopod
    development as development occurs directly within
    the egg.
  • The young cephalopod may be planktonic for a time
    e.g. some octopus usually do not assume a benthic
    life until they reach a larger size.

162
Reproduction in Cephalopods
  • Many cephalopods are not long-lived and die after
    a single spawning e.g. smaller squid, which live
    to only 1-3 years of age.
  • Octopus vulgaris also dies after producing a
    single brood at age 3-4.

163
Cephalopods Giant squid
  • The cephalopods include the largest of all
    invertebrates the giant squid and the colossal
    squid.
  • These deep sea species can measure (including
    tentacles) as much as 60 feet and weigh up to
    1,000 lbs. The mantle length is 15-18 feet with
    the head about 3 feet.

164
Giant squid washed a shore in New Zealand in 1996
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Cephalopods Giant squid
  • Giant squid are known mainly from stranded
    specimens or those trapped in fishing nets and
    live specimens have not been studied, although a
    few photos of live ones have been taken.

166
Photos of live giant squid taken at a depth
of about 3,000 feet off coast of Japan.
167
Cephalopods Giant squid
  • Giant squid are predators and appear to have
    excellent vision (they have the biggest eyes of
    any known organisms up to 10 inches in
    diameter).
  • They are believed to prey mainly on fish and
    other squid.

168
Cephalopods Giant squid
  • For many years giant squid were known only from
    the sucker marks they left on the skin of sperm
    whales and their beaks taken from sperm whale
    stomachs.
  • Based on how often they have turned up in sperm
    whale stomachs, giant squid appear to be an
    important component of their diet.

169
Sperm whale attacking giant squid
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Nautilus
  • The Nautilus is the only cephalopod with an
    external shell, which it uses as a float to keep
    itself upright in the water.
  • In squids and cuttlefish the shell is reduced and
    internal, and completely lacking in the octopus.
  • The familiar cuttlebone that is fed to caged
    birds as a source of calcium is the internal
    shell or pen of a squid.

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Nautilus
172
Nautilus
  • Unlike in gastropods the Nautilus shell is
    divided into internal compartments.
  • The Nautilus lives only in the outermost
    compartment and as it grows it secretes a new one
    and moves into it closing off the compartment
    behind.

173
Nautilus
  • The rear compartments are filled with air, which
    make the shell buoyant and hold it up as the
    animal forages along the bottom.
  • The chambers of a Nautilus shell are connected by
    a tube called a siphuncle, which secretes air
    into the space.

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10.31
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