Molluscs - Doral Academy Preparatory

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Transcript Molluscs - Doral Academy Preparatory

Most Complex of the Invertebrates
Molluscan Success
• Most closely related to Annelids ( segmented
worms) based on protostome characteristics
• Have a coelom, are triploblastic
• Have declined dramatically with the rise of
vertebrates, Currently 100,000 living species
• Subkingdom Lophotrochozoan along with
annelids due to Larva (trochopore) and
feeding structure
Molluscan Characteristics
• Bilateral symmetry, two part body: Head- foot
and Visceral Hump
• Mantle- tissue surrounding visceral mass,
secretes calcerous shell
• Mantle cavity performs excretion, gas
exchange, elimination, and reproductive cells
• Open circulatory system in all but one class
(Cephalopoda )
• Coelom surrounding heart and gonads
Molluscan Shell Structure
• Shell secreted by Mantle, 3 Layers:
• Periostracum- outer layer, rough, Protein
• Prismatic- middle layer, thickest, calcium
carbonate and organic material
• Nacreous Layer- inner layer, thin sheets,
calcium carbonate alternating with organic
matter , Pearly layer
• Some Molluscs have lost shell entirely or
severely reduced
Gastropods (Stomach Footed)
• Largest Class of Molluscs, Includes snails,
limpets (Univalves) and slugs( Nudibranchs)
• Marine, freshwater and Terrestrial varieties
• Torsion of Body 180 degrees in shell- Positions
gills, visceral mass and Anus just behind head
• With torsion the head enters the shell first,
foot last, covered by operculum (calcareous)
• Digestive Tract U shaped, notches in shell
allow wastes to escape
Gastropoda Structure
• Shell Coiling- Asymmetrical in most modern
snails, each successive coil larger than and
ventral to the preceding coil, compact shell
• Flattened foot, ciliated with gland cells, larger
snails use muscular contractions to move
• Modified foot in some for clinging (abalones,
Limpets) or for swimming (Sea Hares)
Subclass Opisthbranchia
• Includes Sea Hares, Sea Slugs etc.
• Shell, mantle cavity and gills are reduced or
lost, if gills are present located on dorsal
surface, may have nematocysts, warning
• Many acquire undischarged nematocysts from
cnidaria they eat, Glaucus open ocean
swimming snail eats man-of-war
• Pterapods have a foot modified for swimming,
large food source in cold water
Gastropoda Feeding
• Radula- Rasping structure chitinous belt with
hooked teeth, muscles pull radula over a
cartilage structure which can be extended from
• Most feed by scraping Algae or small organisms
from substrate (deposit feeders), some
herbivores, scavengers, parasites and predators
• Trap food on mucus strings which extend into
stomach, rotated by cilia, digestion by enzymes
Gastropod Anatomy
• Carnivorous species have modified radula into
a hollow Harpoon for injecting Tetrodoxin
poison( causes respiratory paralysis) Cone
• Siphon- a rolled extension of the mantle,
extends out of shell, brings in water
• Open circulatory system with 2 chambered
Heart, Blood leaves blood vessels directly
bathes tissues in sinuses
Gastropod Nervous System
• Concentration of ganglia in Head
• Eyes may be simple photoreceptors or may
contain a Lens and Cornea, Stalked
• Statocysts- balance organs
• Osphridia- Chemoreceptors anterior wall of
mantle cavity, Smell
Gastropod Reproduction
• Many are Dioecious, discharge gametes into
sea, external fertilization
• Some Monoecious, internal cross fertilization
is the rule, some Protandric where testes
develop first and degenerate, ovaries develop
• Trochophore Larva- free swimming
• Veliger Larva- free swimming with foot, eyes,
shell, settle on bottom undergo
Bivalve Class
• Second largest Molluscan class, includes
Clams, Oysters, mussels and scallops
• Two convex halves to shell (valves), some
produce pearls
• Dorsal margin has proteinaceous hinge
• Teeth- tongue and groove modifications to
shell keep it from twisting
• Most Filter feeders, Sedentary, clean bacteria
from water
Bivalve Anatomy
• Loss of Head and Radula, Expansion of cilia
covered gills, Folded ( increase surface area)
• Water enters mantle cavity thru incurrent
siphon, movement of cilia does pumping,
water circulates thru water tubes in gills
exchange O2 and CO2 with blood, exits thru
excurrent opening in mantle
• Food is trapped by the gills and carried to the
mouth by cilia who sort thru it, expell particles
too large
Cephalopoda (head footed)
• Includes octopus, squid, cuttlefish and
• Anterior portion of foot modified into
tentacles or arms used for capturing prey,
attachment, locomotion and copulation
• Foot incorporated into a funnel attached to
the highly muscular mantle cavity used for
jetting locomotion
• Head in line with the visceral hump
• Nautilus is the only member with an external
shell, last chamber only one occupied the rest
are gas filled, control bouyancy
• Cuttle fish- internal cuttle bone, made of thin
• Squid- internal chitinous structure called a
pen, two long thin rods, also have cartilage
plates in the head, mantle wall, and neck for
protection and support
• Mantle contains radial and circular muscles,
radial bring water in, circular expels it
• Muscles attached to siphon control direction
of animals movement
• Posterior fins aid in steering, propulsion and
• Octopus normally crawl over bottom, jet to
escape predators
Feeding and Digestion
• Excellent Vision, vertebrate like eye, locates
prey, captured by tentacles or extensible arms
• Tentacles equipped with suction cups, some
have edge reinforced with tough protein and
• Mouth has jaws and a radula, Jaws are beak
shaped and radula rasps food into mouth
• Octopus have salivary glands that inject
Cephalopod Anatomy
• Closed circulatory System with arteries, veins
and capillaries, 3 chambered heart
• Brachial hearts at the base of each gill
• Nephridia collect wastes directly from blood
• Complex nervous system, statocyst,
• Chromatophores and irridophores change
color of body, camoflage, warning coloration
Class Polyplacophora
• Chitons- have a reduced head, flattened foot
and a shell divided into 8 segments (valves)
• Muscular mantle extends beyond shell covers
• Muscular foot creates suction holds animal
tight to rocks, if dislodged rolls into ball
• Feed on attached algae with radula
• Body Indicates early ancestral segmentation