Transcript Chapter 12 Molluscan Success

Evolutionary Perspective
 Octopuses, squids, and cuttlefish (all cephalopods) are
some of the most adept predators of the invertebrate
world
 Class Cephalopoda once numbered approx. 9000
species but now only have about 550 species
 Numerically, the mollusks are twice are successful as
vertebrates
 There are nearly 100,000 species of mollusc. Most
belong to two classes: Gastropoda and Bivalvia
E.P. con’t
 Molluscs and all animals from this point on are
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triploblastic.
They are the first to possess a coelom
Coelom performs multiple functions (refer to previous
notes for these functions)
Molluscs are protostomate animals
Coelom forms by the splitting of the mesoderm.
The unique and successful body form of the molluscs
resulted in reduced size and importance of the
coelom.
Molluscan Characteristics
 1. Body of two parts: head-foot and visceral mass
 2. Mantle that secretes a calcareous shell and covers the visceral mass
 3. Mantle cavity functions in excretion, gas exchange, elimination of
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digestive wastes, and release of reproductive products
4. Bilateral symmetry
5. Protostome characteristics – trochophose larve, spiral cleavage, and
schizocoelous coelom formation
6. Coelom reduced to cavities surrounding heart, nephridia, and
gonads
7. Open circulatory system in all but one class (Cephalopods)
8. Radula usually present and used in scraping food
Three Main Regions
1. Head-foot
 Elongate with an anterior head
 Contains mouth and nervous and sensory structures
 Elongate foot – used for attachment and locomotion
2. Visceral Mass
 Contains the organs of digestion, circulation,
reproduction, and excretion
 Positioned dorsal to the head-foot
Con’t
Mantle
 Usually attaches to the visceral mass, enfolds most of the
body, and may secrete a shell that overlies the mantle
 Shell is secreted in 3 layers:
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Outer layer – periostracum. Secreted by the mantle’s outer margin cells
Middle layer – prismatic. Thickest of the three. Consists of calcium
carbonate and organic materials. Secreted by mantle’s outer margin cells
Inner layer – nacreous layer. Formed from thin sheets of calcium
carbonate alternating with organic matter. Secreted by the entire
epithelial border of the mantle. Nacre thickens the shell
Con’t
 In addition to the three main body regions, there is the
mantle cavity and the radula.
 Mantle cavity – space between mantle and the foot. It
is open to the outside. Functions in gas exchange,
excretion, elimination of digestive wastes, and release
of reproductive products
 Radula – made up of achitinous and posteriorly curved
teeth. Overlays a fleshy tongue like structure called an
odontophore. The odontophore can be protruded from
the mouth. Food is scraped off and passed posteriorly
to the digestive tract
Class Gastropoda
 Includes snail, slugs, and limpets
 Over 35,000 living species
 Largest and most varied molluscan class
 Found in marine, freshwater, and terrestrial habitats
 Escargot is considered a delicacy in restaurants
Con’t
 Cone shells are the only members of the gastropod
class that may be seriously harmful to man. The
venomous sting of some cone shell species may be
deadly even for an adult.
Impacts on Humans
 Garden pests
 Serve as an intermediate hosts for trematode parasites
of humans
 Fasciola hepatica, the liver fluke, has a complex life
cycle, requiring water snails as intermediate hosts.
Humans and other mammals acquire the organism by
eating cyst-contaminated water plants. Watercress is a
common source of the parasite for humans. The cysts
release immature flukes that migrate to the liver and
gallbladder. A high load of the parasite may obstruct
the biliary tract.
Torsion
 Torsion positions the gills, anus, and openings from the
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excretory and reproductive systems the head and nerve cords.
Digestive tract becomes U shaped
Three advantages:
1. Head enters shell the first. Provides protection from predators.
Some snails have a operculum that closes the opening of the
shell and prevents dessication
2. Allows clean water from the front of the snail to enter the
mantle cavity
3. It makes the snail more sensitive to stimuli coming from the
direction in which it moves
Shell Coiling
 Asymmetrically coiled into compact forms
 Leaves less room for organs so now organs are single,
not paired
Locomotion
 Flattened foot
 Often ciliated and covered with gland cells
 Creeps across the substrate
 Small gastropods use cilia to propel themselves over
mucous trails
 Larger gastropods use wave-like muscular contractions
 The foot can be used for clinging or for swimming
Feeding and Digestion
 Most feed by scraping algae and other organisms from the
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substrate
Others feed on larger plants
Some are scavengers, parasites, or predators
Some have an extensible proboscis (contains the radula) to
get to those hard-to-reach places
Digestive tract is ciliated.
Digestive gland releases enzymes and acid into the
stomach
Food trapped on the protostyle is freed and digested
Wastes form fecal pellets in the intestines
Maintenance Functions
 Gastropods have one gill
 Some have a siphon. Used by burrowing species to extend
to the surface of the substrate and bring in water
 Gills are lost or reduced in land snails – have a rich vascular
mantle for gas exchanged between blood and air
 Open circulatory system. Blood leaves the vessels and
directly bathes cells in tissue spaces called sinuses. Heart is
a single, muscular mass consisting of ventricle and two
auricles. Some have one ventricle, one auricle due to coiling
Con’t
 Blood acts like a hydraulic skeleton. They contract muscles to force
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fluid into distant structures to push it forward
The nervous system of the modern gastropod is a concentration of
nervous tissues into large ganglia, especially in the head
Well developed sensory structures:
Eyes at/near the base of the tentacles
Statocysts are in the foot (used for equilibrium and balance)
Osphradia – chemoreceptors in the mantle cavity that detect
sediment and chamicals in inhalant water or air. In predatory
gastropods, it helps detect prey
Nephrida – modifies excretory wastes by selectively reabsorbing
certain ions and organis molecules. Aquatic gastropods excrete
ammonia, terrestrial gastropods convert ammonia into to uric acid (a
semisolid form that helps conserve water)
Reproduction/Development
 Many marine snails are dioecious. Ducts release
gametes into the sea for external fertilization.
 Other snails are monoecious. Internal, crossfertilization occurs. Fertilized egg is protected by a
capsule.
 Snails that are monoecious can be protandric. Testes
develop first, and after they degenerate, ovaries
mature.
 Eggs are shed singly or in mass for external
fertilization
Con’t
 Eggs are deposited into moist environments (leaf
litter)
 Calcareous shell may encapsulate them