chapter 11 molluscan success. evolutionary perspective triploblastic coelomate very successful...
TRANSCRIPT
Chapter 11
Molluscan Success
Evolutionary Perspective
• Triploblastic• Coelomate• Very successful
– 100,000 living species• Relationships to other animals
– Lophotrochozoans
Molluscan Characteristics1. Body of two parts: head-foot and visceral mass2. Mantle that secretes a calcareous shell and covers the visceral
mass3. Mantle cavity functions in excretion, gas exchange, elimination of
digestive wastes, and release of reproductive products.4. Bilateral symmetry5. Trochophore larvae, spiral cleavage, and schizocoel coelom
formation6. Coelom reduced 7. Open circulatory system (except Cephalopoda)8. Radula usually present
Body Organization• Head-foot
– Elongate – Mouth– Attachment and locomotion
• Visceral mass– Dorsal to head-foot– Organs of digestion, circulation, reproduction
• Mantle– Enfolds body– Secretes shell
• Mantle cavity– Gas exchange, excretion, elimination of digestive wastes and
reproductive products• Radula
– Supported by odontophore– Rasping food
(a)
(b)
Class Gastropoda• Snails and slugs• 35,000 living species• Torsion
– 180o counterclockwise twisting of visceral mass, mantle, and mantle cavity during development
– Possible adaptive significance• Head enters shell first.• Clean water enters anteriorly oriented
mantle cavity opening.• Mantle sensory organs move to head region.
Class Gastropoda
• Shell coiling– Earliest fossils, one plane– Modern, asymmetrical
• More compact• Internal organs asymmetrical and
sometimes no longer paired
• Locomotion– Flattened foot– Cilia propel over mucous trail– Muscular waves
Class Gastropoda
• Feeding and digestion– Most scrape algae and attached organisms– Herbivores, predators, scavengers– Digestive tract
• Ciliated• Food incorporated into mucous mass called
protostyle.• Gas exchange
– One or two gills in mantle cavity– Land snails (pulmonates)
• Mantle cavity richly vascular for gas exchange with air
(a)
Other Maintenance Functions
• Open circulatory system– Blood bathes tissues in sinuses.– Heart
• Single ventricle and single auricle– Functions
• Transports nutrients and gases• Hydraulic skeleton
• Nervous system– Six ganglia plus nerve cords
• Sensory structures– Eyes at base or end of tentacles– Statocysts in foot– Osphradia in mantle cavity
Other Maintenance Functions
• Excretion– Single nephridium
• Result of shell coiling– Discharges into mantle cavity or
adjacent to mantle cavity (pulmonates)– Ammonia (aquatic species)– Uric acid (pulmonates)
Reproduction• Dioecious or monoecious
– External fertilization• Some dioecious marine species
– Copulation• Sperm transfer may be mutual or one-way.
– Eggs shed singly, in strings, or in masses
– Larval stages• Trochophore• Veliger
Gastropod Diversity• Subclasses
– Prosobranchia• 20,000 species• Mostly marine
– Opisthobranchia• 2,000 species• Mostly marine• Sea hares, sea slugs• Shell, mantle cavity, and gills reduced or lost
– Pulmonata• 17,000 species• Freshwater or terrestrial• Vascular mantle cavity serves as lung
(a) (b)
(c)
Class Bivalvia
• Clams, oysters, mussels, scallops• 30,000 species• Shell and associated structures
– Single shell consisting of two hinged valves
– Mantle sheetlike and covers laterally compressed body.
.
Gas Exchange, Filter Feeding, and Digestion
• Sedentary filter feeders– Loss of head and radula– Expansion of cilia-covered gills into
folded sheets (lamellae)• Cilia create water currents into and through
mantle cavity. – Gas exchange in water tubes – Food trapped along gill surface and transported to
food grooves and labial palps
– Digestion • Crystaline style and gastric shield
Other Maintenance Functions
• Open circulatory system– Mantle and gills oxygenate blood
• Nephridia – Below pericardial cavity– Open to suprabranchial chamber
• Nervous system– Three pairs of interconnected ganglia– Sensory receptors at mantle margin
Reproduction and Development
• Most dioecious• Gonads within visceral mass• External fertilization• Trochophore and veliger larval stages• Freshwater in family Unionidae
– Parasitic larval stage– Glochidium
(c)
.
Gooduck
Class Cephalopoda
• Squid, octopuses, cuttlefish, and nautiluses
• Foot modified into circle of tentacles or arms and incorporated into siphon
• Head in line with visceral mass• Muscular mantle
Class Cephalopoda• Shell
– Reduced or absent except in nautilus• Locomotion
– Jet propulsion using muscles of mantle compressing water within mantle cavity and siphon
• Feeding and Digestion– Predators
• Tentacles, jaws, radula– Digestive tract muscular with large digestive
glands
.
Other Maintenance Functions
• Closed circulatory system• Nervous system
– Large brains– Complex sensory structures
• Eyes• Statocysts
• Chromatophores– Color changes involved with courtship
and other displays• Ink glands
The cephalopod eye.
Learning
• Unparalleled in comparison to any other invertebrate and many vertebrates
• Evolved in response to predatory lifestyles
Reproduction and Development
• Dioecious• Male produces spermatophores
– Transfers to female’s mantle cavity using modified tentacle (hectocotylus)
• Eggs deposited singly or in masses attached to substrate.
• Eggs tended by parents.
Class Polyplacophora• Chitons• Reduced head, flattened foot, shell
consisting of eight dorsal valves, muscular mantle extends beyond margin of shell
• Feed on attached algae• Ladderlike nervous system• Dioecious with external fertilization
(a)
Class Scaphopoda
• Tooth shells or tusk shells• Marine, burrowing• Conical shell open at both ends• Dioecious with trochophore and
veliger larvae
Class Scaphopoda.
Class Monoplacophora
• Marine• Undivided arched shell• Broad, flat foot• Serially repeated pairs of gills and
foot retractor muscles• Dioecious
Class Monoplacophora.
Figure 11.23
Class Solenogastres
• Marine substrates• Lack shell• Crawl on ventral
foot• Minute calcareous
spicules• Carnivores
Class Caudofoveata
• Deep sea• Wormlike • Feed on foraminifera• Lack shell, foot, and nephridia• 120 species
Further Phylogenetic Considerations
• More than 500 million years old• Lophotrochozoa• Shell and muscular foot not ancestral
– Solenogaster spicules may be similar to ancestral “shell”.
– Muscular foot first seen in Polyplacophora.
• Quickly diversified into modern classes
Cladogram showing possible evolutionary relationships among the molluscs.