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Explore the diverse world of molluscs with this introduction to their characteristics, evolutionary contributions, and classes including Gastropoda and Bivalvia. Discover their unique respiratory and circulatory systems, reproductive strategies, and specialized organs like the radula and foot.
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Chapter 16 Molluscs Kingdom: Animalia Phylum: Mollusca Class Gastropoda Class Bivalvia Class Cephalopoda
I. Intro to P. Mollusca • + 90,000 living sp. • 70,000 fossil sp. • Chitons, snails, slugs, clams, mussels, oysters, squids, octopuses, nautiluses • Overview • soft body • microscopic → massive • 80% under 10 cm • giant squid - 900 kg, 20 m long • giant clams - 250 kg, 1.5 m dia. • herbivores/predators/filter-feeders/ parasites • marine/freshwater/terrestrial/tropics/polar • burrowers/borers
Intro to P. Mollusca A. Evolutionary Contributions • respiratory system • gas exchange though body surface + gills or “lungs” • open circulatory system • ♥ that pumps + vessels + blood sinuses • closed circulatory in cephalopods • highly developed direct eye in cephalopods • photosensitive cells in retina face light • derived from skin
Intro to P. Mollusca B. Mollusca Characteristics • bilateral symmetry • bilateral asymmetry • often defined head • ventral body wall forms foot • locomotion • folds of dorsal body wall form mantle w/ mantle cavity • mantle forms gills/lungs and secretes shell • ciliated surface epithelium w/ mucous glands + sensory nerve endings • coelom around ♥, sometimes gonads/kidneys/intestine • complete digestive tract w/ anus • rasping organ (radula)
B. Characteristics Continued… • open circulatory system w/ ♥ (3-chambered) + blood vessels + sinuse • 1-2 kidneys (metanephridia) • nervous system w/ paired cerebral/visceral ganglia w/ nerve cords • sensory organs for touch/smell/taste/equilibrium/vision • internal/external ciliary tracts
Phylum Mollusca • Coelom limited to space around the heart, gonads, and kidneys
P. Mollusca: Form/Func. C. Reproduction and life history • most dioecious, some hermaphroditic • egg hatches and produces a free-swimming larva (trochophore larva) • larva undergoes direct metamorphosis into small juvenile in chitons • many gastropods and bivalves, an intermediate larval stage (veliger) is derived state
P. Mollusca: Form/Func. a. Radula • protruding, rasping, tongue-like organ • unique to molluscs • lacking in bivalves • membrane w/ rows of tiny teeth pointed backward • scrapes/tears/cuts food particles from surface • carries particles to digestive tract • some sp. bore through hard material or harpoon prey
P. Mollusca: Form/Func. b. Foot • func. attachment or locomotion • usually ventral • modifications • hatchet foot in clams • siphon jet in squids • mucus secretions fucn. adhesion or help glide on cilia • uses • snails/bivalves extend foot hydraulically by engorgement w/ blood • burrowers extend foot into substrate, enlarge tip (anchor), draw forward • free-swimming sp. modified foot into wing/fin-like swimming agents
II. 1st Class of Molluscs: Gastropoda A. Class Gastropoda (snails/slugs/conches/sea slugs) • most diverse/abundant • +70,000 living sp., +15,000 fossil sp. • marine → air-breathing terrestrial snails/slugs • sluggish/sedentary animals • terrestrial gastropods restricted by soil mineral content/acidity/ temperature/dryness • intermediate hosts to many parasites • often harmed by larval stages
Gastropoda B. Respiration • performed by ctenidia in mantle cavity • have a highly vascular area in mantle that serves as lung • lung opens to outside by small opening (pneumostome)
Gastropoda C. Nervous/sensory systems • most have well-developed nervous systems • sense organs include eyes/tactile organs/ chemoreceptors • eyes • simple cups w/ photoreceptors → complex eye w/ lens/cornea
Gastropoda D. Reproduction • monoecious/dioecious • some discharge ova/sperm into water • external fertilization • young emerge as veliger larvae or pass this stage inside egg • some sp. ovoviviparous
III. 2nd Class of Molluscs: Bivalvia A. Class Bivalvia Characteristics(mussels/clams/scallops/oysters/shipworms) • 1–2 mm → giant South Pacific clams • most sedentary filter feeders • ciliary currents bring in food • lack head/radula/cephalization • marine/freshwater streams/ponds/lakes • native freshwater clams in U.S. most jeopardized animal group • +300 sp. once present • 12 extinct • 42 threatened/endangered • 88 of concern
Bivalvia B. Bivalvia Form/func. • 2 shells or valves held together by hinge ligament • valves drawn together by adductor muscles • umbo = bulge • oldest part of shell • growth occurs outward in rings • pearls produced when irritant lodged btwn shell/mantle • layers of nacre secreted around foreign material
B. continued… • freshwater clams sensitive to water quality changes (pollution/sedimentation) • Zebra mussels • serious exotic pest • clog water intake pipes • outcompete native sp. • 20,000 animals /m2 • 30,000-1,000,000 eggs/yr • altering biomass distribution
Bivalvia C. Locomotion • slender foot extended out btwn valves • blood pumped into foot • it swells/anchors bivalve in mud • shortening foot pulls clam forward • scallops clap valves to create jet propulsion • mantle edges direct the stream
Bivalvia D. Gills • both mantle/gills perform gaseous exchange • gills derived from primitive ctenidia by lengthening filaments to each side • water mvmt through clam • water enters incurrent siphon • enters water tubes via pores • out excurrent siphon
Classes of Molluscs: Bivalvia E. Circulatory system • 3-chambered ♥ w/ 2 auricles + 1 ventricle • some blood oxygenated in mantle • returns to ventricle via auricles • rest circulates via sinuses/kidneys/gills before back to auricles
Bivalvia F. Feeding • suspended organic matter enters incurrent siphon • gland cells on gills/labial palps secrete mucus to trap particles • food in mucous masses slide via grooves at lower edge of gills • suck in crustaceans by creating sudden inflow of water
Bivalvia G. Digestive system • stomach folded into ciliary tracts for sorting particles • rotating style helps free digestive enzymes, roll mucous food mass • dislodged particles directed to digestive gland or engulfed by amoebocytes
Bivalvia H. Reproduction and Development • dioecious • gametes discharged out in excurrent flow • fertilization usually external • internal fertilization • freshwater clams • sperm enter incurrent siphon to fertilize eggs in water tubes of gills • embryos develop as trochophore, veliger, spat larval stages • eventually sink to begin independent life on streambed
IV. 3rd Class Molluscs: Cephalopoda A. Class Cephalopoda (squid/octopus/nautilus/devilfish/cuttlefish) • 2 cm → giant squid (largest invertebrate known) • mostly marine • octopuses mostly intertidal • squids are deep-sea animals • predators • foot is in head region • modified for expelling water from mantle cavity
A. Class Cephalopoda continued… • cuttlefish shell enclosed in mantle • squid shell is thin strip enclosed in mantle (pen) • octopus has completely lost shell
Cephalopoda B. Locomotion • swim by forcefully expelling water via ventral funnel/siphon • octopuses crawl on bottom/swim backward by spurting water jets • webbing btwn arms allows swimming w/ medusa-like action • control direction/force of water • determines speed • lateral fins of squids/cuttlefishes stabilize • Active life of cephalopods reflected in respiratory/ circulatory/nervous systems
Cephalopoda C. Respiration/circulation • 1 pair of gills • 2 pair in Nautilus • muscular pumping keeps water flowing through mantle cavity when ↑ O2 demand • network of vessels conducting blood through gill filaments • blood goes to systemic circulation before gills • accessory/branchial ♥ at base of each gill ↑ pressure
Cephalopoda D. Nervous/sensory systems • largest brain of any invertebrate • giant nerve fibers • well-developed sense organs • complex eyes (cornea/lens/retina) • learn by reward/punishment/ observation of others • lack hearing • tactile/chemoreceptor cells in arms
Cephalopoda E. Communication • chemical/visual signals • cells in skin contain pigment granules (chromatophores) • contractions of muscle fibers attached to cell boundary cause cell to expand and change color pattern • deep-water cephalopods have luminescent organs • ink sac empties into rectum • ink gland secretes sepia when animal is alarmed
Cephalopoda F. Reproduction • dioecious • ♂ seminal vesicle • 1 arm of ♂ modified as intermittent organ • plucks spermatophore from mantle cavity and inserts it into ♀ • fertilized eggs leave oviduct, attach to substrate • hatch into juveniles w/ no free-swimming larval stage
Human eye Anatomy of the human eye PBS evolution of the eye Read Ken Miller Evolutionary Eye Development @ the PBS website