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Chapter 42: Mollusks and Annelids

Chapter Outline

Chapter 42: Mollusks and Annelids

42.0 Introduction

  1. Higher Invertebrates Possess a Coelom
    1. Mollusks and Annelids Are Two Major Coelomic Phyla fig 42.1
    2. Lophophorate Phyla Are Intermediate Between Protostomes and Deuterostomes

42.1 Mollusks were among the first coelomates

  1. The Mollusks
    1. General Biology of the Mollusks: Phylum Mollusca
      1. Include snails, slugs, clams, scallops, oysters, cuttlefish, octopuses fig 42.2
      2. May possess durable shells
      3. Characterized by a coelom fig 42.3
      4. Widespread and abundant in marine, freshwater and terrestrial habitats
      5. Have successfully adapted to the land
        1. Terrestrial forms occur in seasonally moist places
        2. Surpassed by only arthropods in terms of success on land
    2. Economic Importance
      1. Sources of human food
      2. Production of pearls and shell material
      3. Have deleterious effects as well
        1. Destructive to submerged timbers
        2. Zebra mussels have negatively impacted American aquatic ecosystems
        3. Extensive crop and flower damage caused by snails and slugs
        4. Serve as intermediate hosts for many serious parasitic diseases
      4. Includes largest invertebrates
        1. Giant squid exist in great numbers, but are rarely caught
        2. Giant clam may reach 1.5 meters and 270 kilograms
  2. Body Plan of the Mollusks fig 42.4
    1. Exhibit Distinct Bilateral Symmetry
      1. Possess a visceral mass covered with soft epithelium
      2. Muscular foot is involved in locomotion
      3. May have a well-defined head at the anterior end of the body
      4. Digestive, reproductive and excretory organs located within visceral mass
      5. Folds from dorsal body wall form mantle
        1. Gills or lungs located within mantle cavity
        2. Gills are specialized portion of mantle
          1. Comprised of filamentous projections rich in blood vessels
          2. Highly efficient, extract 50% of oxygen from water
        3. Outer surface of mantle may secrete protective shell
          1. Horny protein outer layer
          2. Calcium carbonate middle layer
          3. Pearly inner layer
          4. Bivalve mollusks may produce pearls of shell material around foreign objects
      6. Some forms can withdraw into shell
      7. Continuous stream of water flows through mantle
        1. Water brings in oxygen and food in the bivalves
        2. Carries out wastes
        3. May carry out gametes when they are formed
      8. Mantle may be modified for propulsion in squid and octopuses
      9. Muscular foot adapted for locomotion, attachment, food capture
        1. Cephalopod foot divided into arms or tentacles
        2. Foot of free-swimming, pelagic forms modified into fins
      10. All mollusks except bivalves possess a rasping, tongue-like radula fig 42.5
        1. Composed if dozens of rows of chitinous teeth
        2. Some gastropods scrape algae with radula
        3. Predatory gastropods use radula to drill holes in shells of their prey
      11. Circulatory system consists of a heart and open flowing system
        1. Three-chambered heart: Two collect from gills, third pumps to body
        2. Cephalopods have a closed system of vessels and auxiliary hearts
      12. Coelom is primarily represented by small area around the heart
      13. Excretory system is more efficient than that of lower invertebrates
        1. Nitrogenous wastes removed by tubular nephridia
        2. Funnel-shaped, cilia-lined nephrostome collects waste from coelom
        3. Coiled tube from nephrostome connects to bladder
        4. Bladder connected to excretory pore
        5. Wastes collected from only coelom around the heart
        6. Waste discharged into mantle cavity
        7. Nutrients and salts reabsorbed to maintain osmotic balance
      14. Advantages of a closed circulatory system
        1. Coiled tubule of nephridium surrounded by capillary network
          1. Wastes extracted from circulatory system through capillaries
          2. Transferred to nephridium and discharged
          3. Salts, water, other materials reabsorbed from nephridium tubule
          4. Returned to capillary circulation
        2. Far more efficient than flame cells of acoelomates
        3. Flame cells pick up wastes only from body fluids
        4. One of earliest lines to develop efficient circulatory system
    2. Reproduction in Mollusks
      1. Most have separate sexes, few hermaphroditic forms
        1. Cross-fertilization is the rule, even in hermaphrodites
        2. Some may change sex within one season
      2. Mollusks dispersed through larval forms
        1. Many form free-swimming trochophore larvae fig 42.6a
        2. A second free-swimming veliger stage may precede adult form fig 42.6b
  3. The Classes of Mollusks
    1. Seven Classes of Mollusks
      1. Smaller classes provide information on evolutionary relationships
        1. Probable ancestor was dorsoventrally flattened and unsegmented
        2. May have chitinous cuticle and overlapping calcareous plates
        3. Ancestor may have been segmented even though current forms are unsegmented
      2. Many characteristics present in today's chitons: Class Polyplacophora fig 42.7
        1. Have oval bodies with eight overlapping plates
          1. Plates believed to be evidence of prior segmentation
          2. Underneath plates body is not segmented
        2. Chitons creep along on broad, flat foot
          1. Foot surrounded by groove or mantle cavity
          2. Gills arranged within mantle cavity
        3. Most chitons are shallow water grazing herbivores, some live in depths
      3. Three classes examined in detail
        1. Gastropoda: Snails, slugs, limpets
        2. Bivalvia: Clams, oysters, scallops
        3. Cephalopoda: Squids, octopuses, cuttlefishes, nautilus
    2. Class Gastropoda: The Snails and Slugs
      1. Basic features
        1. Primarily marine, also freshwater and terrestrial forms fig 42.8
        2. Possess single shell or are derived from shelled forms
        3. Body divided into head, foot and visceral mass
        4. Shell of marine forms closed by a door-like operculum
        5. Head possess paired tentacles that may have terminal eyes
        6. Mouth may be simple or modified into proboscis
      2. Visceral mass asymmetrical because of torsion during development
        1. Lateral muscles of embryo grows disproportionately, relocate anus 120ø
        2. Twisting of shell due to one side of larva growing faster than other
        3. Associated with other anatomical changes, loss of right gill and nephridium
      3. Display varied feeding habits
        1. Terrestrial herbivores are serious garden pests
        2. Oyster drills bore into other mollusk shells, suck out insides
        3. Cone shells, predator with harpoon-like radula fig 42.8a
        4. Some nudibranchs possess nematocysts from cnidarian polyps fig 42.8d
      4. Terrestrial forms evolved a rudimentary lung under mantle
        1. Animals live in environment with plenty of oxygen
        2. More efficient than a gill would under similar conditions
    3. Class Bivalvia: The Bivalves
      1. Includes clams, scallops, mussels, oysters
      2. Basic features
        1. Foot is wedge-shaped
        2. Have two shells hinged together dorsally (left and right sides)
          1. Held together by a ligament
          2. Close with contraction of adductor muscles
        3. Mantle secretes shells and ligament, encloses internal organs within shells
          1. Mantle often forms incurrent and excurrent siphons
          2. Pair of gills located under folds of the mantle
        4. Lack distinct heads and radulas fig 42.4
        5. Foot adapted for locomotion, burrowing and anchoring
      3. Most forms are filter feeders with palps located on sides of mouth
        1. Food particles entangled in mucus secreted by glands
        2. Cilia convey food to mouth
      4. Disperse from place to place in larval stage
      5. Scallops are unique, very mobile forms fig 42.9a
        1. Adductor muscles are what is eaten by humans, not whole organism
        2. One shell larger than other
        3. Edge of body lined with tentaclelike projections
        4. Complex eyes can differentiate light and dark, amy see shadows of predators
        5. Also detect predators by chemical signals
      6. Abundant in marine and freshwater habitats
        1. Pearly freshwater mussels distributed worldwide fig 42.10
        2. More than 500 species live in lakes and rivers of North America
        3. Larvae brooded in special pouch in gill
        4. Parasite fish in part of life cycle
    4. Class Cephalopoda: The Octopuses, Squids and Nautilus
      1. Most intelligent invertebrates
      2. Active predators that compete successfully with fish
      3. Foot has evolved into a series of tentacles fig 42.11
        1. Squids have 10 tentacles
        2. Octopuses have eight tentacles
        3. Nautilus have 80 to 90 tentacles
        4. Tentacles snare prey, paired beaklike jaws bite, pulled into mouth by radula
      4. Have highly developed nervous systems
        1. Rapid responses result from giant nerve fibers attached to mantle
        2. Eyes are elaborate with retina similar to that in vertebrates
        3. Exhibit complex behaviors and high level of intelligence
      5. Most have closed circulatory system
      6. Lack external shells except for the few nautilus species
      7. Take water into mantle and expel it through siphon for propulsion
        1. Change direction of movement with siphon
        2. May eject dark fluid to confuse predators
      8. Sexes are separate
        1. Specialized tentacle transmits spermatophore to female
        2. Eggs fertilized as they leave the oviduct

42.2 Annelids were the first segmented animals

  1. The Annelids
    1. Early Innovation in Coelomates Was Segmentation
      1. Body built from series of similar segment like prefabricated building
      2. Segmentation offers evolutionary flexibility
        1. Small change in a segment can produce segment with new function
        2. Segments can be modified for various activities
    2. Segmentation First Evolved in Annelid Worms fig 42.12
      1. Two-thirds are marine, rest are terrestrial
      2. Characterized by three principle features
        1. Repeated segments
          1. Visible as ring-like structures along body length
          2. Separated internally by partitions called septa
          3. Each segment contains excretory and locomotor organs
          4. Fluid in segments creates hydrostatic skeleton that gives the segment rigidity
          5. Each segment can expand or contract independently
        2. Specialized segments
          1. Anterior segments modified with sensory organs
          2. Well-developed brain contained within one anterior segment
        3. Connections
          1. Provide ways for materials to pass between segments
          2. Closed circulatory system carries blood between segments
          3. Ventral nerve cords connect ganglia in each segment
  2. Body Plan of Annelids fig 42.13
    1. Annelid Tube within a Tube Design
      1. Digestive tract tube within a coelom tube, runs from mouth to anus
      2. Each segment has setae: External bristles of chitin
        1. Help provide anchorage during movement
        2. Annelids often called bristleworms
      3. Possess a more efficient closed circulatory system
        1. Lack respiratory systems and exchange gases across body surfaces
        2. Earthworms have five pulsating blood vessels that serve as hearts
      4. Excretory units, ciliated funnel-shaped nephridia, similar to those of mollusks
        1. Repeated in each segment
        2. Transport waste out of coelom
  3. Classes of Annelids
    1. Diverse Forms Appear in Many Different Habitats
      1. Three classes
        1. Polychaetes: Free-living, marine,bristleworms
        2. Oligochaetes: Terrestrial earthworms, marine and freshwater forms
        3. Hirudinea: Freshwater predatory or bloodsucking leeches
      2. Evolutionary trends
        1. Evolved in sea, polychaetes most primitive
        2. Oligochaetes evolved from polychaetes
        3. Leeches possess clitellum as do oligochaetes
        4. Evolved from oligochaetes by specialization in habits
    2. Class Polychaeta: The Polychaetes
      1. Great variety of marine worms fig 42.14
        1. Crucial part of marine food chains
        2. Many commensal with sponges, mollusks, echinoderms, crustaceans
      2. Well-developed heads with specialized sense organs
      3. Possess distinct paddle-like parapodia
        1. Function in locomotion
        2. Provide increased surface area for gas exchange
      4. Sexes separate, fertilization generally external
        1. Usually lack permanent gonads
        2. Gametes produced directly from cells lining coelom or on septa
        3. Produce mobile trochophore larvae
    3. Class Oligochaeta: The Earthworms
      1. Literally eat their way through the soil
        1. Contraction of pharynx sucks in organic debris
        2. Muscular gizzard grinds food with aid of soil particles
        3. Castings (undigested materials) are deposited outside burrows
      2. Lack eyes, but have light- touch- and moisture-sensitive organs
      3. Have fewer setae than polychaetes, no parapodia
      4. Are hermaphroditic, individuals trade gametes during mating fig 42.15
        1. Mucus from clitellum holds worms together, forms cocoon
        2. Passes along body after separation, picks up deposited sperm
        3. Contains fertilized eggs which ultimately hatch into young worms
    4. Class Hirudinea: The Leeches
      1. Most are freshwater, few marine and terrestrial forms
      2. Usually dorsoventrally flattened
      3. Are hermaphroditic, seasonally develop a clitellum, cross-fertilize
      4. Coelom is reduced, continuous through the body, and unsegmented
      5. Have a sucker at one or both ends, for attachment and locomotion
      6. Lack setae, except for one species
      7. Most are predators or scavengers, some suck blood
      8. Example: Hirudo medicinalis, medicinal leech
        1. Mouth has chitinous teeth, secretes an anticoagulant
        2. Used to remove blood after special surgery

42.3 Lophophorates appear to be a transitional group

  1. Lophophorates
    1. Include Three Phyla of Marine Animals
      1. Ectoprocts
      2. Brachiopoda
      3. Phoronida
    2. Biology of the Lophophorates
      1. Lophophore is a circular or U-shaped ridge around the mouth fig 42.16
        1. Coelomic cavity lies within lophophore and its tentacles
        2. Functions in gas exchange and food collection
      2. Share features of protostomes and deuterostomes
        1. Cleavage is radial as in deuterostomes
        2. Formation of coelom varies, resembles deutero- or protostomes
        3. Ribosomes of all are protostomic
    3. Phylum Phoronida: The Phoronids
      1. Superficially resemble tube worms
      2. Individuals secrete a tube made of chitin
      3. Possess U-shaped gut within a sac
      4. Development
        1. Develop as protostomes with radial cleavage
        2. Anus develops secondarily
    4. Phylum Ectoprocta: The Bryozoans
      1. Commonly called moss animals
      2. Anus ("proct") is external to lophophore ("ecto")
      3. Form colonies, freshwater and marine forms
      4. Secrete a zooecium chamber and live within it
      5. Development
        1. Develop as deuterostomes
        2. Mouth develops secondarily
        3. Exhibit radial cleavage
    5. Phylum Brachiopoda: The Brachiopods
      1. Superficially resemble clams
      2. Some attach to substrate with a stalk
      3. Lophophore located within shell
      4. Few living species, many extinct species
      5. Example: Lingula, most ancient surviving genus of all animals
      6. Development
        1. Develop as deuterostomes
        2. Exhibit radial cleavage

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