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Invertebrates II: Mollusca, Arthropoda, Echinodermata, Chordata (non-vertebrate chordates) ©2004 Amanda Demopoulos I. Phylum Mollusca More species (200,000+) than any other animal phylum except Arthropoda Soft body – often protected by CaCO 3 shell

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invertebrates ii mollusca arthropoda echinodermata chordata non vertebrate chordates
Invertebrates II:Mollusca, Arthropoda, Echinodermata, Chordata (non-vertebrate chordates)

©2004 Amanda Demopoulos

I. Phylum Mollusca

  • More species (200,000+) than any other animal phylum except Arthropoda
  • Soft body – often protected by CaCO3 shell
  • Unsegmented, typically bilaterally symmetrical


    • Head (eyes/sensory), visceral mass (organs), muscular foot
    • Mantle – covers visceral mass, secretes shell
    • Radula – Ribbon of small chitin teeth used in feeding
    • Gills (ctenidia-comb like) for gas exchange in mantle cavity

Fig. 7.19

i phylum mollusca
I. Phylum Mollusca
  • Exceptions to basic plan
    • Bivalves lack radulas
    • Squids have internal shells
    • Octopuses have no shells
  • Reproduction (for some gastropods and bivalves)
    • 2 larval forms
      • Trocophore – looks similar to annelid larva
      • Veliger – ciliated, wing-like, with shell




©2004 Amanda Demopoulos

  • Phylum Mollusca
  • 4 classes

A. Class Gastropoda (=stomach foot)

  • Largest, most common, most diverse group (40K-75K sp)
  • Coiled mass of vital organs=visceral mass enclosed by a shell
    • Snails
    • Limpets
    • Abalones
    • Nudibranchs – lose shell in larval stage
  • Ventral creeping foot
  • Diet
    • Many vegetarian (scrape algae off rocks with radula)
    • Some predatory
      • Prey on bivalves, worms, fishes, sponges
i phylum mollusca7
I. Phylum Mollusca

A. Class Gastropoda

  • Torsion : during late veliger stage, twisting of visceral mass 180º, counter-clockwise, resulting in coiled shell
  • Operculum : leathery, trap door
  • *Drawing: radula slide, live lettuce slugs-order Sacoglossa, marine snails*






Anterior adductor muscle

Posterior adductor muscle




I. Phylum Mollusca

Giant clam

B. Class Bivalvia (=2 valved)

  • Laterally compressed
  • Hinged, 2-valved shell – (oysters, clams, mussels, scallops) muscles keep closed
  • No head , no radula
  • Sedentary lifestyle
  • Some have muscular foot for burrowing (e.g., clams)
  • Gills used for suspension feeding (active) & respiration
    • Water enters and leaves through siphons (incurrent and excurrent)
  • Some anchor to substrate with byssal threads (e.g., mussels)
  • Scallops – Swim! (repeated clapping of valves)
  • *Drawing-Scallop shell, live flame scallop

I. Phylum Mollusca

C. Class Cephalopoda (=head footed)

  • Octopuses (5 cm – 9 m)– no shell; Cryptic, bite prey with beak-like jaw
  • Squid (giant ~ 1000 kg, 18 m) – Shell reduced to pen made of chitin
  • Cuttlefish – Carbonate shell=cuttlebone, aid in buoyancy
  • Nautilus – Shell with chambers (septa) filled with gas, buoyancy organ
    • Exclusively marine
    • Fast moving, highly mobile predators
      • Large eyes
      • Shell reduced or absent
      • Foot modified as arms and tentacles bearing suckers
      • Siphon – Directs water released from mantle cavity
      • Move by jet propulsion-move in any direction
    • Ink sac for defense-distraction

**Drawing-squid and nautilus**

Phylum MolluscaClass Cephalopoda- Octopus and squid have beak-like jaws- Mouth is central among tentacles


8 arms + 2 tentacles non-retractable w/suckers and hooks


8 arms, non-retractable w/ 2 rows of suckers


60-90 suckerless tentacles


8 arms + 2 tentacles, retractable w/ suckers, no hooks



2 –extensible tentacle stalks




tentacle clubs



I. Phylum Mollusca

C. Class Polyplacophora (Many-plate-bearing=Chitons, 800 species)

  • Mostly graze algae on rocky shorelines
  • Exclusively marine
  • 8 overlapping plates
  • *drawing-chiton*

D. Class Scaphopoda (=spade foot, Tusk shells)

  • Predators on foraminifera and juvenile bivalves
  • Most common in deep water, buried in sediment

© Charlotte M. Lloyd


II. Phylum Arthropoda

  • Most species of any phylum (1 million+), 75% of all animals species described
  • Insects, spiders, centipedes, crabs, lobsters, shrimp, barnacles, etc.
  • Segmented, bilaterally symmetrical body
    • Jointed appendages
    • Exoskeleton made of chitin
    • Growth requires molting - defenseless
  • Body segments
    • Head, thorax, abdomen
      • Some groups have head and thorax fused = cephalothorax
  • Discuss 3 subphyla and representative classes
subphylum trilobitomorpha
Subphylum Trilobitomorpha

Class Trilobita

  • Marine
  • Extinct since 250 mya
  • Rich fossil history because of exoskeleton
  • Dorso-ventrally flattened
  • *drawing-trilobite fossil*

Subphylum Chelicerata

A. Class Merostomata (=legs attached to mouth, Horseshoe crabs)

  • Not true crabs
    • Named for feeding appendages – chelicerae
  • Distinctive, horseshoe-shaped carapace
  • No antennae
  • Benthic predators/scavengers on clams and small invertebrates
  • No jaws - Grind food with bristles on walking legs (must be walking to “chew”)
  • Much of what we know understand about vision based on horseshoe crab eyes
  • Blood is used to test injectable pharmaceutical solutions for bacterial contamination
  • **drawing-horseshoe crab**

Subphylum Chelicerata

B. Class Pycnogonida (= thick knees, Sea spiders)

  • All marine
  • Superficially resemble spiders
  • Mouth at end of large proboscis
  • Carnivores
    • Feed on sea anemones, hydrozoans,
    • other soft inverts
  • Legs much longer than body, more than 8
  • *Drawing-sea spider*

PHOTO: Bill Rudman

Subphylum Crustacea (40K species)
  • Contains majority of marine arthropod species
  • Gills + two pairs of antennae (sensory)
  • Larval forms – nauplius and zoea

A. Class Copepoda (=oar foot, Copepods)

  • Extremely abundant holoplankton (always plankton)
  • Some live on/in substrates (benthos)
  • **Among most abundant animals on earth**
  • Important primary consumers of phytoplankton
  • Small << 1-2 mm
  • Some parasitic forms
  • *drawing-copepod slide*

Subphylum Crustacea

B. Class Cirripedia (= hairy foot, Barnacles)

  • Active suspension feeders (filter feeders)
    • Use feathery cirri (modified swimming appendages)
  • Sessile (attached to surfaces-whales, piers)
  • Fouling organisms (boats, whales)
  • Resemble mollusks superficially – calcareous plates
  • *drawing-barnacle nauplii, cypris*

Subphylum Crustacea

C. Class Malacostraca(=soft shell, 75% crustacean species-Discuss 4 Orders)

1. Order Amphipoda (Amphipods) beach hoppers, sand fleas, whale lice

    • Laterally compressed
      • Generally small (< 2 cm), but larger in deep ocean
      • Head and tail downward
    • Widespread distribution
      • Generally free living
      • Important scavengers
  • *drawing-Gammarus slide, Daphnia slide*

©2004 Amanda Demopoulos


Subphylum Crustacea

C. Class Malacostraca

2. Order Isopoda (Isopods-rock lice, fish lice)

  • Dorsoventrally compressed
    • Generally small (< 2 cm), but larger in deep ocean
  • Related to terrestrial pill bugs
  • Widespread distribution
    • Generally free living
    • Important scavengers
    • Some parasites

Subphylum Crustacea

C. Class Malacostraca

3. Order Euphausiacea (Krill) - holoplankton

  • Laterally compressed
    • Up to 10 cm long (usually smaller)
  • Head and anterior segments fused to form distinct carapace
  • Widespread distribution
    • Important primary consumers and predators
    • Important prey for larger consumers (whales, penguins, fish)
    • Keystone species in some ecosystems (Polar, Southern Ocean)
  • Aggregate in schools (billions of individuals)
  • *Drawing: euphausid (krill) specimens*

Subphylum Crustacea

C. Class Malacostraca

4. Order Decapoda (=10 legs, Crabs, Lobsters, Shrimps)

  • Most species in Crustacea (~10,000)
    • Scavengers/Predators/Both
  • Largest crustaceans
  • Five pairs of walking legs (deca=10)
    • First pair usually modified as claws for feeding/defense
  • Well-developed carapace = cephalothorax
    • Rest of body = abdomen, tail
    • Laterally compressed, except crabs – abdomen under cephalothorax
  • *drawing-crab zoea slide, lobster
phylum arthropoda subphylum uniramia
Phylum Arthropoda Subphylum-Uniramia

Class Insecta=Hexapoda (6 footed)

  • 1 million species described to date
  • Found in every known terrestrial + freshwater habitat, some marine except deep sea
  • Diversity attributable to
    • Feeding specialization
    • Dispersal capabilities
    • Predator-avoidance possibilities (flight)
  • *Drawing-insect leg types

a) walking legs

b) swimming legs


Phylum Echinodermata –

  • spiny skin
  • 6000 species
  • Sea lilies, feather stars, brittle stars, sea stars, sand dollars, sea urchins, sea biscuits, sea cucumbers
  • Radial symmetry
    • Pentaradial symmetry in adults
      • Oral/aboral
  • Endoskeleton = hard plates, ~95% calcium carbonate, covered by skin
  • Complete digestive, nervous systems, and reproductive organs
  • Regeneration
  • Water vascular system = internal hydraulic system
    • Unique to echinoderms
    • Tube feet (podia) extended by pressure from ampullae (muscular sacs)
    • Tube feet used for locomotion, feeding, sensory functions
    • Connected to exterior through madreporite (porous plate)

Phylum Echinodermata

  • Class Stelleroidea = a star

A. Subclass Asteroidea (=star like, Sea stars)

  • Most species have five arms (some more),
  • Tube feet on oral surface in ambulacral grooves
  • Endoskeleton composed of CaCO3 plates
    • Flexible skeleton – permits movement
  • Aboral surface often covered with pedicellariae
    • Small claws used for grooming surface
  • Predators
    • Feed on bivalves, snails, barnacles
    • Pry shells of bivalve apart and insert stomach
  • *drawing-sea star, starfish young slide*

Crown of Thorns

Acanthaster planci


Phylum Echinodermata

  • Class Stelleroidea

B. Subclass Ophiuroidea (Brittle stars, Serpent stars)

  • Arms long and very flexible
    • May resemble writhing snakes
  • Tube feet lack suckers (used for feeding)
  • Central disk distinct
  • 1 Mouth, no anus
  • Cryptic – Usually not in open areas
  • Scavengers/Detritivores
    • Particles collected by tube feet and passed to mouth
  • (*drawing brittle star*)

III. Phylum Echinodermata

C. Class Echinoidea (=spine like, Sea urchins, sea biscuits, sand dollars), (Echinus = Gr. Hedgehog)

  • Round, rigid test with movable spines and pedicellariae
    • Spines and tube feet used for locomotion
  • Tube feet in shallow ambulacral grooves (5 rows) along outside of test
  • Complete digestive system
    • Mouth on bottom, anus on top
  • Herbivores
    • Feed on seaweeds and seagrasses (especially drifting) plus attached encrusting organisms
      • Mouth includes Aristotle’s lantern (system of jaws and muscles used to bite off algae, other food from bottom)
  • *drawing-sea urchin, sand dollar*