Notes for Chapter 16:
Mollusca (Molluscs or Mollusks)

Click link to return to Lecture Schedule
or back to Chapter 14 or Chapter 15
or ahead to Chapter 17

Chapter 16 Assignment: All; RQ 16: 2,5,6,8,9,11,13
   (2 Lectures)

Introduction: A Significant Space (coelom)


Source of Image

Featured organism: Fluted giant clam (Tridacna Squamosa)
   More Links: 1 - 2 - 3 - 4 - 5 - 6

I. Molluscs Squids, Snails, Bivalves, and Chitons
    More Links

II. Form and Function

Key Terms:
visceral mass

mantle cavity
coiling vs. torsion

Common molluscan features (none in all):

muscular creeping foot
visceral mass (gut, blood, gonads)
mantle cavity
ctenidia (gills)
trochophore larva
spiral cleavage
coelomic heart cavity
hemocyanin blood pigments

Molluscs and Humans

1. Positive aspects

Food (e.g., oyster farm produces 20 times meat/acre as a cattle farm)
Rich cultural history, inspirations in art
Currency (NW Native Americans used scaphopods – "wompom")

2. Negative aspects

Introduced pests

carnivorous snails in Pacific islands
Corbicula – a clam introduced "for fish bait" from Asia, clogs dams
zebra mussel – larvae transported in bilge water to Great Lakes
            – huge ecological and financial impact

Bankia – bivalve "shipworm" bores into wood, tremendous destruction
Schistosomiasis flatworm parasite uses snails for intermediate host
    – widespread in tropical areas with primitive sewage treatment
venomous – snail Conus and blue-ringed octopus can be deadly

III. Classes of Molluscs


    a) Chitons, Tusk Shells, Monoplacophorans, and Solenogastors
        (Classes Polyplacophora, Scaphopoda, Monoplacophora, and
     Prof. Eernisse works on Chitons!
     Here is his compilation of Web Images

(More links: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8  - 9 - 10 - 11)


mostly in deep sea and poorly known
lack shell but have spicules in mantle, like a chiton
with or without foot groove
with or without gill (ctenidium)
have radula (secondarily lost in some)
little evidence of serial repetition


eight shell plates
repeated gills, shell muscles
creeping foot
radula like a limpet/monoplacophoran
teeth mineralized with magnetite (iron)
many sensory organs in shells and girdle
serial repetition (ancestral or derived?)


single shell, discrete shell gland
prismatic and nacreous shell layers
mantle margin with three folds
crystalline style
no spicules in mantle
some have serial repetition (monoplacophorans, Nautilus)


    b) Snails and Their Relatives (Class Gastropoda)
        terms: trochophore and veliger larval stages, velum,
            protoconch (larval shell)

        1) Prosobranch snails are grazers, predators, suspension feeders.

        2) Opisthobranchs (sea slugs including nudibranchs) are
            few in species, diverse in life-styles.

    Featured Pelagic Molluscs:
     floating purple snail (Janthina janthina)
     heteropods (planktonic swimming snails)
     pteropods (planktonic opisthobranchs)
     blue dragon "solar powered" sea slug (Pteraeolidia janthina)

Gastropods (snails, slugs: > 50,000 species)

Paraphyletic group: "prosobranchs"
    (all snails except the clade of opisthobranchs + pulmonates)
    about half of all snail species
    includes, limpets, abalones, keyholes,
    diverse marine snails including trochaceans, neogastropods
Opisthobranchs –diverse but few species
    shell internal or absent
includes Aplysia, nudibranchs
   (these are very attractive "sea slugs"
     – Links: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9)

    half of all snail species
    lungs instead of gills
    lining of mantle cavity takes over
    coincide with invasion of land

Evolutionary Trends in Gastropod Gills (see Fig. 16-17)
       two gills (abalone, key hole limpets)
       reduce right gill
       loss of right gill (limpets)
       monopectinate gill fused to mantle
   opisthobranchs –secondary gill
   pulmonates – lungs

Gastropod Features:

1. Torsion – important synapomorphy (does not = coiling)
     upper part is twisted nearly 180° from lower part (See Fig. 16-13)
     mantle cavity moves from back to front
       helps larval "veliger" pull its head inside its shell
(Walter Garstang proposed this as a poem:
        "Ballad of the veliger or how the gastropod got its twist")
       helps the adult carry weight of shell
        (Proposed by German biologists even earlier)

2. Coiling
     snails protect themselves:
     clamping, withdrawing, locomotion
     coiling helps
     a high uncoiled shell would be hard to carry
         high center of gravity, frontal cross-section great
         shell is usually tilted (See Fig. 16-14)
         most shells coil to right (dextral)
         some coil to left (sinistral)

Parameters of coiling (pioneered by David Raup in 1960s;
     see p. 329 or this online shell-generating Java applet):

amount of translation (T)
   isotrophic (T = 0) [planispiral]
   orthostrophic (T is negative)
   hyperstrophic (T is positive)
whorl expansion rate (W)
   long pointed shells (W is low)
   broad shells (W is high)
distance from axis of coiling (D)
   hollow center as D increases
Of possible combinations of T, W, D
    snail examples for many but not all
here are some example combinations:


   c) Clams, Oysters, Mussels, Scallops and Their Relatives
        (Class Bivalvia)
        terms: siphon

     d) Octopuses, Squids, Cuttlefish, and Nautiluses
        (Class Cephalopoda)
        terms: pen

         Featured organisms:
     Chambered nautilus (Nautilus) -
            More Links: 1 - 2 - 3 - 4 - 5 - 6
     Cuttlefish (Sepia)
     Giant squid (Architeuthis dux) -
            More Links: 1 - 2
     Giant Pacific octopus (Octopus dofleini)


have large head like their snail relatives
all predators
use muscles, not cilia, for locomotion (siphon)
synapomorphy is siphuncle, but this is lost as shell is internalized, reduced
Nautilus –> Spirula –> Sepia –> Loligo
Nautilus has eyes like pinhole cameras
Squids have much more complex eyes
eye design is direct, not indirect as in vertebrates

IV. Phylogeny and Evolution

Classification based on Fig. 16-42

Mollusca (> 100,000 species – esp. snails)
  Caudofoveata (burrowers)
  Solenogasters (cnidarian-feeders)
  Testaria (shelled molluscs)
    Polyplacophora (chitons)
    Conchifera (discrete shell gland)
      (incl. snails, squids, clams, etc.)

Note: Testaria and Conchifera are not labeled in Fig. 16-42
   – Can you find them?


Click link to return to Lecture Schedule
or back to Chapter 14 or Chapter 15
or ahead to Chapter 17

This page created 9/28/01 © D.J. Eernisse, Last Modified 9/28/01, Links Last Completely Checked 9/28/01