15.8: Phylum Mollusca - Geosciences

15.8: Phylum Mollusca - Geosciences


The phylum Mollusca is defined by several special characteristics. These defining characteristics include a mantle with a mantle cavity, a shell (except where lost), visceral mass, foot, and radula. The odontophore is in the mouth of most mollusks and it supports the radula (a ribbon of teeth). More features of molluscs include bilateral symmetry, soft or unsegmented bodies, respiration via ctenidium, ganglia/nerve comprised nervous system, haemocoel body cavity, etc. [1].

“Anatomy of a Common Land Snail”, by Jeff Dahl [CC BY-SA 4.0]

Although when most people think of a mollusc they imagine a typical clam or snail with the shell on the exterior of the organism, there are actually variations of these characteristics. One example of this is the Flamingo Tongue. The Flamingo Tongue is a small marine snail. It is a fascinating organism, in that their mantle, when they are at rest, covers the outer shell on the exterior of the organism. When threatened the Flamingo Tongue will retract all of the exposed tissue back into the shell in a very interesting manner.

“Flamingo Tongue, 2018” by Alana Olendorf [CC by 4.0]


Chitons are the common name for species in the Class Polyplacophora. Chitons are considered more primitive in relation to other groups within the phylum such as bivalves or cephalopods. There are 8 dorsal plates lining the dorsal surface of these organisms. There are many primitive species of the class Polyplacophora that are still extant. However, only a few species of Monoplacophorans still exist in the world today.

“Dorsal side of chiton specimen under dissection microscope 2019” by Jason Charbonneau [CC by 4.0]


Gastropoda is a very diverse class of molluscs. The subclass Prosobranchia, are often identified by their coiled, cone shaped or tubular shells [1]. The mantel cavity is typically located on the anterior of the organism. Important distinctions of these organisms are the variations or the absence of radula. These organisms utilize Nephridia for excretion of nitrogenous waste. One characteristic of gastropods is the presence of a calcareous operculum. This structure acts as a shielded plate that protects the organism by covering the opening when it is retracted into its shell.

In the photo below, the dark oval in the center is the operculum. They are calciferous and rough to the touch, protecting the opening to the shell like a man-hole cover.

“Gastropod under dissection microscope” by Jason Charbonneau [CC by 4.0]


Nudibranchs are typically categorized as sea snails which lack shells. Often they are richly colored and captivating to the eye. Their magnificent beauty is the result of aposematism which is the bright coloration exhibited by these organisms that warns predators that they are toxic, distasteful or dangerous. The toxicity of these organisms differentiates from species to species, with some being exponentially more dangerous than others. Their toxicity is usually contingent upon their evolutionary specialization and their genus’s specific niche.

“Flabellina iodinea” by Jerry Kirkhart [CC by 2.0]


Class Bivalvia consists of molluscs that have two connected shells such as Oysters, Clams, Mussels, Scallops and many more. Most bivalves are enjoyed as delicacies, despite the fact that they are benthic level filter-feeders. As others in the phylum Mollusca, bivalves have a shell that is made up of deposits of Calcium Carbonate. These deposits are derived from substances in the water and harden over-time.

“Clam Shells” by Jason Charbonneau [CC by 4.0]

Visible among the many aspects of an open clam are the posterior and anterior adductor muscles. The function of these muscles is to hold the shell closed as a defense from predators. The strength of these muscles given their relative size is immense. The mantle is a layer of tissue that overlays the visceral mass of these organisms and is directly connected to the shell. The foot of the bivalve is directly responsible for its movement. The muscular foot will emerge when the shell is opened and pushes the organism along or into the benthos or substrate.


Cephalopods exhibit several similarities but also distinguishable differences from other molluscs. They sometimes exhibit a calcium carbonate shell. Squids and Octopus lack this feature, but the more primitive Nautilus does have this feature. Species in the Class Cephalopoda contain a large closed circulatory system and prehensile arms/tentacles that encompass a mouth, complete with a beak and radula. One of the most important evolutionary aspects of these organisms are their large complex eyes. These eyes are specialized for improved sight at depths where little ambient light reaches. The mantle of these organisms forms a sizable ventral cavity containing ctenidia. A portion of the mantle also forms a muscular funnel. Water is taken up and forced through these chambers under pressure creating a unique form of jet propulsion. There are over 900 living species of cephalopods that inhabit the world today.

“Nautilus shell” by Jason Charbonneau [CC by 4.0]

“Squid, December 9, 2013” [CC 0 Public Domain]


  1. Brusca, Gary J., Brusca, Richard C. 2003, Invertebrates 2nd ed. ISBN 0-87893-097-3

The information in this chapter in thanks to content contributions from Alana Olendorf and Jason Charbonneau

15.8: Phylum Mollusca - Geosciences


Make great contribution to America's food supply - 130 million lbs./yr
Cephalopods have the capability to learn
Nudibranchia (order) have lost their shells
Size range 1.0 mm to 50 ft. (15.24 m)
weight to 2 tons
giant clams 5 feet long, 500 pounds
Unsegmented (except Neopilina)
Bilateral symmetry - except gastropods
Body - Head - with tentacles and eyes, lost in pelecypods
foot - ventral and muscular
visceral mass - coiled in gastropods, internal organs concentrated in this mass
mantle - (pallium) a soft skin or sheet of tissue overgrowing visceral mass. Mantle also
secretes calcareous shell with organic matrix.
Mantle cavity - space at posterior end of visceral mass, contains paired gills (respiratory chamber),
anus, excretory and reproductive systems open into mantle cavity. Exhalent currents
remove waste products.
Gills - cilia on gills create water movement. In class Pelecypoda, gills also used as feeding structure.
Digestive tract - Mouth, jaws, radula, stomach. Pelecypods lack jaws and radula.
Organs - 3 chambered heart (ventricle and two auricles), capillaries in cephalopods
gills - one pair, except in class Amphineura (up to 80 pairs)
kidneys or paricardinal glands
sexes separate, hermaphroditism widespread, fertilization external.
Shell - calcareous (aragonite and calcite)
prismatic - twinning of calcite or aragonite
foliated - calcite - resembles x-bedded Ss, inner shell layers
nacreous - aragonite separated by thin leaves or organic material cross-laminar
homogeneous - no structure under plane light. Under X nicols, small xls seen.

Fossil Molluscs
Monoplacophora - molluscs with cap, spoon shaped or arched single shell, (mammilated
shell) (L.C. - Holocene). Some groups bilaterally symmetrical
Polyplacophora - Amphineura - class chitins (U. Cambrian - Holocene)
shore-living molluscs, radula
8 calcareous articulating plates
Scaphopoda - benthonic ("scaphe" = bowl, "podas" = foot)
neritic and bathyal realms
Modern forms appear in early K
Gastropoda - ("gastro = stomach) (L. Cambrian - Holocene)
head - distinct, fused or partially fused to foot, eyes and
tentacles unspecialized
foot - solelike, adapted for creeping, modified in pelagic forms
Radula - normally present
Nervous system - cerebral and pleural ganglia
Torsion -
Symmetry - varying degrees of bilateral asymmetry
Shell - single, univalve, calcareous (with aragonite), closed apically, lacks chambers

Classification of Gastropods

Subclass PROSOBRANCHIA : torsion, varishaped shells, conispiral,
planispiral, operculum. High-spired shells rare
visceral loop twisted into figure 8
mantle cavity opens to front
freshwater, terrestrial, marine
(L. Cambrian - Holocene)

lacks siphon
male without prostate and penis
heart with two auricles
inner layer of shells often nacreous
(L. Cambrian - Holocene)

varishaped, conispiral
siphonal notch
marine, freshwater, terrestrial
(Ord. - Holocene), not common until the Mesozoic/Cenozoic
Helmet shells
Moon snails

Conispiral, with siphonal notch or canal
not as diverse as Mesogastropods
(Cretaceous - Holocene)
Muricids (Murex)

gastropods display detorsion - straightening out of the nervous
shell reduced or absent
operculum commonly absent
exclusively marine
(Dev.? Miss. - Holocene)
Where shell present
aciculate - slender, tapered to a sharp point, low spired
miolute - last whorl envelopes earlier ones
convolute - last whorl completely embraces and conceals
earlier ones
heterostrophic - Apical whorls of embryonic shell (protoconch) coiled in
opposite direction to rest of shell
poorly represented as fossils, common today

Order PLEUROCOELA - (Tectibranchia)
shell dextral (right-handed)
involute or convolute
shell and mantle cavity become obsolete
(Miss. - Holocene)

Order Uncertain
Superfamily Pyramidallacea: living opisthobranchs
(Dev.? Miss. - Holocene)

Pelagic molluscs: naked or covered with thin, generally transparent varishaped
shell, often operculate
(Mesozoic - Holocene)

Subclass PULMONATA: Origin of pulmonates through family Ellobidae
Mantle cavity acts as lung
Nervous system in adults becomes symmetrical following torsion
well developed shell
radula with teeth reduced
Terrestrial and fresh water
(Mesozoic to Holocene)

pulmonates with shell (spiral, cap, bowl-shape)
single pair noninvaginable tentacles with eyes at base
intertidal marine, coastal and inland terrestrial, freshwater

Watch the video: Palaeontology: -Lamellibranchia, Gastropoda and Cephalopoda