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Biology 320
Invertebrate Zoology
Fall 2005
Chapter 12 – Phylum Mollusca
Part One
Introduction

Second largest phylum at
100,000 described spp.

Chitons, snails, clams, and
squids

Rich fossil record, as
calcareous shell preserves
easily

Seven classes

Mostly marine, but some
freshwater and terrestrial spp.
Generalized Mollusc

Also sometimes referred to
as HAM (hypothetical
ancestral mollusc)

Seven classes share
common features



Amendments to general body
plan
Decent with modification
General mollusc shape:



Bilaterally symmetrical
Dorsoventrally compressed
Oval outline

General mollusc structures:

Poorly defined anterior head

Dorsal visceral mass

Ventral muscular foot

Feeding apparatus known as
radula

Mantle

Shell
Foot

Broad, flat, and muscular

Located ventrally

Functions in adhesion and
locomotion

Contains secretory mucus glands
which facilitate locomotion

Posses several pairs of pedal
retractor muscles


Connect foot to shell
Contractions of theses muscles
allow animal to pull shell over
visceral mass and foot, or vice
versa
Mantle

Dorsal covering of visceral mass

Secretes shell

Forms mantle cavity



Dorsal pocket that seawater flows
through
Gills are housed here
Important for many processes such as
respiration, excretion, and feeding in
some cases
Shell

Mantle epidermis secretes
proteins and calcium salts
that form the shell

Three layers to the shell




Periostracum – outermost and
proteinaceous
Ostracum – middle and
calcareous
Hypostracum – innermost and
calcareous; may be nacreous
Shell increases in size as
animal grows
Respiration

Gills are termed ctenidia



Several pairs in HAM
One pair or one gill in modern
molluscs
Housed in mantle cavity

Gills are attached to mantle
via axis

Axis houses branchial blood
vessels


Afferent – delivers deoxygenated
blood from body to gills
Efferent – delivers newly
oxygenated blood from gills to
heart; then on to body

Leaf-like gill filaments
radiate from axis


Bipectinate if radiate from
both sides of axis
Monopectinate if only one
row of gill filaments

Interfilamentary water
spaces separate individual
gill filaments

The position of the gills in
the mantle cavity divides the
cavity into inhalant and
exhalant chambers

Gill filaments have cilia that generate water
currents




Water enters inhalant chamber ventrally
Passes between gill filaments
Exits through the dorsal exhalant chamber
This system is an example of countercurrent
gas exchange, which is common in aquatic
animals


Water and blood flow opposite of each other
Maintains diffusion gradients for O2 and CO2
Digestive System and Nutrition

HAM is a browser; scrapes algae and small organisms off
of substratum

As a result of this feeding strategy, a great deal of
substrate, minerals, and other inorganic particles are
ingested

Digestive system is adapted for browsing



Ciliary sorting fields to separate food from non-food
Foregut and hindgut are lined with cuticle to protect from abrasion
Gut is less complicated in molluscs that feed on larger organic particles

Foregut – mainly for ingestion


Mouth
Buccal cavity



Radula housed here
Salivary glands – particles are trapped in mucus and transported in
strings by cilia
Pharynx

Radula

Feeding apparatus

Long chitinous structure consisting
of many rows of curved teeth

Supported by a connective tissue
structure called the odontophore

Protractor and retractor muscles
control odontophore

Food is scraped from substrate and
pulled towards mouth

Anterior radular teeth are oldest,
and one to five new rows grow
daily

Midgut – sorting, digestion,
and absorption

Esophagus (considered to be
part of foregut in Ch. 9)

Stomach





Located in visceral mass
Site of extracellular digestion
Possesses chitinous gastric
shield
Sorting fields that sort
particles by type / size
Digestive ceca



Connect to stomach
Produce enzymes and deliver
to stomach
Site of absorption,
intracellular digestion, and
nutrient storage

Hindgut – elimination


Intestine
Anus


Located in exhalant chamber
Wastes are swept away during ventilation
Coelom

Coelomate




Protostomes




Small coelom
Not used as hydrostat as most have a shell that serves as an exoskeleton
Coelom houses heart, and gonads in HAM
Spiral cleavage
Schizocoely
Blastopore becomes mouth
Have not encountered pseudocoelomate animals yet

We will discuss Phylum Nematoda later
Hemal System

Classified as an open system



Blood is not always contained in vessels
Hemocoelic tissues and organs are bathed in blood
Hemal system components



Heart
Vessels
Hemocoel sinuses




Head
Foot
Visceral mass
Hemolymph containing hemocyanin
Excretion

Pair of metanephridia in
close proximity to
pericardial cavity (coelom)

Hemolymph filtration
occurs in visceral sinus of
hemocoel

Urine is dumped into
exhalant chamber of mantle
cavity via nephridiopores
Nervous System

CNS



Esophageal nerve ring with pairs of ganglia
Two pedal and two visceral nerve cords
Additional pairs of ganglia





Cerebral ganglia – brain that receives sensory input from eyes, tentacles, and
statocysts
Buccal ganglia – controls odontophore
Pedal ganglia – controls muscular foot
Pleural ganglia – controls mantle
Sensory organs




Cephalic tentacles – located on head
Ocelli – located on head
Statocysts – located on foot
Osphradia – located in inhalant chamber. Monitor incoming water for
chemicals and sediment. Ciliary beating cessates if conditions are unfavorable
Reproduction

Gonochoric

External fertilization

Gonads attached to coelom (pericardial cavity)

Gametes released into coelom, enter nephridia,
and exit nephridiopores into exhalant chamber

Most produce a trochophore larva






Top shaped
Girdle of cilia called prototroch
Apical tuft of cilia
Planktotrophic
Complete gut
Some have veliger larvae or direct
development
Class Polyplacophora

Chitons

Marine; many live
intertidally

Physically challenging habitat

In some ways resemble
HAM

Shell composed of 8
overlapping plates

Allows flexibility when
conforming to shape of
substrata

Indistinct head lacking
eyes and tentacles

800 spp.

3 mm to 40 cm
(gumshoe, Cryptochiton)

Can be red, brown,
yellow, or green in color
Mantle

Mantle covers entire
dorsal surface, including
(partially or entirely) the
valves

Thick and stiff

Has lateral overhangs
around mantle cavity

Referred to as the girdle
Shell

Eight overlapping valves

Name Polyplacophora
means “bearer of many
plates”

Lateral insertion plates of
valves are embedded in
mantle tissue

Variation in the amount of
valve exposed

Pair of pedal retractor
muscles for each valve

Four layers to each
valve

Second outermost layer
possesses sense organs
called esthetes (more on
this later)
Locomotion

Similar to HAM

Negatively phototactic, so creep away from
light

Found in crevices; under ledges and rocks

If dislodged from substrate (by a wave,
predator, etc.), chitons can contract their
longitudinal enrollment muscles, and roll
into a ball

During adhesion, mantle and foot make
contact with substrate


Great deal of suction
Almost impossible to remove animal without
harming it
Respiration

Chitons have two lateral mantle
cavities, as opposed to HAM’s one
dorsal cavity

Located in groove between the foot
and mantle

6 to 88 bipectinate gills are located on
each side

When animal lifts anterior girdle, two
inhalant apertures form

Gill cilia beat to draw water
posteriorly

Water exits via one medial exhalant
aperture
Nutrition

Most are browsers

Up to 75% of gut contents may be
sediment

Some feed on seaweeds

Some are carnivorous and may
use mantle to trap small
animals

Chitons posses very long
radulas

Teeth may be capped with an
iron-containing mineral called
magnetite

Digestive system is similar to HAM, with exceptions

Subradular organ




Located in buccal cavity
Chemosensory organ that is extended like a tongue
If food is detected, odontophore is extended
Two salivary glands


Also associated with buccal cavity
Secretes mucus into which particles are trapped

Strings of mucus with particles are moved to esophagus

Esophagus

Two esophageal glands dump amylase into esophagus
Circulation and Excretion

Hemal system is similar
to HAM

Posterior coelom
(pericardial cavity)

Two huge nephridia in
lateral hemocoel
Nervous System

Unlike HAM, chitons lack ganglia

Posses an anterior nerve ring that surrounds the
anterior gut

Four longitudinal nerve cords



Two pedal - ventral
Two visceral - lateral
Commissures give nervous system a ladder-like
appearance

Sensory organs

Subradular organ

One osphradium in each mantle cavity

Esthetes





Unique to chitons
Found on valves
Consist of many canals that traverse the layers of the
valves
High density of canals: up to 1750 / mm2
Function is disputed
Reproduction

Gonochoric

Single, large, median gonad



Located in dorsal hemocoel, just
anterior to coelom
Two gonoducts that empty directly
into exhalant chamber
Gametes do not pass through coelom
or nephridia

Mostly external fertilization, but some
internal fertilization occurs in mantle
cavity of female

Trochophore larva or direct
development