Download Biology 320 Invertebrate Zoology Fall 2005

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

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Amendments to general body
plan
Decent with modification
General mollusc shape:

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Bilaterally symmetrical
Dorsoventrally compressed
Oval outline
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General mollusc structures:
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Poorly defined anterior head
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Dorsal visceral mass
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Ventral muscular foot
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Feeding apparatus known as
radula
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Mantle
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Shell
Foot
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Broad, flat, and muscular
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Located ventrally
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Functions in adhesion and
locomotion
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Contains secretory mucus glands
which facilitate locomotion
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Posses several pairs of pedal
retractor muscles

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Connect foot to shell
Contractions of theses muscles
allow animal to pull shell over
visceral mass and foot, or vice
versa
Mantle
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Dorsal covering of visceral mass
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Secretes shell
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Forms mantle cavity
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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
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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


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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

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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

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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

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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

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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
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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
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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

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Mouth
Buccal cavity

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Radula housed here
Salivary glands – particles are trapped in mucus and transported in
strings by cilia
Pharynx

Radula
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Feeding apparatus
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Long chitinous structure consisting
of many rows of curved teeth
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Supported by a connective tissue
structure called the odontophore
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Protractor and retractor muscles
control odontophore

Food is scraped from substrate and
pulled towards mouth
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Anterior radular teeth are oldest,
and one to five new rows grow
daily
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Midgut – sorting, digestion,
and absorption
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Esophagus (considered to be
part of foregut in Ch. 9)
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Stomach
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Located in visceral mass
Site of extracellular digestion
Possesses chitinous gastric
shield
Sorting fields that sort
particles by type / size
Digestive ceca
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Connect to stomach
Produce enzymes and deliver
to stomach
Site of absorption,
intracellular digestion, and
nutrient storage
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Hindgut – elimination
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Intestine
Anus
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Located in exhalant chamber
Wastes are swept away during ventilation
Coelom
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Coelomate
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Protostomes
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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

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Blood is not always contained in vessels
Hemocoelic tissues and organs are bathed in blood
Hemal system components

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Heart
Vessels
Hemocoel sinuses
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Head
Foot
Visceral mass
Hemolymph containing hemocyanin
Excretion

Pair of metanephridia in
close proximity to
pericardial cavity (coelom)
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Hemolymph filtration
occurs in visceral sinus of
hemocoel
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Urine is dumped into
exhalant chamber of mantle
cavity via nephridiopores
Nervous System
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CNS
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Esophageal nerve ring with pairs of ganglia
Two pedal and two visceral nerve cords
Additional pairs of ganglia
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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
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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
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External fertilization
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Gonads attached to coelom (pericardial cavity)
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Gametes released into coelom, enter nephridia,
and exit nephridiopores into exhalant chamber
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Most produce a trochophore larva
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Top shaped
Girdle of cilia called prototroch
Apical tuft of cilia
Planktotrophic
Complete gut
Some have veliger larvae or direct
development
Class Polyplacophora
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Chitons
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Marine; many live
intertidally
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Physically challenging habitat
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In some ways resemble
HAM
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Shell composed of 8
overlapping plates
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Allows flexibility when
conforming to shape of
substrata
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Indistinct head lacking
eyes and tentacles
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800 spp.

3 mm to 40 cm
(gumshoe, Cryptochiton)
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Can be red, brown,
yellow, or green in color
Mantle
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Mantle covers entire
dorsal surface, including
(partially or entirely) the
valves
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Thick and stiff
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Has lateral overhangs
around mantle cavity
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Referred to as the girdle
Shell
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Eight overlapping valves
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Name Polyplacophora
means “bearer of many
plates”
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Lateral insertion plates of
valves are embedded in
mantle tissue
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Variation in the amount of
valve exposed
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Pair of pedal retractor
muscles for each valve
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Four layers to each
valve
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Second outermost layer
possesses sense organs
called esthetes (more on
this later)
Locomotion
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Similar to HAM
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Negatively phototactic, so creep away from
light
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Found in crevices; under ledges and rocks
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If dislodged from substrate (by a wave,
predator, etc.), chitons can contract their
longitudinal enrollment muscles, and roll
into a ball
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During adhesion, mantle and foot make
contact with substrate
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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
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Located in groove between the foot
and mantle
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6 to 88 bipectinate gills are located on
each side
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When animal lifts anterior girdle, two
inhalant apertures form
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Gill cilia beat to draw water
posteriorly

Water exits via one medial exhalant
aperture
Nutrition
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Most are browsers
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Up to 75% of gut contents may be
sediment
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Some feed on seaweeds
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Some are carnivorous and may
use mantle to trap small
animals
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Chitons posses very long
radulas
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Teeth may be capped with an
iron-containing mineral called
magnetite
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Digestive system is similar to HAM, with exceptions
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Subradular organ
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Located in buccal cavity
Chemosensory organ that is extended like a tongue
If food is detected, odontophore is extended
Two salivary glands
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Also associated with buccal cavity
Secretes mucus into which particles are trapped
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Strings of mucus with particles are moved to esophagus
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Esophagus
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Two esophageal glands dump amylase into esophagus
Circulation and Excretion
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Hemal system is similar
to HAM
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Posterior coelom
(pericardial cavity)
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Two huge nephridia in
lateral hemocoel
Nervous System
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Unlike HAM, chitons lack ganglia
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Posses an anterior nerve ring that surrounds the
anterior gut
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Four longitudinal nerve cords
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Two pedal - ventral
Two visceral - lateral
Commissures give nervous system a ladder-like
appearance
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Sensory organs
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Subradular organ
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One osphradium in each mantle cavity
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Esthetes
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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
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Gonochoric
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Single, large, median gonad
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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
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Trochophore larva or direct
development