Download Chapter 16 Molluscs

Z202-Unit4
Chapter 16
Molluscs
I.
Phylum Mollusca
A. Evolution
- First animals with a true coelom = Eucoelomates
- Fluid-filled space, lined with mesoderm, organs suspended
by mesenteries
- fossil evidence indicates molluscs evolved in sea, most
have remained marine (freshwater & brackish)
- only snails (Gastropods) have successfully invaded land
but limited to moist sheltered habitats w/ Calcium
- Cephalopods evolved to become relatively intelligent
- Coelom limited to chamber @ heart, gonads, nephridia
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B. Economics – Beneficial & Harmful
- Food source
- culturing pearls & pearl buttons
- burrowing shipworms destroy
wooden ships & docks
- snails & slugs garden pests
- some snails serve as
intermediate hosts for parasites
C. General Characteristics
- All 90,000 living species & 70,000 fossil species; all are "soft
bodied"
- Size varies: weigh up to 450kg, grow to 18m
(80% are under 5cm)
- Varied modes of nutrition: herbivorous grazers, predaceous
carnivores, filter feeders, & parasites
II.
Form and Function
A. Three Part Molluscan Body Plan
1. Head-foot portion contains feeding, cephalic
sensory & locomotor organs
2. Visceral mass portion contains digestive,
circulatory, respiratory, & reproductive organs
3. Mantle forms a protective covering
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Mantle Cavity
-
2 folds of skin form the protective mantle (pallium)
space between known as mantle cavity
mantle cavity houses gills (ctenidia) or lung
mantle may secrete protective shell over the visceral mass
B. Head-Foot
-
well-developed head bears mouth & sensory organs in most
photosensory receptors range from simple to complex eyes
tentacles may be present
Foot (chief locomotor organ) located posterior to mouth
- Radula
1. unique to molluscs (found in all except bivalves &
solengasters)
2. protruding, rasping, tongue-like organ scrapes fine
particles of food from substrate
3. ribbon-like membrane has rows of tiny teeth (up to
25,000) pointed backwards
4. serves as a conveyor belt to move particles
5. pattern & number of teeth used in classification
6. some specialized to bore through hard material or
harpoon prey
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- Foot
1. usually ventral
2. function for attachment to substrate or locomotion
3. modification: attachment disc of limpets; hatchet foot on
clams & siphon jet of squid
4. Snails & bivalves extend the foot hydraulically by
engorgement with blood
5. Burrowers extend foot into mud, enlarge tip & pull it
forward
6. Free-swimming forms have modified foot into wing or finlike apparatus
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C. Visceral Mass
- Mantle & Mantle Cavity
1. Mantle - sheath of skin on each side of body; secretes
shell when present
2. Cavity houses gills or lungs that develop from mantle
3. Continuous water flow through cavity brings oxygen &
food; removes waste
4. Products of digestive, reproductive, & excretory empty
into cavity
5. Cephalopods use head & mantle cavity to create jet
propulsion
6. Counter-current blood flow increases diffusion of
gases
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- Shell
1. when present, secreted by mantle & lined by it
- Internal Structures & Function
1. Open circulatory system includes pumping heart,
blood vessels, & blood sinuses
2. Most Cephalopods have Closed circulatory system
with heart, vessels, & capillaries
3. Most molluscs have pair of kidneys or metanephridia
4. “Kidney” ducts also discharge sperm & eggs
5. Nervous system w/ paired ganglion (simpler than
annelids)
6. Sense organs vary and may be highly specialized
D. Reproduction & Life History
- most are dioecious, some hermaphroditic
- internal & external fertilization occurs, though all lay eggs
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- egg hatches & produce free-swimming larvae (trochopore)
- larvae undergoes metamorphosis into juvenile mollusk
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III.
Major Classes of Molluscs
3 Major Classes to Know:
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A. Class Gastropoda
(snail, slug, limpet, whelk, conches, periwinkle, sea slug,
sea hare, sea butterflies)
Characters:
- most diverse (70,000 living/15,000 fossil sp)
- Gastropod shell univalved, coiled or uncoiled
- Terrestrial individuals limited by
mineral content, temp, moisture,
pH level
- Snails have Operculum covering
shell aperture
- Snails serve as intermediate hosts
of “worm” parasites
- Only gastropods undergo Torsion
(moves the mantle cavity, twist the visceral organs 90180o rotation to join at head)
- variation in feeding due to radula:
*herbivorous & graze or feed on plankton
*scavenge on decaying flesh
*active carnivores tear prey w/ radula
*boring carnivores drill holes in shells & use
venom of neurotoxins
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*extracellular complete digestion
- respiration occurs in ctenidia in mantle cavity (requires
they live in moist habitats)
- most have single nephridium, well developed circulatory
& nervous systems
- sense organs include eyes,
statocysts, tactile organs, &
chemoreceptors
- eyes vary from simple to
complex lensed eye
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- gastropods include both monoecious & dioecious
B. Class Bivalvia
(Pelecypoda)
(mussels, clams, scallops, oysters, shipworms)
Characters:
- Most marine, some freshwater
- Freshwater species sensitive to water quality, including
pollution & sedimentation
- Zebra mussels are a serious exotic invader
- range from 1-2mm in length to giant S. Pacific clam
- most sedentary filter feeders dependent on ciliary currents
to bring in food
- bivalves lack a head, radula, or other aspects of
cephalization
- 2 shells (valves) held by hinge
ligament
- strong adductor muscles hold
valves together
- shell grows in outward rings
- pearls produced in response to
irritants
- slender foot extended out from between valves
- blood pumped into foot, it swells & anchors bivalve in mud
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- shortening of foot pulls bivalve forward (clams)
- Scallops clap valves to create
propulsion
- Mantle & gills perform gas exchange coupled with food
collection
- Circulatory system: 3-chambered heart
(2 atria, 1 ventricle)
- Excretory: Pair of Ushaped Kidneys
(nephridium concentrated
into organs)
- Nervous system has 3
pairs of ganglion
- Sense organs poorly
developed:
*some mantle eyes advanced
- Tentacles may have tactile & chemoreceptor cells
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- Sexes usually separate (dioecious);
- Most gametes are broadcast out with excurrent flow,
externally fertilize & develop, few use mother’s brood
pouch to develop internally
C. Class Cephalopoda
(squid, octopus, nautilus, devilfish, cuttlefish)
Characters:
- all are marine predators
- range from 2cm to giant squid (largest invertebrate known)
- foot in head region is modified for expelling water from
mantle cavity (funnel/siphon)
- fossils go back to Cambrium
- 2 types of ancestors:
*straight shelled ancestor - squid, octopus
*coiled shelled ancestor - nautilus
- Shell Types:
1. Nautiloid shells has gas chambers
(unlike gastropod)
2. Squid shell is a thin strip called the pen, enclosed
in mantle
3. Octopi has completely lost the shell (beak)
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- Locomotion (swim by forcefully expelling water through a
ventral funnel or siphon)
- Lateral fins of squids & cuttlefish are stabilizers
- Active life of cephalopods is reflected in their
advancements in respiratory, circulatory, & nervous
systems
- Cephalopods have 1 pair of gills (except nautiloids) w/
dedicated blood vessels
- Cephalopod brain largest of any invertebrate
*squid have giant nerve fibers
- Sense organs are well-developed, eyes complex
(complete w/ cornea, lens, retina)
- "Learn" by reward/punishment & observation
- lack sense of hearing but have tactile & chemoreceptors
in arms
- Communicate using chemical & visual signals (deep-sea
sp have luminescent organs)
- Ink sacs used when alarmed
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- Dioecious
- Spermatophores
(sperm sacs) stored
in mantle cavity and
inserted into female
using modified arm
- Eggs laid & develop
externally (some
females tend to
eggs)
IV.
Phylogeny & Classification
A. Phylogeny
- 1st Molluscs probably arose in Precambrian times
- Mollusca considered Protostomes allied with annelids &
arthropods (due to: spiral cleavage, mesoderm development,
& trochophore larvae)
- Molecular evidence suggests Molluscs more closely related to
Annelids than either is to Arthropods
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B. Classification of Classes:
Class Caudofoveata
Class Solenogastres
Class Monoplacophora
Class Polyplacophora
(Chitons)
Class Scaphopoda
Class Gastropoda
(snail, slug, limpet, whelk, conches, periwinkle,
sea slug, sea hare, sea butterflies)
Class Bivalvia
(Pelecypoda)
Class Cephalopoda
(squid, octopus, nautilus, devilfish, cuttlefish)
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