MOLLUSKS
Chapter 16
Molluscs are triploblastic which means three tissue
layers.
II.
Molluscs are coelomates: a fluid filled
body cavity lined by mesoderm. Have a complete
digestive system.
2 GROUPS OF COELOMATES:
1. Protostomes: Molluscs and Annelids
Spiral, determinate cleavage
The blastopore forms the mouth
Deuterostomes: echinoderms,
chordates
v Radial, indeterminate cleavage.
v The blastopore forms the anus.
2.
The coelom may have formed from
splitting of the mesoderm or from outpocketing of the gut.
PHYLUM MOLLUSCA SHARES CHARACTERISTICS
. 8 different classes
Body composed of a head-foot and visceral
mass (organ systems)
A mantle – covers visceral mass and may
secrete a calcareous shell
III
OTHER
COMMON
CHARACTERISTICS
Bilateral symmetry
Protostome characteristics
A trochophore larva
A reduced coelom
Open circulatory system
(exception: cephalopods which have a closed
circulatory system)
Many molluscs feed via a radula
Class
Examples
Feeding/digestion
Respiratory
Gastropoda
Snails and Slugs
Herbivores, feeding with a radula
Gills; mantle contractions circulate air; have
a siphon which serves as an inhalant tube.
Circulatory
Open (blood flows from heart leaves the
vessels and goes directly into sinuses.)
Skeletory
Hydraulic (blood confined to tissue spaces
for support.)
6 ganglia in the head foot and a visceral
mass
a. eyes on tentacles
b. statocysts on the foot for equilibrium
c. ophradia in the anterior wall of the
mantle that helps detect prey.
Nervous system
Excretory
Nephridium – converts ammonia to uric acid
Reproduction
Locomotion
Special
Features
External fertilization with some
being monoecious and others
being dioecious.
A ciliated, flattened foot covered
with glands used to creep across
the substrate.
Torsion – gives shell the coiled
appearance
Class
Bivalvia
Examples
Feeding/digestion
Respiratory
Clams, oysters, mussels, and scallops
Filter-feeders; loss of head and radula
Cilia covered gills forming folded sheets called
lamellae; cilia moves water over an incurrent
siphon
Open
External shell
3 pairs of ganglia, sensory cells on the margin
of the mantle, photoreceptors, statocysts and
osphradium
Nephridium
Most dioecious; external fertilization
Sedentary
Two convex halves of shell called valves
Circulatory
Skeletory
Nervous system
Excretory
Reproduction
Locomotion
Special Features
Class
Cephalopoda
Examples
Octopuses, squids, cuttlefish, and nautili
Feeding/digestion
Locate prey by sight and capture prey with tentacles that have
adhesive cups; radula and beak for tearing food; peristalsis
replaces ciliary action
Respiratory
Circulatory
Skeletory
Nervous system
Excretory
Reproduction
Locomotion
Special Features
Gills
Closed – blood confined to vessels; more
efficient
Shell is reduced or absent; except nautilus
Large brain – memory and decision making;
can form images, distinguish shape & some
color; statocysts by brain; osphradia only in
nautilus
Have a kidney which allows waste to filter
directly from the blood stream which is very
efficient
Dioecious; males have a hectocotylus for
spermatophore transfer into the female near
the opening of the oviduct; eggs deposited
onto a substrate and hatchlings are miniature
adults.
Jet propulsion (water forced through funnel)
Modified foot into tentacles or arms. They
have chromatophores (pigment cells) which
allow animal to change color and discharge
ink for defense; confuses predator allowing