Download 37-1 Mollusks

37-1 Mollusks
 Invertebrates like clams, snails, slugs
and octopuses
 Phylum mollusca
 112,000 species. Some are predators,
others are filter feeders
 Have bilateral symmetry
Common Features
Coelomates
 Have a true coelom (hollow fluid
filled cavity)
Advantage is muscles of body wall
are separate from those of the gut,
so body wall muscles can contract
without hindering the digestion
process. Also transports blood
Trochophore
 Shared by aquatic
mollusks and
annelids during the
larval stage
 Contains cilia at
both ends and in the
middle
 Cilia contribute to
dispersal of
trochophore
Body Plan of Mollusks
2 regions
1.Head-foot
2.Visceral mass
1. Head-foot (fig. 37-2)
 head, mouth, sensory structures,foot for
locomotion
2. Visceral mass (fig. 37-2)
 heart, digestive organs, excretory organs,
reproductive organs
Mantle
Covers visceral mass, which secretes 1
or more hard shells made of calcium
carbonate. Found in both sexes. Protects
entire animal. The disadvantage is that
the animal cannot exchange gases, so
they had to evolve gills, which
exchange gases with water. The gills are
protected by the mantle cavity.
Ganglia
 Located in the head-foot
region
 Connected by 2 pairs of
nerve cords
Radula
 A feeding adaptation. It is a
flexible, tongue-like strip
covered with abrasive teeth.
(fig. 37-3)
3 Classes of Mollusks
1. Gastropoda
 Snails, abalones, conchs
2. Bivalvia
 Clams, oysters, scallops
3. Cephalopoda
ctopuses, squids, chambered nautilus
*Only mollusk to have closed
circulatory system
1. Class Gastropoda
 Body undergoes torsion during larval
stage. The visceral mass twists 180
degrees towards head, so everything like
gills, mantle cavity and anus are near
head. (fig. 37-4)
 Have open circulatory system. A
circulatory fluid called hemolymph passes
through body into tissue spaces called
hemocoels then the hemolymph goes back
to the heart.
Class Gastropoda
 Snails (land and
aquatic, fresh water
and salt water)
 Aquatic snails respire
through gills
 Land snails respire
through mantle cavity
and are hermaphrodites
 Move around for food
Abalone
Conch
2. Class Bivalvia
 Shell is divided
into 2 halves (or
valves) connected
by a hinge
 Use adductor
muscles to close
shell
 Has open
circulatory system
Valves
 Each valve contains 3 layers of cells
secreted by mantle
a. Outer layer protects shell against acidic
conditions
b. Middle layer is calcium carbonate
c. Inner layer is smooth and protects soft
body. This is what forms a pearl.
Bivalves
 Bivalves are sessile
hey use muscular foot to dig into sand
and become filter feeders
 Only mollusk not to have a radula
 Have 3 pairs of ganglia (nerves)
*near mouth
*near digestive tract
*near foot
Clams
 Buried in mud or sand
 Use siphons
1. Incurrent siphon allows food to enter,
sticks to mucus on gills which enters mouth.
Gases are also exchanged and sperm also
enters here.
2. Excurrent siphons allow all waste
materials to exit the clam
Reproduction
 Reproduction in fresh water clams vs. saltwater clams
1. Fresh water
 Sperm enters incurrent siphon and
fertilization is internal
2. Salt-water
 Egg and sperm are released into the water.
Fertilization is external.
Cephalopoda
Chambered
Nautilus
Show video
Hand out clam terms and
clam anatomy
Dissect clam and define all
terms