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Chapter 35
Mollusks and Annelids
I.
Phylum Mollusca
Things you should know about mollusks after this chapter
• Key characteristics of mollusks.
• The body plan of different mollusks.
• Name three classes of mollusks.
– Gastropods
– Bivalves
– Cephalopods
A. Characteristics of Mollusks:
•
2nd largest animal phylum (Arthropoda = largest)
–
•
from Latin molluscus, which means “soft.
–
•
Many have soft bodies /some have a hard shell.)
Feeding
–
–
•
Invertebrates such as clams, snails, slugs, squids, and octopuses.
Some are sedentary filter feeders
others are fast-moving predators.
Mollusks are true coelomates.
• Trochophore larval stage
–
Most aquatic mollusks and annelids (NOT ALL) have this stage
B.
Mollusk Body Plan
• Divided into 2 parts:
– Visceral mass
• which contains the heart and the organs of digestion, excretion,
and reproduction
– Head-foot, which consists of:
• the head, contains the mouth (including Radula- a tongue
like feeding adaptation with tiny teeth that point
backwards) and sensory structures
• the foot, a large, muscular organ for locomotion
• Coelom -limited to space around heart.
• Mantle -a layer of epidermis that covers the
visceral mass. (secretes the shell)
C. Classes
1. Gastropoda
(snails, slugs, albalone,
conch)
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2. Bivalvia
(clams, oysters,
scallops)
www.mcaorals.co.uk
3. Cephalopoda
(octopuses, squid,
cuttlefish, chambered
nautiluses)
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Class Gastropoda
• The largest and most diverse class of mollusks
• Most, including snails, abalones, and conchs,
have a single shell. Slugs have no shell.
• Undergo the process of torsion during larval
development.
• Open circulatory system in which a heart pumps
hemolymph from gills or lungs into the
hemocoel.
Class Bivalvia
• Aquatic mollusks
(clams, oysters, &
scallops)
• Shell is divided into
two valves connected
by a hinge.
• Most are sessile filter
feeders.
• Bivalves lack a
distinct head
Clams
• The mantle cavity of a clam is sealed except for
a pair of hollow, fleshy tubes called siphons.
– Water enters through the incurrent siphon.
– Water leaves through the excurrent siphon.
Other Bivalves
• Oysters attach to a hard surface
• Scallops can move through the water by
repeatedly opening their valves and snapping
them shut.
Class Cephalopoda
• Means “head-foot.”
• Free-swimming predators
• Have tentacles & beaklike
jaws on the head.
• Nervous system is very
advanced.
• Have a closed
circulatory system.
• Many use pigments to
hide and disguise
themselves.
Squids
• 10 tentacles.
• Propels by pumping jets of water with the mantle
through an excurrent siphon.
Octopuses
• 8 tentacles
• They often crawl along the ocean bottom or lie in
wait for prey.
Chambered Nautiluses
• The only existing cephalopod with external shell.
• Shell is coiled & divided into chambers.
• The body is confined to the outermost chamber.
Cephalopod Intelligence
• Considered the most intelligent invertebrates.
• Examples:
– Dexterity, tool use and manipulation. Suction cups & arms are as
efficient as human’s hand.
– Can learn through observation (choose colored balls)
– Can “Hide” & act like sea weed,
– Assess their prey.
– Scientists saw octopus complete & remember mazes & patterns
• Notes John Messenger, a neurobiologist at the University of
Sheffield. They learn these things rather faster than a
vertebrate will – like a pigeon or a rat.“ "That is quite
impressive." He also points out, however, that although
cephalopods learn faster at first, their skills level off. A
trained octopus will always make more mistakes than a
trained rodent, he says.
http://en.wikipedia.org/wiki/Cephalopod_intelligence ,
http://www.fortunecity.com/emachines/e11/86/cephpod.html
II. Phylum Annelida
Things you should know:
• Advantages of body segmentation.
• Structures used to divide annelids into
three classes.
• Structures in earthworms
• Name 3 classes of annelids.
A. Characteristics of Annelids
• Bilaterally symmetrical, segmented worms.
(earthworms, feather-duster worms, & bloodsucking
leeches.
• Annelids means “little rings” (many body segments).
• True coelom that is divided into separate
compartments by partitions.
• Most annelids have external bristles called setae
• Some have fleshy protrusions called parapodia
Class Oligochaeta
• Live in the soil or in fresh water
• Have no parapodia.
• Oligochaeta means “few bristles”; have a few setae
on each segment.
• Most familiar is the earthworm.
Structure and Movement
• An earthworm’s body has over 100 nearly-identical
segments.
• Circular and longitudinal muscles line the
interior body wall.
• Locomotion is made possible by segmentation.
Earthworms Feeding /Digestion
Ingest soil as they burrow through it.
• Soil is moved through these structures:
– mouth
– pharynx
– esophagus
– crop
– gizzard
– intestine
• includes the typhlosole
– anus
• Earthworms play an important role in the condition of soil.
Closed circulatory system.
• Contractions of the aortic arches and the dorsal
blood vessel force blood through body vessels
Respiration and Excretion
• O & CO2 diffuse through moist skin,
• Cellular wastes and excess water are excreted
through nephridia.
Neural Control
• Consists of a chain of ganglia connected by a ventral
nerve cord.
• Sensory structures are found in all segments but are
concentrated at the anterior end.
Reproduction
• Earthworms are hermaphrodites,
but an individual worm cannot fertilize
its own eggs.
– During mating, earthworms press their
ventral surfaces together.
– Held together by their setae and by a film of
mucus secreted by each worm’s
clitellum.
Class Polychaeta
• Most annelids are members of the class Polychaeta,
which means “many bristles.”
• Polychaetes differ from other annelids in that they have
antennae and specialized mouthparts.
• Are the only annelids that have a trochophore stage.
Class Hirudinea
• Hirudinea is the smallest class, consisting of about 500
species of leeches.
• Leeches have no setae or parapodia.
• Many leeches are carnivores but some are parasites that
suck blood from other animals.
Earthworm diagrams
Haemophagic Leeches
• Attach to their hosts & remain until
they become full & they fall off to
digest.
• Bodies are 34 segments.
• Have an anterior (oral) sucker formed
from the first six segments of their
body, used to connect to a host for
feeding and releases an anaesthetic
to remain unnoticed by the host.
• Use a combination of mucus and
suction to stay attached and secrete
an anti-clotting enzyme into the host's
blood stream.
• http://en.wikipedia.org/wiki/Leech
•Reading assignment: page 712
Comparing some Mollusks &
Annelids
Same in both:
– Both phyla are true coelomates
– Most aquatic forms develop from a trochophore
larvae
• Some Differences:
– Annelids are segmented, flexible & soft
– Mollusks are not segmented but divided into 2 areasa head/foot & a visceral mass
– Many mollusks have a shell (not octopus or slug) ,
annelids –no shell