Snail in Amber
Feather duster worms
•Sipuncula are the sister taxon of Mollusca
• (2) the two aplacophoran taxa,
Neomeniomorpha and Chaetodermomorpha, are
monophyletic with a common
neomenioid-like ancestor, and of the two taxa,
• Chaetodermomorpha are more divided,
Aplacophora and Polyplacophora are sister taxa
and form a clade, Aculifera
•Aculifera are the sister group of the remaining
extant mollusks, Conchifera
•Aplacophora are progenetic Aculifera.
•Some of the earliest animals burrowed in the soft
were segmented worms, as shown by fossil
worm tunnelling tracks, dated to Ediacaran times.
of these earliest worms can be found
today living in the muddy substrate of quiet bays -
•The body symmetry of Mollusca and Annelida is a figure
of the same bilateral appearance when it is cut in half
The body symmetry of Mollusca and Annelida when cut
in half is the same mirror image of the other half.
body symmetry of Mollusca and Annelida is typically
a short soft body with a very small coelom .
This is the coelom, found in Mollusks and
• The basic body structure, consisting of a foot,
visceral mass, and mantle, is similar in most mollusks.
foot is located below the visceral mass.
external shell consists of three layers. The thin
outer layer, called the periostracum, is made of a
tough hornlike material and serves to protect the
body usually round.
For polychaetes and clitellidae, head has prostomium and
peristomium (which usually has the mouth); anus is in
Trunk segmentation reflected in external annuli, internally by
series of coelomic compartments separated by septae
(mostly in oligochaetes, less so in others).
•Some are homonomous, with body segments mostly similar,
others are heterononmous, with specialized segments.
• Process and Structures
• macrophagy (predation and herbivory) – use either radula to scrape food or
harpoon-like poisoned teeth
• microphagy (filter feeding)- modifications of gills, which are covered by sheets of
mucus which trap food, then are passed into the stomach
• A Mollusk has food brought in through a siphon system and then to its mouth. It
then goes to a digestive gland and to its intestine. Wastes leave via the anus.
• They eat plants, animals, and dead things
• Many cephalopods eat fish
• Bivalves usually eat phytoplankton
• Structures and Process
They have either a straight or coiled gut.
Some have no known digestive system.
Some use a raptorial method by capturing and swallowing prey
Deposit feeders ingest sand/dirt and extract nutrients from it.
Suspension feeders use tentacles or mucus webs to filter nutrients from fluid
• The sediment is ingested and any digestible organic material is assimilated as
it passes through the alimentary canal
• Many eat sediment
• Uses non-selective deposit feeding
• 3-chambered heart
• Circulatory system (except for cephalopods) is open
• Cephalopods have a closed circulatory system and an extra pair of
powerful gill hearts.
• Uses one large foot muscle
• Blood from the paired gills enters the atria, leaves the ventricle.
• the artery from the ventricle opens and releases blood into tissue-bathing
sinuses, eventually draining back into gills.
Well-developed and closed in most annelids
Some have some vessels capable of functioning as "hearts".
Some have no mesodermally derived blood vessels, but have blood "channels”
The nervous system has a solid, ventral nerve cord from which lateral nerves
arise in each segment. This controls movement.
• Carries blood from the dorsal to the ventral vessel in the head region.
• Every segment has an autonomy; however, they unite to perform as a single
body for functions such as locomotion
• Structures and Process
• Molluscs use true gills, with extensive capillary network and countercurrent
flow for gas exchange.
• A Mollusk has tubes called siphons that bring water in and out of their bodies
so that they can get oxygen and give off carbon dioxide. They have gills
that help with this.
• Structures and process
• Respiration may occur through gills
• In other types, the cilia are the most likely site for respiration.
• Many also respire through the skin
Annelids may reproduce asexually or sexually.
• Most annelids can regenerate in many different ways. Certain species, such as
Dodecaceria can regenerate an entire individual from a single segment.
• Asexual reproduction is usually by fragmentation.
• Most polychaetes, chiurans, and pogonophorans are gonochoristic.
• Gonochorism is the differentiation of male and females of a species by their
reproductive organs. For example, most mammals are gonochoristic
Mollusks come in hermaphroditic and gonochoristic forms. Gastropods
can be either.
When a Gastropod is Hermaphroditic, they usually have only one kind of
gonad, and are protandrous.
• Protandrous means that the male sex organs are
produced before female organs.
Most bivalves are gonochoristic; though a few are hermaphroditic, like
mollusks, the brain and one or two large ventral nerve cords, are connected
by commissures (which is a ladder form).
Cephalopods can be said to have a true brain, capable of learning and complex
annelids show a degree of cephalization. A well-developed
circumesophageal ganglion in the head region leads to paired ventral
nerves with ganglia in each segment controlling the activity of that segment
via lateral nerves and coordinating with other segments. Many use giant
axons (e.g., Lumbricus) for escape reactions.
have a soft body that is protected by a hard shell
•Mollusks are bilaterally symmetrical.
•Mollusks are capable of living on either land or in water.
• They have lungs that can function in both water and air.
mollusks, annelids are bilaterally symmetrical.
•The Annelid moves by alternating circular and longitudinal muscles.
• The circular muscles surround the body wall and contractions cause the
body to become long and thin.
• The longitudinal muscles run the length of the body, and cause the body
to become shorter and fatter.
•The body cavity is a true coelom, divided by internal septa.
•Body possesses 3 separate sections, a prosomium, a trunk and a pygidium.
•They live in most environments.
BC Learning Network, "Characteristics of Annelids." Cool School. BC Learning Network. 12 Apr 2009
Cavendish, Marshall. "Slug."International Wildlife Encyclopedia. 3rd. 2002.
Forces of Nature, "Characteristics Of Mollusks." Forces of Nature. Forces of Nature. 12 Apr 2009 <http://www.forces-ofnature.net/topics/molluscum/Characteristics_Of_Mollusca.htm>.
Ramel, Gordon. "The Annelids." Earth Life. 29 Sep 2008. Earth Life. 12 Apr 2009
Mehaffey, Leathem. " Phylum Annelida" Vassar College. 1 Apr 2001. Vassar College. 26 Mar 2009
Mehaffey, Leathem. "Mollusca." Vassar College. 1 Apr 2001. Vassar College. 26 Mar 2009
"Mollusks." UXL Encyclopedia of Science. Ed. Rob Nagel. Vol. 7. 2nd ed. Detroit: UXL, 2002. 1288-1290. Gale Virtual
Reference Library. Gale. VERNON HILLS HIGH SCHOOL. 24 Mar. 2009
Russel, Bruce. "Branches on the Tree of Life: Annelids ." Biomedia Associations. 7 June 2000. Biomedia Associations. 26
Mar 2009 <http://ebiomedia.com/prod/BOanelids.html>.
Saladin, Kenneth S. "Mollusk." Biology. Ed. Richard Robinson. Vol. 3. New York: Macmillan Reference USA, 2002. 105-106.
Gale Virtual Reference Library. Gale. VERNON HILLS HIGH SCHOOL. 24 Mar. 2009
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