1. Molluscs and Annelids Bonneville High School
CHAPTER 16

2. Characteristics Phylum Mollusca
Over 90,000 living species and 70,000 fossil species
Soft body and protostomes
Include chitons, tusk shells, snails, slugs, nudibranchs, clams, mussels, oysters, squids, octopuses, and nautiluses

3. Marine Snail
Nudibranch
Chiton
Pacific Giant Clam
Octopus

4. Characteristics Characteristics:
Herbivorous grazers, predaceous carnivores, filter feeders, and parasites
Most are marine, but some are terrestrial or freshwater aquatic

5. Characteristics Evolution Fossil evidence Some bivalves and gastropods
Indicates molluscs evolved in the sea
Most have remained marine
Some bivalves and gastropods
Moved to brackish and freshwater
Snails (gastropods) successfully invaded land
Limited to moist, sheltered habitats with calcium in the soil
Cephalopods
Evolved to become relatively intelligent

6. Mollusc Body Plan: Head-Foot and Visceral Mass Portions
Form and Function
Mollusc Body Plan: Head-Foot and
Visceral Mass Portions
Head-foot region contains feeding, sensory, and locomotor organs (foot)
Visceral mass contains digestive, circulatory, and reproductive organs
Mantle Cavity
Space between mantle (sheath of skin) and body wall is the mantle cavity
Mantle cavity houses the gills or a lung
In most molluscs
Mantle secretes a shell over the visceral mass

7. Form and Function Radula Unique to molluscs
Found in all except bivalves
Protruding, rasping, tongue-like organ
Ribbon-like membrane has rows of tiny teeth (up to 250,000) pointed backward
Radula rasps off particles of food from surfaces
Serves as a conveyor belt to move particles to digestive tract
New rows of teeth replace those that wear away
Pattern and number of teeth are used in classification of molluscs
Some specialized to bore through hard material or harpoon prey

8. Form and Function Foot Functions in attachment or locomotion
Modifications include
Hatchet foot of clams
Siphon jet of squids
Secreted mucus aids in adhesion or helps molluscs glide
Snails and bivalves extend the foot by engorgement with blood

9. Form and Function Shell If present, secreted by the mantle
Periostracum
Outer layer - wears away
Composed of hardened protein
Middle prismatic layer
Closely packed prisms of calcium carbonate
Increases with animal growth
Inner nacreous layer
Next to the mantle; the nacre is laid down in thin layers
Aids in Pearl formation
Shiny layer in abalone, nautilus, and bivalve

10. Reproduction and Life History
Form and Function
Reproduction and Life History
Most dioecious, some hermaphroditic (monoecious)
Egg hatches and produces a free-swimming trochophore larva
In many gastropods and bivalves
Trochophore is followed by intermediate larval stage, the veliger.

11. Internal Form and Function (Obj 4)
-Torsion: A twisting process that makes the Mollusc visceral mass asymmetrical.
-Coiling: The spiral winding of the shell and visceral mass. It is NOT the same as torsion.
-Ctenidium: Located in the mantle cavity, this acts as gills for some Molluscs
-Lung: A highly vascular area in some Molluscs which aids in respiration it has an opening called a pneumostome.
-Dioecious: Having male and female gonads in separate individuals.
-Monoecious: Having both male and female gonads in the same organism: Hermaphrodite
-Pulmonates: Molluscs which show detorsion and include land and most fresheater snails and slugs. They lost their ctenidia and their vascularized mantle wall has become a lung. They are monoecious.
-Chromatophores: Cells in the skin that contain pigment granules. This can change the color of the skin of some mollusks. They are used for camouflage and communication.
-Ink gland: A gland which most cephalopods and nautiloids have that secretes sepia, a dark fluid containing the pigment melanin. The pigment is deposited into an ink sac that empties into the rectum. The Mollusc can then release the dark cloud to confuse or distract a predator.
Open circulatory system: a pumping heart, blood vessels and blood sinuses. Blood is pumped from the heart into open spaces.
Closed circulatory system: Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. 

12. Annelids: Segmented Worms
Invertebrate Zoology
Phylum Annelida means
“Little Rings”

13. Phylum Annelida
Annelids are segmented worms. Segments marked with circular rings called annuli or metameres
Examples:
earthworms
leeches
clam worms

14. The evolutionary milestone shown by annelids is
Phylum Annelida
The evolutionary milestone shown by annelids is
segmentation (metamerism).
Segmentation evolved separately in annelids, arthropods, and chordates.
The body is divided into a series of segments, each having similar components of all major organ systems.
Built in fail-safe (if parts of body fail!).
Allows for specialization.

15. Phylum Annelida
Many annelids have chitinous bristles called setae.
Help in locomotion
Anchor worm in place
Deter predators

16. Phylum Annelida – Body Plan
Peritonium (layer of mesodermal epithelium) of adjacent segments meet to form septa (dividing walls).
Fluid-filled coelom acts as a hydrostatic skeleton.
Coelomic lining holds in the organs

17. Traditionally, annelids are divided among 3 classes:
Phylogeny
Traditionally, annelids are divided among 3 classes:
Class Polychaeta (marine worms)
Class Oligochaeta (segmented worms)
Class Hirudinida (leeches)
Oligochaeta and Hirudinida form a monophyletic group called Clitellata.
Characterized by reproductive structure called a clitellum.