Chapter 27
Mollusks and segmented worms
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Slugs, snails, and animals that once lived in the shell on the beach are all mollusks
Phylum: Moluska
They range from the slow moving slug to the jet-propelled octopus
Most live in the ocean; others live in fresh water and on land
Some have shells, slugs and squids are adapted to life without shells
All have bilateral symmetry, a coelom, 2 body openings, a muscular foot, and a mantle
The mantle is a thin membrane that surrounds the internal organ of the mollusks
In shelled mollusks, the mantle makes the shell
The phylum is diverse, however they all share similar development patterns
The larval stages of all mollusks are similar- have different appearance as adults
Where do mollusks live?
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In a wide variety of habitats
Most live in marine or freshwater or on land
Some in the cold polar regions
Aquatic mollusks, oysters, and mussels attach to the ocean floor- or base of docks
Many live in warm tropical
Octopus swim freely in the ocean
Land dwellers- slugs and snails
Classes of mollusks- 3 classes
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Gastropods- clam
Bivalvia- snails
Cephalopods- squid
While members of these 3 classes look different on the outside, they share many similarities on
the inside
Gastropods- one shelled mollusks
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The largest class of mollusks is Gastropoda or stomach- footed
The name comes from the way the animals large foot is positioned under the rest of the body
Most have a single shell and are sometimes called univalves
Other gastropods have no shell
Snails, slugs, and sea slugs belong to this class
Gastropod adaptations- snail
1. Radula- is a tongue-like organ with loads of teeth located within the mouth- it used to scrape,
grate, or cut food
2. Nervous system
A. Has a small brain with associated nerves to coordinate the animals movement and behavior
3. Circulatory system
A. Blood is pumped by the heart in an open circulatory system
B. An open circulatory system- blood moves through vessels and into open spaces around body
organs
C. It exposes body organs directly to blood that contains nutrients, O2, and removes metabolic
wastes
4. Respiratory structure
A. Mollusks are the first animals to have evolved respiratory systems
B. These structures are called gills
C. Gills increase the surface area through which gasses diffuse
D. Gills supply a rich supply of blood
E. The mantle cavity of land snails and slugs has evolved into a primitive lung
5. Excretory Structures
A. Nephridia are organs that remove metabolic wastes
B. Gastropods usually have one nephridia
6. Reproduction
A. Many gastropods that live on land are hermaphrodites
B. Most aquatic gastropods are male and female
C. No mating- external fertilization
Shelled gastropods
1. Snails, abalones, and conches are examples
2. May e creeping plant eaters, dangerous predators, or parasites
Gastropods
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Slugs are one type
Their body is protected by a thick layer of mucus
Some sea slugs secrete a strong unpleasant smelling mucus
Some secrete a poisonous mucus
Bivalves- Two shelled mollusks
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Clams, oysters, scallops, etc
Bivalves have no distinct head or radula
Most use their foot for burrowing in the mud or the sand
A strong ligament like a hinge holds their shells together
Muscles allow the shell to open and close
Bivalves are filter feeders
They have cilia that beat to draw water in through an incurrent siphon
Water moves over gills and exits through an excurrent siphon
9. Food and sediments are trapped in mucus then cilia push food particles to the stomach
10. Large particles, sediment, and anything else that is rejected and transported to the mantle or to
the foot where it is eliminated
Cephalopods- head- footed mollusks
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Includes the octopus, squid, etc.
They are the most complex and most recently evolved of all mollusks
Cephalopods have tentacles with sucker for movement and capturing food
The radula and sharp beaklike jaw tear apart fish, bivalve and crabs
Have a closed circulatory system
Blood moves through the body enclosed entirely in a series of blood vessels
Closed- is a more efficient means of gas exchange
Segmented worms
1. The phylum in annelid are the segmented worms
2. 3 classes of annelids
A. Oligochita- earthworms
B. Polychaeta- bristleworms
C. Hirudinea- leeches
3. Includes earthworms and leeches
4. Annelid means tiny rings
5. Are bilateral and have coelom and 2 body openings
6. Some have a larval stage
All annelids are made up of segments
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Their bodies are divided into a series of ringed segments
This segmentation continues internally
Each segment has its own muscles
The segments also allow for specialization
Most segments contain excretory organs and nerve center
A few segments also contain organs for digestion and reproduction
The gizzard is a sac with muscular walls and hard particles that grind soil before it passes into
the intestines
8. Some organs of the digestive, nervous, and circulatory systems do run the length of the body
Where do segmented worms live?
1. Everywhere except polar regions and dry sand of the desert
2. Earthworms are nocturnal- primarily at night
How do annelids reproduce?
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Earthworms are hermaphrodites
They mate and exchange sperm
Each worm forms a capsule
The capsule(slime ring) is slipped off inside the soil- 2 to 3 weeks the young emerge
Leeches
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Are segmented worms with flattened bodies and no bristles
Most live in fresh water and are parasites
They suck blood or other body fluids from their host- ducks, turtles, fish, people, etc
Bite- not painful- their saliva acts as an anesthetic
Can ingest 2-5 times their weight- may not eat for a year
The Earthworm
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Mouth
Esophagus
Pharynx- sucking
Crop- for storage
Gizzard- grinds
Setae- Tiny bristles (legs)
Nephridia- excretory structures for metabolic waste in each segment
Circulatory system- is closed- 5 pairs of hearts- they have a dorsal and ventral blood vessel
Nervous system- they have a ventral nerve cord, a simple brain, and nerve fibers in each
segment
10. The clittellum- segments 9-15 is for reproduction