Download Chpt 27 Mollusks and Annelids - Phylum Mollusca o Are soft bodied

Chpt 27
Mollusks and Annelids
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Phylum Mollusca –
o Are soft bodied animals
o Have (or had) a shell
Shell may be exterior or interior
o Are very diverse
There are 7 classes (you will need to know 3 of them)
Habitat –
o Aquatic – marine and fresh
o Terrestrial
Body Plan –
o There are 4 basic structures found in all mollusks
1 - Foot – a soft muscle
• Can be long and flat
o Ex) snails and slugs
• Can be a spade shape for burrowing
o Ex) clams
• Can be divided into tentacles for feeding
o Ex) squid and octopus
2 – Mantle – a soft layer of tissue
• Usually just inside or around the shell
• Its function is to secrete and make the shell
3 – Shell – a hard case
• Can be outside the body
o Ex) snails and clams
• Can be inside the body
o Ex) cuttlefish or squid
• Can be not there (absent)
o Ex) slugs and octopus
4 – Visceral mass – internal organs
o Note : the foot and shell are used to classify mollusks
Feeding –
o Very diverse feeding styles
Herbivores – snails and slugs
Carnivores – octopus
Filter feeders – clams, mussels
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Detritus feeders – sea slugs
Parasites – snails
Digestion –
o Tube digestive system
Starts at the mouth, ends at the anus
• Has other digestive organs in between
o May have other unique features
Radula – a small layer of skin with layers of tiny teeth
• Sand paper
o Herbivores scrape off plant bits
o Carnivores scrape through hard shells
• Mostly found in snails
Gills – filter feeders use their gills to trap food.
Respiration
o Aquatic mollusks use gills inside the mantle
O2 and CO2 diffuse at the gills
o Terrestrial mollusks have an adapted mantle cavity for respiration
The cavity has many moist folds to increase the efficiency of diffusion
Internal transport –
o There are 2 types of circulatory systems in mollusks
1 – Open circulatory system –
• Blood is not in vessels
• Blood circulates in open cavities called sinuses
• There is a heart that pumps the blood
o But the blood is not directed
• Ex) slow moving (or sessile) and small mollusks
o Snails, slugs and clams
2 – Closed circulatory system –
• Blood is in vessels
• Has a heart to pump the blood
o Blood goes in one direction
Circuit
o Ex) large and fast moving mollusks
Ex) octopus, squid and cuttlefish
Excretion –
o Metabolic wastes are removed from the blood in a tube shaped organ called the
nephridia
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Responses –
o The nervous systems level of complexity varies depending on the mollusks life
style. (mobility)
Ex) sessile
• Have little sensory organs
o Some ocelli – light
o Chemoreceptors – food
o Touch receptors – touch
o Statocysts – balance
Ex) free moving
• Have a very active and intelligent nervous system
o Have a very well developed brain
Good memory
o Have complex sensory organs
Eyes
• Can detect texture, colour and movement
Movement –
o Have very well developed muscles
Use a hydrostatic system to contract the muscles
• This uses water pressure to move
Some mollusks use their shell to aid in movement
Some mollusks will expel water from a syphon to propel them in the
water
Reproduction –
o Most mollusks have separate sexes
There are some hermaphrodites
o Aquatic mollusks use external fertilization
o Terrestrial mollusks use internal fertilization
o After fertilization, the zygote develops into –
A trochophore larva
• Common in mollusks and annelids
There are 3 classes of mollusks that you need to know.
1 – Class Gastropoda –
o Gastro = stomach
o Poda = foot
Therefore gastropods have a broad muscular foot on the ventral side
Ex) snails, slugs and nudibranchs (sea slugs)
They are aquatic – marine and fresh
And terrestrial
There are 2 main groups
A – with a shell
A hard cover
Can be a simple shield
• Ex) Limpets
Can be coiled
• Ex) snails
Some snails have a hard disk used to close the opening called an
operculum
• Aids in defence
o B – without a shell
Terrestrial slugs and aquatic nudibranchs (sea slugs)
These have developed other defense methods
• Fowl tasting ink or poisons in the skin (bright colours)
o Gastropods have bilateral symmetry at the larval stage only
o As they pass from larva to adult, they go through a twisting called torsin
This produces a twisted intestine with the anus at the anterior end
o Gastropods have cephalization with one or 2 pairs of tentacles with ocelli and
touch receptors
2 – Class Bivalvia
o Bi = 2
o Valvia = shells
o Have 2 shells hinged together and held by 1 or 2 muscles
Ex) oysters, clams, mussels and scallops
o They are sessile or slow moving
Exception – scallops are quick moving and able to swim
o Bivalves lack any real cephalization
o Their nervous system is very simple
No real sensory organs
Exception – scallops have ocelli and touch receptors
3 – Class Cephalopoda –
o Cephalo = head
o Poda = foot
o Have a foot near or attached to the head with tentacles
Ex) Squid, octopus, nautilus
o Most cephalopods have no shell or an internal shell
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Exception – the nautilus still has an external shell
o Some cephalopods use water squirted from a siphon for jet propulsion.
o Many cephalopods have chromatophores – specialized pigment cells that allow
them to change colour
Used for defence and communication
o Cephalopods have the most developed nervous system
o They also have a closed circulatory system
Importance of Mollusks
o Mollusks have many roles in the ecosystem
Clean up dead organism
Some symbiotic relationships
Food chain (eat and be eaten)
Filter feeders are often used as environmental indicators
Studied for medical research
Homework = work on animal review book
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