1. Ch. 35 – Mollusks & Annelids
Mollusks and annelids are grouped together because they were the 1st organisms to develop a true coelom.
This suggests that these two groups of animals may have evolved from a common ancestor.
The coelom provides space for a circulatory system to function without interference from other organs. They can contract muscles without hindering the movement of food through the gut.
Both have the same 1st stage in larva development which is called a trochophore.

2. Phylum: Mollusca
Snails, clams, octopuses, oysters, squids
Video
Mollusk is Latin for “soft”
Most have a shell to protect them and are soft inside the shell.
Develop from a trochophore larva which is a free-swimmer. Cilia helps them swim.

3. Mollusk Characteristics
Most are filter feeders (100,000 + species).
True coelom
3 main body parts: muscular foot, head, & visceral mass.
Circulation, respiration, digestion, excretion, reproduction, nerve impulses.

4. Body Arrangement 1. Muscular foot – used for locomotion.
2. Head – has a cerebral ganglia & mouth.
3. Visceral Mass – contains many organs.
4. Mantle – protects the visceral mass. It secretes a shell in most mollusks.
Shell is made of calcium carbonate.
How does the mollusk breathe with a shell?
Gills are used for respiration.

5. 4 Classes of Mollusks 1. Class: Gastropoda – snails & slugs
Largest class of mollusks
Gastropoda means “stomach foot”
Slugs have no shell
Snails have a single shell
The body mass can turn 180 degrees this is called torsion

6. Snails Gills for respiration Survive in moist environments
Open Circulatory System
Hemocoel – open body cavity for blood circulation.
Radula – a saw-like tongue made of chitin.
Use foot to move with wavelike contractions.

10. 2. Class: Polyplacophora – chitons
Polyplacophora means “many plates”
Their shells consist of 8 overlapping plates.

12. 3. Class: Cephalopoda – squids & octopus
Cephalopod means “head foot”
They have tentacles and a sharp radula to kill prey.
Closed Circulatory System – blood circulates in vessels.

13. Squids Largest invertebrate Giant squid are up to 3 meters long.
Squid have the ability to squirt ink.
They have the ability to change colors.
Video
Squids have separate sexes
Squid & octopus use jet propulsion to move.

14. 4. Class: Bivalvia – clams & oysters
Bivalvia means “two halves”
They are sessile & filter food from the water.
They have a muscular foot
They don’t have a distinct head

15. If a piece of sand grain gets into its shell the mantle coats the sand grain with secretion to form a pearl.
Clams
Filter feeders
Have incurrent siphon for water to enter and an excurrent siphon for water to exit.
Have separate sexes like most mollusks.

16. Body parts of a clam
Dissection
Clam Video

17. Annelida Phylum: Annelida – earthworms & leeches Characteristics
Video
Characteristics
Body is divided into segments  metamerism
True coelom
Well developed organ system
Most have setae  external bristles
Some have parapodia  fleshy appendages

18. 1. Class: Polychaeta – marine annelids Have setae & parapodia
3 Classes
1. Class: Polychaeta – marine annelids
Have setae & parapodia
Have antannae (the only class)
Some are free-swimmers

19. 2. Class: Hirudinea – leeches No setae
Live in calm bodies of freshwater
Sucker at each end
Smallest class of annelids
Video

20. 3. Class: Oligochaeta – earthworm
Have setae – used to anchor their segments to soil as they burrow through the soil.
Contract and expand in order to move
Video

21. Main Systems of an Earthworm
Digestive system
Worms feed on organic matter in the soil.
Worms loosen & aerate soil.
Travels in the following pathway: prostomium (lip)  mouth  pharynx  esophagus  crop  gizzard  intestine
Crop  stores food, gizzard  grinds food

22. Circulatory system
Closed system – have blood vessels
Aortic arches – serve as a heart
5 pair
Have a dorsal & ventral blood vessel

23. Respiratory & Excretory systems
No lungs or gill  directly through the skin
When it rains why do worms migrate to the surface?
Nephridia is the excretory organ that eliminates waste.

24. Reproductive system
Hermaphrodites
However, they can’t fertilize their own eggs.
They attach at the clitellum & secrete a mucus layer in which the egg & sperm are placed in.

25. Sperm is injected from the seminal receptacle.
Eggs are released from the oviducts.