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Ch. 35 – Mollusks & Annelids

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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.

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