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.

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.

Mollusk CharacteristicsMost are filter feeders (100,000

+ species).True coelom3 main body parts: muscular

foot, head, & visceral mass.Circulation, respiration,

digestion, excretion, reproduction, nerve impulses.

Body Arrangement1. 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.

4 Classes of Mollusks1. 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

SnailsGills for respirationSurvive in moist environmentsOpen Circulatory SystemHemocoel – open body cavity for

blood circulation.Radula – a saw-like tongue

made of chitin.Use foot to move with wavelike

contractions.

2. Class: Polyplacophora – chitons–Polyplacophora means

“many plates”–Their shells consist of 8

overlapping plates.

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.

SquidsLargest invertebrateGiant 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 sexesSquid & octopus use jet propulsion

to move.Video

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

–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.

Body parts of a clamDissectionClam Video

Annelida Phylum: Annelida – earthworms &

leeches– Video

Characteristics– Body is divided into segments

metamerism– True coelom– Well developed organ system– Most have setae external bristles– Some have parapodia fleshy

appendages– Video

3 Classes1. Class: Polychaeta –

marine annelids–Have setae & parapodia–Have antannae (the only class)

–Some are free-swimmers

2. Class: Hirudinea – leeches–No setae–Live in calm bodies of freshwater

–Sucker at each end–Smallest class of annelids

•Video

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

Main Systems of an EarthwormDigestive systemWorms 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

Circulatory systemClosed system – have

blood vesselsAortic arches – serve as a

heart–5 pair

Have a dorsal & ventral blood vessel

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.

Reproductive systemHermaphroditesHowever, they can’t

fertilize their own eggs.They attach at the

clitellum & secrete a mucus layer in which the egg & sperm are placed in.

Sperm is injected from the seminal receptacle.

Eggs are released from the oviducts.