Kingdom Animalia

Ch. 12- Phylum Mollusca

Key Characteristics of Mollusks

• 2nd largest animal phylum• Marine, freshwater, and terrestrial• Seven classes• True coelem- gut and internal

organs suspend from body wall; cushioned by fluid

• Trochophore- Trochophore- Free-swimming larval stage propels with cilia

Key Characteristics of Mollusks• Body cavity• Bilateral symmetry• 3-part body plan:

– Visceral massVisceral mass- central section with organs– Mantle- Mantle- Heavy fold of tissue forms outer layer of body– Foot- Foot- Muscular region for locomotion

• Organ systems: Excretion, circulation, respiration, digestion, and reproduction

• Shell- Exoskeleton made of protein, calcium carbonate, and hard minerals– Univalve- one part shell– Bivalve- two part shell

• Radula- Tongue-like organ in mouth with thousands of backward-curving teeth in rows; Not in bivalves

Mollusk Body Plan

Organ Systems• Excretion:

– Coelem collects waste fluid

– Nephridia- Nephridia- tubular structures recover useful molecules to be reabsorbed by tissues

– Fluid waste leaves through pores in mantle

• Circulation:– Digestive tube

surrounded by mesoderm; barrier for diffusion of nutrients into tissues

– Blood carries nutrients and oxygen to tissues and removes waste and carbon dioxide

– 3 chambered heart; open circulatory system

– Exception: Octopus and squid; closed circulatory system

Organ Systems• Respiration:

– Most have gills; extract 50% or more oxygen

– Sea snail gas exchange through skin

– Terrestrial snails- no gills; thin moist membrane acts as primitive lung

– Snails most active at night and after rain

– When dry, pull into shell and plug opening with mucus

• Reproduction:– Most distinct male and

female individuals– Some snails and slugs

hermaphrodites– Some oysters and sea

slugs change back and forth from male to female

– Octopus, freshwater snails, some freshwater mussels have no free-swimming larvae; juvenile stage hatches from egg

http://player.discoveryeducation.com/index.cfm?guidAssetId=C0CD06F4-1E3D-4C34-96E5-4F6BAA4B7DFE&blnFromSearch=1

Class Gastropoda• Snails and slugs• Microscopic to 1 m in length• Mostly univalve; slugs and nudibranchs lost shell completely• Foot secretes mucus fro slimy path to glide on• Pair of tentacles on head with eyes on end• Herbivores use radula to scrape algae or tear leaves• Some active predators: sea slugs, whelks, and oyster drills

(radula bores holes in other mollusk shells), and cone shells (use poison-tipped radula)– Whelks used by native-americans to make beaded wampum belts for

trading or gifts to tribal leaders

• Land snails, genus Helix, raised on snail farms for food• Few marine species, such as conchs, delicacies

Gastropods

Gastropods

Class Bivalvia• Oysters, clams, scallops and mussels• Two-part hinged shell secreted by mantle• Adductor muscles- Adductor muscles- Connect valves and allow for

opening and closing; water propels when snapped shut

• No distinct head region nor radula• Nerve ganglion is simple brain; simple sense organs• Either male or female; few hermaphrodites• Sperm and egg release into water for fertilization

– Trochophore larvae– Mussels brood in pouches on gills– Larvae are parasites on fish

Class Bivalvia• Filter feeders; cilia on gills draws water in through

siphons siphons (hollow tubes)• Mucus covering gills traps marine organisms and

organic matter which is directed to the mouth• Mussels attach to rocks in intertidal zones• Scallops swim• Oysters permanently attach to rocks

– Produce pearls when tiny foreign objects get trapped between mantle and shell

– Coat them with nacre (nacre (mother-of-pearl) ) in successive layers

– Fine pearls contain mineral crystals which act like prisms for color

Bivalves

Class Cephalopoda• Squid, octopus, cuttlefish, and nautiluses• Body with large head attached to tentaclestentacles

(foot divided into numerous parts)• Tentacles equipped with suction cups or hooks

– Squids- 10 tentacles– Octopus- 8 tentacles– Nautilus- Between 80 and 90 tentacles

• Lack an external shell; except nautilus• Squid and cuttlefish have small internal shell

– Cuttlefish bones sometimes used in bird cages for calcium

http://player.discoveryeducation.com/index.cfm?guidAssetId=AE2DAA67-15DE-44E7-94DD-4D985B263D1E&blnFromSearch=1

Class Cephalopoda• Most intelligent of all invertebrates• Complex nervous system with well-developed brain• Octopuses can be trained to classify objects; square

from cross• Some have color vision• Squid have very large eyes for collecting light; Giant

squid is largest invertebrate with largest animal eyes• Move by water propulsion• Some release dark fluid when threatened to conceal

their escape – Cuttlefish ink used by many famous historical

artists

Class Cephalopoda

• Active marine predators• Feed on fish, mollusks, crustaceans,

and worms• Tentacles nab prey and pull into mouth• Strong, beak-like jaws tear apart• Radula pulls pieces into mouth• Blue-ringed octopus: Extremely

poisonous

Cephalopodshttp://player.discoveryeducation.com/index.cfm?guidAssetId=E4CA93EE-D862-4F4E-8500-6B1CAA1BA7BA&blnFromSearch=1

Ammonites

• Extinct cephalopod

• Fossil records • Had conical shell• Only living

cephalopod is the nautilus

Squid vs. Octopus

Squid Octopus

10 tentacles 8 tentacles

Inner shell No shell

Change color- camouflage No ability to change color

Swim tentacles first Swim head first

Class Caudofoveata

• Wormlike mollusks with cylindrical, shell-less body

• Deep-water marine burrowers

• No eyes• Chaetoderma

Class Aplacophora

• Shell, mantle and foot lacking

• Worm-like• Marine and

burrowing• Neomenia

Class Monoplacophora

• Single, arched shell

• Broad, flat foot• Marine • Neopilina

Class Scaphopoda

• Body enclosed in a tubular shell

• Open on both ends

• No head• Tentacles used for

deposit feeding• Dentalium

Class Polyplacophora

• Chitons– Reduced head– Flattened foot– Shell with eight

articulating dorsal valves

– Most feed on algae