Mollusks

Chapter 27

Mollusk characteristics

• Soft-bodied animals with an internal or external shell

• Trochophore: free-swimming larvae stage• Body plan

– Foot: crawling, burrowing, tentacles– Mantle: thin tissue layer that covers body– Shell: made from glands that secrete calcium

carbonate, may be reduced or lost in some– Visceral mass: internal organs

Section 27-4

Shell

Mantle cavity

Foot

Gills

Digestive tract

Snail

Earlymollusk

Clam

Squid

The Mollusk Body Plan

Figure 27–21 

Feeding

• Herbivores, carnivores, filter feeders, detritivores, parasites

• Siphon: tubelike structure where water enters and leaves the body, washing water over gills and trapping plankton

Radula

• flexible, tongue-shaped structure with tiny teeth for drilling through shells or scraping algae off rocks

Hypselodoris bilineata

Digestive system

Respiration

• Gills within mantle cavity

• Large surface area with blood vessels

Orange peel nudibranch (about 25 cm length) with protruding white gills on the ventral side

Circulation

• Circulatory system to carry oxygen, nutrients, & waste

• Open circulatory system

Excretion and Reproduction

• Nephridia: tube shaped structures remove ammonia from blood

• Release waste outside of body

• Sexual reproduction

• External fertilization

Laying squid eggs

Response

• Clams and other bivalves live sedentary lives and have simple nervous systems.

• Cephalopods have highly developed nervous systems.

octopus eye

Movement

• Snails: secrete mucus and crawl slowly

• Cephalopods: fast moving, drawing water into mantle cavity and forcing water out of siphon (like jet propulsion)

excessive amount of snail mucus secreted

Snail mucus trail

Groups of mollusks

• Gastropods: shell-less or single-shelled mollusks with muscular feet

Sea snail

Mexican turbo snail

Nudibranch sea slug

Nudibranch sea slug

Groups of mollusks

• Bivalves: two shells held together by one or two powerful muscles

Section 27-4

Mouth

Shell

Stomach CoelomHeart

Nephridium

Adductor muscle

Anus

Excurrentsiphon

Incurrentsiphon

Gills

Mantle cavity

Foot

Intestine

Mantle cavity

Adductormuscle

The Anatomy of a Clam

Figure 27–23 

Groups of mollusks

• Cephalopods: soft-bodied mollusks where the head is attached to a foot, divided into arms and tentacles

Squid

Octopus

Cuttlefish

Vampire squid

Octopus eye

Nautilus