Mollusks

Section 29.1

Soft-bodied Animals

Snails, slugs, oysters, clams, scallops, octopuses, squid

A larval stage called a trochophore Second largest phylum of animals Found in almost all marine, freshwater

and land ecosystems Three major classes: 1. Gastropods2. Bivalves3. Cephalopods

Key Characteristics

1. Body cavity – a true coelom; in most species it is a small area around the heart

2. Symmetry – bilateral3. Three-part body plan – a.) visceral mass: central

section with organs, b.) mantle: heavy outer layer of the body, c.) foot: muscular organ used for movement

4. Organ systems – excretion, circulation, respiration, digestion, reproduction

5. Shell – many have an exoskeleton of 1 or 2 shells made of protein and calcium carbonate

6. Radula – most have a tongue-like organ in the mouth with rows of pointed teeth to scrape food off rocks

Organ Systems

Excretion:

1. Body waste collects in the coelom

2. Cilia moves the fluid into nephridia

3. Fluid is filtered – useful molecules returned to body & fluid wastes leave

Organ Systems, continued…

Circulation: Most have a three-chambered heart

and an open circulatory system Octopuses and squid have a closed

circulatory system

Organ Systems, continued…

Respiration: Most have gills located in the mantle

cavity Land snails have no gills – but the

mantle cavity acts as a primitive lung

Organ Systems, continued…

Reproduction Most have males and females – some

snails and slugs are hermaphrodites Some species can change back and

forth between sexes

Gastropods

“Stomach-foot” Snails and slugs Mainly marine, some in freshwater or

on land Most are herbivores

Gastropod means ‘stomach-foot’

Gastropods have a radula

Bivalves

“Two shells” Clams, oysters, mussels, scallops Two shells are connected to adductor

muscle which can close shell and may also help in swimming

Most are filter feeders which use tubes called siphons to remove food from water

Many of these are important food sources for people

Bivalve means ‘two-shells’

The sea scallop – a typical bivalve

Bivalves have a muscular foot for movement

Cephalopods

“Head-foot” Squids, octopuses, cuttlefish, nautiluses Large head attached to tentacles Tentacles have suction cups or hooks The most intelligent of all invertebrates Well-developed eyes similar to

vertebrates

Cephalopods – ‘head-foot’ squid, chambered nautilus, octopus

Cephalopods have well-developed eyes

Cephalopod have tentacles with suckers