Phylum Mollusca Mollusk General Characteristics

• • Are a very diverse phylum – Most have a specific type of larvae (trochophore) Are all soft-bodied with an internal or external shell

Body Plan

• • Have a coelom (true body cavity), with a one way digestive system Are bilaterally symmetrical with 4 basic body parts: 1. Foot- usually contains mouth and feeding parts 2. Mantle- tissue layer that covers most of the body 3. Shell- may be internal or external 4. Visceral Mass- contains the internal organs

Mollusk Body Plan

Feeding

• Many contain a tongue shaped structure called a radula that has hundreds of teeth • • Some have jaws surrounding radula Others use gills to filter-feed (eg bivalves)

Respiration

• Aquatic mollusks use gills (ctenidia) • Terrestrial mollusks use mantle cavity (adapted to resemble a lung)

Internal Transport

• Many have an open circulatory system • Faster moving mollusks have a closed circulatory system

Elimination and Excretion

• • Solid waste leaves through anus as feces Nitrogen waste is excreted by nephridia

Response

• Greatly varies within the phylum – Many have simple nervous systems (eg. clams) – Some have highly developed nervous systems with a well developed brain (eg. octopuses) • Octopuses have well developed eyes

Reproduction

• • Sexes are separate in most Fertilization is external in most

Major Classes

1.

Gastropods- have stomach next to foot • • Move via a muscular foot on ventral side Have one shell, a reduced shell, or no shell • Snails and slugs

Major Classes

2.

Bivalves- have two shells • Held together by one or two powerful muscles • Most are sessile • Clams, oysters, mussels, scallops

3.

Cephalopods- have head next to foot • Foot is divided into tentacles • Most have small internal shells or no shells • Octopuses, squids, cuttlefish, nautiluses

4. Polyplacophora- chitons - bear a number of plates

27-1 Section Review

1.

• Mollusks are soft bodied animals with an internal or external shell.

Characterized by having a trochophore larvae during life cycle

2. Effect of Mollusks on Humans • Mollusks as food- snails, clams, oysters, scallops, squid, etc (can cause illness during red tides).

• Mollusks as pests- snails and slugs damage crops, shipworms drill through wood • Used in biological research- eg. Cancer research

3. Uses of radulla • Herbivores use radula to scrape algae off rocks and twigs, or to eat plants • Carnivores use radula to drill through the shell of other animals • Cone shells use radula as poisonous darts

4. Protection • • • • • • Some burrow or hide Some have well developed brains and rely on senses and intelligence to avoid predation Some are fast swimmers e.g. sea butterfly or flap their shells eg. scallops Some rely on tasting bad (or poisons) to predators eg. Snails and slugs Some release ink eg. Squid Some are able to change colours to blend into surroundings eg. Octopi