Chapter 29

Mollusks and Annelids

Section 1: Mollusks

Section 2: Annelids

Section 1

Mollusks

Objectives:•Summarize the evolutionary relationship between mollusks and annelids.•Describe the key characteristics of mollusks.•Describe excretion, circulation, respiration, and reproduction in mollusks.•Compare the body plans and feeding adaptations of gastropods, bivalves, and cephalopods.

Section 1

Mollusks

A True Coelom•Coelom and Trochophore Mollusks and annelids have a true body cavity and many mollusks and annelids have a larval form called a trochophore.

Section 1

Mollusks

Key Characteristics of Mollusks•Key Traits The mollusk body has three distinct parts: a visceral mass, a mantle, and foot. All mollusks except bivalves have a rasping tonguelike radula. Mollusks have a true coelom and well-developed organs. •Organ Systems Most mollusks respire with gills but some respire with a primitive lung. Nephridia enable mollusks to recover the useful substances from their bodily wastes.

Section 1

MollusksBody Plans of Mollusks•Gastropods Gastropods (snails and slugs) live in oceans, in fresh water, and on land. Gastropods have an open circulatory system.•Bivalves Bivalves (clams, oysters, and their kin) are aquatic and have hard shells called valves that protect their soft bodies. Bivalves have an open circulatory system.•Cephalopods Cephalopods (octopuses, squids, and their kin) have a well-developed head region, many tentacles, and a closed circulatory system. Most cephalopods have no external shell.

Section 2

Annelids

Objectives:•Identify the major change in body plan that distinguishes annelids from mollusks.•Describe the basic annelid body plan.•Describe the annelid digestive system.•Compare the three classes of annelids.

Section 2

Annelids

The First Segmented Animals•Characteristics of Annelids Annelids are coelomate worms that have segmented bodies and complex nervous systems. Annelids are classified according to the presence or absence of setae and parapodia. Annelids respire through their skin, and they have a closed circulatory system.

Section 2

AnnelidsAnnelid Groups•Marine Worms Marine polychaetes have parapodia and setae. Some are active predators and others are filter feeders.•Earthworms Earthworms burrow through the soil, ingesting it as they crawl.•Hydrostatic Skeleton The fluids within the coelom of each body segment creates a hydrostatic skeleton that supports the segment. •Leeches Leeches lack parapodia and setae, and their segments are not separated internally. They may be aquatic or terrestrial, and some are parasites.