Chapter 35 Mollusks and Annelids Table of...

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Resources Chapter menu

Mollusks and Annelids Chapter 35

Table of Contents

Section 1 Mollusca

Section 2 Annelida



Section 1 Mollusca

Chapter 35

Objectives

• Describe the key characteristics of mollusks.

• Describe the body plan of mollusks.

• Name the characteristics of three major classes of

mollusks.

• Compare the body plans of gastropods, bivalves,

and cephalopods.



Section 1 Mollusca

Chapter 35

Characteristics of Mollusks

• Phylum Mollusca is a diverse group of invertebrates

such as clams, snails, slugs, squids, and octopuses.

• They are called mollusks, from the Latin molluscus,

which means “soft.” Many mollusks have soft bodies

and some have a hard shell.

• Some mollusks are sedentary filter feeders, while

others are fast-moving predators.

• Mollusks are coelomates.

• Most aquatic mollusks and annelids have a larval

stage called a trochophore.



Section 1 Mollusca

Chapter 35

Body Plan of Mollusks

• The body of a mollusk is generally divided into:

– the visceral mass, which contains the heart and the

organs of digestion, excretion, and reproduction

– the head-foot, which consists of:

• the head, which contains the mouth and sensory

structures

• the foot, a large, muscular organ for locomotion

• The coelom is limited to a space around the heart.

• A layer of epidermis called the mantle covers the

visceral mass.



Section 1 Mollusca

Chapter 35

Body Plan of Mollusks, continued

• In most mollusks, the mantle secretes one or more hard shells containing calcium carbonate.

• This disadvantage of a shell is offset by having gills.

• The gills are protected within the mantle cavity.

• Most mollusks are bilaterally symmetrical.

• The nervous system consists of paired clusters of nerve cells called ganglia.

• The main feeding adaptation of many mollusks is the radula.



Section 1 Mollusca

Chapter 35

Class Gastropoda

• The largest and most diverse class of mollusks is

Gastropoda, whose members are called

gastropods.

• Most gastropods, including snails, abalones, and

conchs, have a single shell. Others, such as slugs

and nudibranchs, have no shell.

• Gastropods undergo the process of torsion during

larval development.

• Gastropods have an open circulatory system in which

a heart pumps hemolymph from gills or lungs into

the hemocoel.



Section 1 Mollusca

Chapter 35

Class Gastropoda, continued

Snails • Snails live in moist and aquatic environments. • Most snails locate food using eyes at the end of

tentacles. • Snails survive dry periods by retreating into their

shells and sealing the opening with a mucous plug. Other Gastropods • Slugs are terrestrial and lack shells.

• Nudibranchs are marine and lack shells.

• Pteropods, or “sea butterflies,” have a foot that is modified into a winglike flap and used for swimming rather than crawling.



Section 1 Mollusca

Chapter 35

Class Bivalvia

• The class Bivalvia includes aquatic mollusks such as

clams, oysters, and scallops.

• They are called bivalves because their shell is

divided into two valves connected by a hinge.

• Each valve consists of three layers.

• Most bivalves are sessile filter feeders.

• Bivalves lack a distinct head and have three pairs of

nerve ganglia.



Section 1 Mollusca

Chapter 35

Class Bivalvia, continued

Clams

• Clams are bivalves that live buried in mud or sand.

• The mantle cavity of a clam is sealed except for a pair of

hollow, fleshy tubes called siphons.

– Water enters through the incurrent siphon.

– Water leaves through the excurrent siphon.

Other Bivalves

• Oysters become attached to a hard surface early in their

development.

• Scallops can move through the water by repeatedly

opening their valves and snapping them shut.



Section 1 Mollusca

Chapter 35

Class Cephalopoda

• The class Cephalopoda includes octopuses, squids,

cuttlefishes, and chambered nautiluses.

• They are called cephalopods, which means “head-

foot.”

• They are specialized as free-swimming predators.

• They have tentacles and beaklike jaws on the head.

• Their nervous system is very advanced.

• They have a closed circulatory system.

• Many use pigments to hide and disguise

themselves.



Section 1 Mollusca

Chapter 35

Class Cephalopoda, continued

Squids

• Squids are cephalopods with ten tentacles.

• They propels themselves by pumping jets of water with the

mantle through an excurrent siphon.

Octopuses

• Octopuses have eight tentacles and are similar to squids.

• They often crawl along the ocean bottom or lie in wait for prey.

Chambered Nautiluses

• The chambered nautilus is the only existing cephalopod that has

retained its external shell.

• The shell is coiled and divided into a series of chambers. The

body is confined to the outermost chamber.



Chapter 35

Mollusk Body Plan

Section 1 Mollusca



Chapter 35

Characteristics of Mollusks

Section 1 Mollusca



Chapter 35

Anatomy of a Trochophore

Section 1 Mollusca



Chapter 35

Anatomy of a Mollusk

Section 1 Mollusca



Chapter 35

Radula

Section 1 Mollusca



Chapter 35

Types of Mollusks

Section 1 Mollusca



Chapter 35

Characteristics of Gastropods

Section 1 Mollusca



Chapter 35

Anatomy of a Snail

Section 1 Mollusca



Chapter 35

Characteristics of Bivalves

Section 1 Mollusca



Chapter 35

Anatomy of a Clam

Section 1 Mollusca



Chapter 35

Characteristics of Cephalopods

Section 1 Mollusca



Section 2 Annelida

Chapter 35

Objectives

• Identify the structures that provide the basis for

dividing annelids into three classes.

• List the advantages of body segmentation.

• Describe the structural adaptations of earthworms.

• Compare the three classes of annelids.



Section 2 Annelida

Chapter 35

Characteristics of Annelids

• The phylum Annelida is made up of bilaterally

symmetrical, segmented worms.

• This phylum includes common earthworms, feather-

duster worms, and bloodsucking leeches.

• They are called annelids, which means “little rings” and

refers to the many body segments.

• Annelids have a true coelom that is divided into

separate compartments by partitions.

• Most annelids have external bristles called setae

(singular, seta), and some have fleshy protrusions

called parapodia (singular, parapodium).



Section 2 Annelida

Chapter 35

Class Oligochaeta

• Annelids of the class Oligochaeta generally live in the soil or

in fresh water and have no parapodia.

• Oligochaeta means “few bristles”; these annelids have a few

setae on each segment.

• The most familiar member is the earthworm.

Structure and Movement

• An earthworm’s body has over 100 nearly-identical

segments.

• Circular and longitudinal muscles line the interior body wall.

• Locomotion is made possible by segmentation.



Section 2 Annelida

Chapter 35

Class Oligochaeta, continued

Feeding and Digestion

• Earthworms ingest soil as they burrow through it.

• Soil is moved through these structures:

– mouth

– pharynx

– esophagus

– crop

– gizzard

– intestine

• includes the typhlosole

– anus

• Earthworms play an important role in the condition of soil.



Section 2 Annelida

Chapter 35


Circulation

• Contractions of the aortic arches and the dorsal blood vessel force

blood through the closed circulatory system.

Respiration and Excretion

• Oxygen and carbon dioxide diffuse through the skin, which contains

many small blood vessels and must be kept moist.

• Cellular wastes and excess water are excreted through nephridia.

Neural Control

• The nervous system of an earthworm consists of a chain of ganglia

connected by a ventral nerve cord.

• Sensory structures are found in all segments but are

concentrated at the anterior end.



Section 2 Annelida

Chapter 35


Reproduction

• Earthworms are hermaphrodites, but an individual worm

cannot fertilize its own eggs.

• During mating, earthworms press their ventral surfaces

together.

• They are held together by their setae and by a film of mucus

secreted by each worm’s clitellum.

• The sperm from each worm move through the mucus to the

seminal receptacle of the other.

• The clitellum secretes a tube of mucus and chitin.

• Fertilization occurs inside the tube, which forms a

protective case for the young worms.



Section 2 Annelida

Chapter 35

Class Polychaeta and Hirudinea

• Most annelids are members of the class Polychaeta, which

means “many bristles.”

• Polychaetes differ from other annelids in that they have

antennae and specialized mouthparts.

• They are the only annelids that have a trochophore stage.

• Hirudinea is the smallest class of annelids, consisting of about

500 species of leeches.

• Leeches have no setae or parapodia.

• Many leeches are carnivores but some are parasites that suck

blood from other animals.



Chapter 35

Anatomy of the Earthworm

Section 2 Annelida



Chapter 35

Characteristics of Annelids

Section 2 Annelida



Chapter 35

Segmentation

Section 2 Annelida



Chapter 35

Types of Annelids

Section 2 Annelida



Chapter 35

Anatomy of an Earthworm

Section 2 Annelida



Chapter 35

Movement of an Earthworm

Section 2 Annelida



Chapter 35

Feeding Habits of an Earthworm

Section 2 Annelida



Multiple Choice

1. Why do terrestrial snails and slugs need an

environment with a high moisture content?

A. to avoid drying out

B. to see and hear better

C. to swim more efficiently

D. to avoid being eaten by birds

Standardized Test Prep Chapter 35



Multiple Choice, continued

1. Why do terrestrial snails and slugs need an

environment with a high moisture content?

A. to avoid drying out

B. to see and hear better

C. to swim more efficiently

D. to avoid being eaten by birds





2. Which of the following are the only mollusks with a

closed circulatory system?

F. snails

G. bivalves

H. gastropods

J. cephalopods





3. Annelids are divided into three classes based partly

on the number of which of the following?

A. setae

B. segments

C. nephridia

D. aortic arches





4. Parapodia are distinguishing characteristics of which

class of annelids?

F. Bivalvia

G. Hirudinea

H. Polychaeta

J. Oligochaeta





The diagram below shows the internal structure of a

bivalve. Use the diagram to answer the questions

that follow.





5. Which of the structures is involved in respiration and

feeding?

A. A

B. B

C. C

D. D





6. What is structure B?

F. a gill

G. a siphon

H. the mantle

J. an adductor muscle





7. Nephridium : excretion :: clitellum :

A. digestion

B. circulation

C. respiration

D. reproduction





The diagram below shows the mouth structure of a

class of organisms. Use the diagram to answer the

question that follows.





8. In which class of organisms is the mouth structure

shown above most likely to be found?

F. bivalves

G. hirudines

H. gastropods

J. polychaetes




Short Response

During larval development, gastropods undergo a

process called torsion.

Describe the outcome of torsion.




Short Response, continued

During larval development, gastropods undergo a

process called torsion.

Describe the outcome of torsion.

Answer: Torsion is a developmental process in

which the visceral mass twists around 180

degrees in relation to the head, bringing the

mantle cavity to the anterior end of the snail.





Extended Response

Base your answers to parts A & B on the

information below.

Earthworms have three main structural features that

enable movement.

Part A Describe these features.

Part B Explain how they work together to enable

movement.




Extended Response, continued

Answer:

Part A Circular muscles and longitudinal muscles line the

interior body wall of an earthworm. Setae, or external

bristles, line the ventral surface of the earthworm.

Coelomic fluid is found in individual segments.

Part B: Contractions of the circular muscles increase the

pressure of the coelomic fluid. This increased pressure

causes the animal to elongate and pushes the anterior

end forward. The animal then uses its setae to grip the

ground as the longitudinal muscle contracts, pulling the

posterior end forward.

Chapter 35 Mollusks and Annelids Table of...

Documents

Transcript of Chapter 35 Mollusks and Annelids Table of...