Download Mollusks Annelids

GUIDE FOR READING
After you read the following
sections, you will be able to
CHAPTER
27-1 Mollusks
• Explain how mollusks perform
their essential life functions.
• Describe and give examples
of the three major classes
Mollusks
27-1 Mollusks
Guide For Reading
¦ What are mollusks, and how do they perform
essential life functions?
sa What are the three major classes of mollusks?
¦ How do mollusks fit into the world?
of mollusks.
• Discuss how mollusks affect
humans and other living things.
and
27-2 Annelids
• Describe how annelids perform
their essential life functions.
• List and give examples of three
Annelids
classes of annelids.
Members of the phylum Mollusca are known as mollusks.
Mollusks evolved in the sea more than 600 million years ago
and have experienced a long and successful adaptive radiation.
Today there are more than 100,000 mollusk species, which are
divided into seven classes. Mollusks live everywhere—from
deep ocean trenches to mountain brooks to the tops of trees.
They range in size from snails as small as a grain of sand to
giant squids that may grow more than 20 meters long. And as
you can see in Figure 27-1, mollusks come in a wide range of
forms and colors.
Journal Activity
" YOU AND YOUR WORLD
The organisms that seem to be exotic flowers swaying
in the breeze are actually annelid worms. The worms
use their feathery gills for feeding and respiration. The
spotted nudibranch, or sea slug, (inset) is a mollusk
from the Great Barrier Reef in Australia.
Have you ever observed a snail or a
slug? In your journal, describe the
animal and how it moves. What were
you doing when you noticed the ani¬
mal? What characteristic intrigues
you most about the animal?
H,lave you ever eaten fried dams, broiled scallops, or calamari in
tomato sauce? Have you ever gone fishing with live worms as bait?
If you have, you are already familiar with some of the more common
members of the two phyla that we shall study in this chapter:
mollusks and annelids (segmented worms). Both of these phyla are
ancient, very large, and remarkably diverse. Both provide many
examples of how evolution can mold a single basic animal body
plan into many different shapes. And both remind us that animals
with ancient and simple body plans can be very well adapted to
their environments.
What are mollusks and annelids? How are they related to one
another? How are they adapted to their environments? What
relationships do they have with other living things? You will find the
answers to these questions in the pages that follow.
Figure 27-1 Chitons are
relatively primitive marine
mollusks that have a shell made
up of a number of plates (inset).
Snails are more specialized
mollusks that have a one-part
shell. The tree snail is creeping
over a red Heliconia flower.
What Is a Mollusk?
Why are animals that look and act so differently grouped in
the same phylum? One reason mollusks are classified together
is that they share similar developmental patterns. (As you may
recall from Chapter 15, many animals are classified on the
basis of shared features during early development.) Most mol¬
lusks have a special kind of larva called a trochophore (TROHkoh-for). See Figure 27-2 on page 586. Trochophore larvae
swim in open water and feed on tiny floating plants.
Trochophore larvae are also seen in segmented worms
which belong to the phylum Annelida. Biologists believe that
this indicates that mollusks and annelids evolved from a com¬
mon ancestor that existed during the Precambrian Period
(more than 580 million years ago) and had a trochophore
Figure 27-2 The trochophore larva
of a chiton, like other trochophore
larvae, has a tuft of cilia making up
the "handle" on its top-shaped body
and a band of cilia encircling its
body.
Figure 27-3 The basic body parts
of mollusks are the foot, mantle,
shell, and visceral mass. Note that
the form and function of the foot
and shell vary greatly among
larva. Because the phyla Mollusca and Annelida are closely re¬
lated to each other, we shall discuss them both in this chapter.
Another reason mollusks are placed in a single phylum is
that their different forms are the results of variations on the
same basic body plan. Mollusks are defined as soft-bodied
animals that have an internal or external shell. Their name is
derived from the Latin word molluscus, meaning soft. Although
a few present-day mollusks lack shells, they are thought to
have evolved from shelled ancestors.
Form and Function in Mollusks
As you can see in Figure 27-3, the body plan of almost all
mollusks consists of four basic parts: foot, mantle, shell, and
visceral mass. The soft muscular foot usually contains the
mouth and other structures associated with feeding. The foot
takes many different shapes in mollusks: Flat surfaces adapted
to crawling, spade-shaped structures for burrowing, and tenta¬
cles for capturing prey are a few examples. The mantle is a
thin, delicate tissue layer that covers most of a mollusk's body,
much like a cloak. The shell, which is found in almost all mol¬
lusks, is made by glands in the mantle that secrete calcium car¬
bonate (CaCOs). Just beneath the mantle in most mollusks is
the visceral mass, which contains the internal organs.
mollusks.
These basic body parts have taken on different forms as
mollusks evolved adaptations to different habitats. The type of
foot and the kind of shell that mollusks have are used to group
them into classes. Later in this chapter we shall examine the
three major classes of mollusks.
FEEDING Mollusks have evolved many types of feeding
mechanisms and feed on many kinds of food. In fact it would be
simpler to list the few things these animals do not eat than it
would be to describe everything they can feed on! Every mode
of feeding is seen in the phylum Mollusca. Most mollusks are
herbivores, carnivores, or filter feeders, but a few species are
detritus feeders and others are parasites.
Many mollusks—snails and slugs, for example—feed with a
tongue-shaped structure called a radula (RAJ-oo-lah). The radula is a layer of flexible skin that carries hundreds of tiny
teeth, which make it look and feel like sandpaper. Inside the
radula is a stiff supporting rod of cartilage. When the mollusk
feeds, it places the tip of the radula on the food and pulls the
sandpapery skin back and forth over the cartilage. Mollusks
that are herbivores use their radula to scrape algae off rocks
and twigs in the water or to eat the buds, roots, and flowers of
land plants. Mollusks that are carnivores use their radula to
drill through the shells of other animals. Once they have made
a hole through the shell, these carnivores extend their mouth
and radula into the shell and tear up and swallow the prey's
soft tissue. In the carnivorous snails called cone shells, the tiny
rasping teeth of the radula have evolved into long hollow darts
that are attached to poison glands. A cone shell uses these
darts to stab and poison prey such as small fish.
Although they may have a radula, carnivorous mollusks
such as octopi and certain sea slugs typically use sharp jaws to
eat their prey. Like cone shells, some octopi produce poison to
subdue their prey. Although cone shells and octopi generally
feed on fish and other small animals, the poisons produced by
some species are strong enough to hurt or even kill humans.
Mollusks such as clams, oysters, and scallops are filter
feeders. They use their feathery gills to sift food from the
water. As these animals pass water over their gills, phytoplankton (tiny photosynthetic organisms) in the water become
trapped in a layer of sticky mucus. Cilia on the gills move the
mixture of mucus and food into the mouth.
Figure 27-4 Many mollusks scrape
bits of food into their mouth by
pulling the tooth-covered skin of the
radula back and forth over a
supporting rod of cartilage. The
scanning electron micrograph shows
the teeth on the radula of a land
snail.
RESPIRATION Gills serve as organs of respiration as well
Shell
as filters for food. In fact, in most species gills are used only for
breathing. Aquatic mollusks such as snails, clams, and octopi
breathe by using gills located inside their mantle cavities. But
Mantle cavity
land snails and slugs breathe by using a specially adapted
Visceral mass
mantle cavity that is lined with many blood vessels. The mantle
is wrinkled or folded to fit a larger surface within the limited
Radula teeth
Figure 27-5 The nudibmnch (right) breathes through its skin
and tuft of gills. Janthina (left), which uses a raft of air bubbles
to float at the ocean surface, breathes with gills inside its shell.
Many land snails (bottom) use their mantle cavity as a lung.
space of the cavity. This surface is constantly kept moist so
that oxygen can enter the cells. Because the mantle loses water
in dry air, most land snails and slugs must live in moist places.
They prefer to move around at night, during rainstorms, and at
other times when the air is humid.
INTERNAL TRANSPORT Oxygen that is taken in by the
respiratory system and nutrients that are the products of di¬
gestion are carried by the blood to all parts of a mollusk's
body. The blood is pumped by a simple heart through what is
called an open circulatory system. "Open" does not mean that
blood can spill to the outside of the animal! It means that blood
does not always travel inside blood vessels. Instead, blood
works its way through body tissues in open spaces called sin¬
uses. These sinuses lead to vessels that pass first through the
gills, where oxygen and carbon dioxide are exchanged, and
then back to the heart. Open circulatory systems work well for
slow-moving or sessile (attached to one spot) mollusks like
snails and clams. But the flow of blood through sinuses is not
efficient enough for fast-moving octopi and squids. Those ani¬
mals have closed circulatory systems, in which blood always
moves inside blood vessels.
EXCRETION Like other animals, mollusks must eliminate
waste products. Undigested food becomes solid waste that
leaves through the anus in the form of feces. Cellular metabo¬
lism produces nitrogen-containing waste in the form of ammo¬
nia. Because ammonia is poisonous, it must be removed from
body fluids. Mollusks get rid of ammonia by using simple tubeshaped organs called nephridia (neh-FRiHD-ee-ah; singular:
nephridium). Nephridia remove ammonia from the blood and
release it to the outside.
RESPONSE Mollusks vary greatly in the complexities of
their nervous systems and their abilities to respond to environ¬
mental conditions. Clams and other two-shelled mollusks,
many of which lead basically inactive lives burrowing in mud
or sand, have simple nervous systems. They have several small
ganglia near the mouth, a few nerve cords, and simple sense
organs such as chemical and touch receptors, statocysts (sim¬
ple organs for balance), and ocelli (eyespots). Octopi and
other tentacied mollusks, on the other hand, are active and in¬
telligent predators that have the most highly developed ner¬
vous systems of all members of their phylum. Because of their
well-developed brain, these animals can remember things for
long periods of time, and they may even be more intelligent
than some vertebrates. The numerous complex sense organs
these mollusks possess help them distinguish shapes by sight
and texture by touch. Octopi can be trained to perform differ¬
ent tasks in order to obtain a reward or avoid punishment. Be¬
cause of these abilities, octopi are often studied by
psychologists interested in the way animals learn.
REPRODUCTION As with almost all other essential func¬
tions, mollusks accomplish the function of reproduction in dif¬
ferent ways. In most mollusks, the sexes are separate and
fertilization is external. These mollusks—which include many
snails, almost all two-shelled mollusks, and most of the species
in the four minor classes of mollusks—release eggs and sperm
into open water in enormous numbers. Eggs and sperm find
each other by chance, and free-swimming larvae develop from
the resulting fertilized eggs. In tentacied mollusks and certain
snails, fertilization takes place inside the body of the female.
Fertilization is also internal in some hermaphrodites (organ¬
isms that have both male and female reproductive organs). For
example, many hermaphroditic snails get together in pairs and
fertilize each other's eggs at the same time. Some other her¬
maphroditic mollusks, such as certain oysters, switch from one
sex to the other. Sometimes they are male (and thus produce
sperm) and sometimes female (and thus produce eggs)!
Figure 27-6 Like most mollusks,
coquina clams (inset) have an open
circulatory system. Cephalopod
mollusks, such as cuttlefish, have a
closed circulatory system. Clams
move slowly, whereas cuttlefish can
jet backwards through the water at
high speeds.
Figure 27-7 A scallop gathers
information about its environment
with tiny round eyespots and sensory
tentacles. Using those eyespots and
other sense organs, scallops stay
alert for enemies, such as starfish.
Unlike most bivalves, scallops can
sometimes escape from enemies by
clapping their shells together rapidly,
using jet propulsion to help them
jump off the ocean bottom and
scuttle away. See Figure 27-10.
Figure 27-8 The ringed top snail
(bottom, left) is found in the oceans
of the Pacific Northwest. Despite
their lack of a protective shell, the
marine nudibranch (bottom, right)
and terrestrial banana slug (top) are
not likely to be eaten by predators—
their bright colors and patterns
indicate that these gastropods are
poisonous.
Snails, Slugs, and Their Relatives
colors of the nudibranch and avoid it in the future! (While this
Members of the class Gastropoda are called gastropods
(GAS-troh-pahdz). The name gastropod literally means stomach
foot. This name is quite appropriate because most gastropods
move by means of a broad, muscular foot located on their ven¬
does not help the first nudibranch, it does protect others of its
kind.) Thus shell-less gastropods do have means of protection.
tral (stomach) side. Gastropods include the familiar pond
snails and land slugs as well as more exotic mollusks such as
abalones, sea butterflies, sea hares, and nudibranchs.
Many gastropods have a one-piece shell that protects their
soft bodies. This shell may be simple and shieldlike, as in lim¬
pets, or coiled, as in snails. When threatened, many snails can
pull up completely into their coiled shells. Some snails are ad¬
ditionally protected by a hard disk on their foot that forms a
solid "door" at the mouth of their shell when they withdraw.
Some gastropods have small shells or, as is the case with
slugs, lack shells completely. This would seem to make them
easy prey for hungry predators. However, these gastropods are
not entirely helpless. Most land slugs are protected by their be¬
havior—they spend the daylight hours hiding under rocks and
logs, hidden from birds and other animals that might eat them.
Some sea hares have a special ink-producing gland that they
use when threatened to squirt ink into the surrounding water.
This confuses predators and allows the sea hare to escape
under its "smoke screen." Some gastropods, such as sea but¬
terflies, escape predators by swimming rapidly. Many nudi¬
branchs, or sea slugs, have chemicals in their bodies that taste
bad or are poisonous. When a predator nibbles on one of these
bad-tasting morsels, it gets sick. In addition, some nudibranchs
use nematocysts from the cnidarians they eat to sting preda¬
tors. The bad-tasting, poisonous, stinging, or otherwise booby-
trapped nudibranchs are usually brightly colored. The bright
colors warn predators to stay away. If a predator ignores the
warning colors and eats a nudibranch, the consequences
usually guarantee that the predator will remember the bright
Two-Shelled Mollusks
Members of the class Bivalvia (bi- means two; valve means
shell) have two shells that are hinged together at the back and
held together by one or two powerful muscles. Common
bivalves include clams, oysters, and scallops. Bivalves may be
tiny or as large as the giant clam Tridacno, which has been
known to grow as large as 1.9 meters in length.
Although bivalve larvae are free-swimming, they soon set¬
tle down to a relatively quiet life on the bottom of a body of
water. Some bivalves, such as clams, burrow in mud or sand.
Others, such as mussels, secrete sticky threads to attach them¬
selves to rocks. Although most adult bivalves are sessile, some,
such as scallops, can move around rapidly by flapping their
shells when threatened.
The mantles of bivalves, like those of most other mollusks,
contain glands that manufacture the shells. These mantle
glands also keep the shell's inside surfaces smooth and com¬
fortable by secreting layers of mother-of-pearl. If a foreign ob¬
ject—a sand grain or small pebble, for example—gets caught
between mantle and shell, the mantle glands cover it with this
secretion. After many years these objects become completely
coated and are called pearls.
Figure 27-10 The internal structures of a clam, a typical bivalve,
are shown in the diagram on the left. Another bivalve, the
scallop, can swim by rapidly opening and closing its shell (top).
Pearls—objects coated by smooth, shiny secretions of a bivalve's
mantle—may be beautiful and valuable gems or they may be
fascinating curios, like the pearl fish (bottom).
Nephridium
Posterior
adductor
muscle
Anterior
muscle
Excurrent
Mouth
siphon
ncurrent
siphon
Intestine
Gonad (reproductive organ)
Figure 27-9 When threatened, a
sea hare releases purple ink into the
water. This confuses predators and
allows the sea hare to make its
escape.
Tentacled Mollusks
How Mollusks Fit into the World
Cephalopods (SEHF-uh-loh-pahdz)—members of the class
Cephalopoda—are among the most active and interesting mol¬
lusks. This class includes octopi, squids, cuttlefish, and nauti¬
luses. Cephalopoda means head-foot (cephalo- means head-pod means foot). This name refers to the fact that a cephalopod's head is attached to its foot, which is divided into tenta¬
cles, or arms. Cephalopods range in size from tiny cuttlefish
less than 2 centimeters long to giant squids, which are thought
to grow to more than 20 meters long.
Mollusks play many different roles in living systems. For
example, they feed on plants, prey on animals, and "clean up"
their surroundings by eating detritus. Some of them are hosts
to symbiotic algae or to parasites; others are themselves para¬
sites. In addition, mollusks are an important source of food for
many organisms, including humans.
Modern-day scientists have found some new uses for mol¬
lusks. Because filter-feeding bivalves concentrate dangerous
pollutants and microorganisms in their tissues, careful checks
Most cephalopods have eight flexible tentacles equipped
of bivalves can warn biologists and public health officials of
with a number of round sucking disks that are used to grab and
hold fish and other prey. In addition to these tentacles, cuttle¬
fish and squids also have two long, slender arms with suckers
on the end. Nautiluses have many more tentacles (38 to 90)
than other cephalopods. Their tentacles lack suckers but are
made sticky by a mucuslike covering.
Although fossil evidence indicates that their ancestors had
large external cone-shaped or coiled shells, most modern ce¬
phalopods have small internal shells or no shells at all. The
only present-day cephalopods with shells are the few species
of nautiluses. These cephalopods look much like fossil cepha¬
lopods from the beginning of the Cambrian Period, more than
500 million years ago. Cuttlefish have small shells that are
found inside their bodies. The shells of some cuttlefish are thin
and coiled, whereas others (which serve as the cuttlebone on
health problems long before scientists can detect these
dangers in the open water. Besides acting as environmental
monitors, mollusks also serve as subjects in biological re¬
search. Some current investigations are based on the observa¬
tion that snails and other mollusks never seem to develop any
form of cancer. If scientists can determine what protects the
cells of these animals from cancer, they will gain valuable in¬
sights into how to fight cancer in humans.
Although mollusks are beneficial in many ways, they do
have some negative relationships with humans. For example,
land slugs and snails are plant eaters that can do much damage
to gardens and crops. The bivalves called shipworms, which
which pet birds condition their beaks) are flat, platelike, and
do not resemble shells at all. In both nautiluses and cuttlefish,
gases in the shell help the cephalopod remain upright and
allow it to float in the water. A squid's internal shell has
evolved into a thin, flexible supporting rod known as a pen. Oc¬
topi have lost their shells completely.
Although most cephalopods lack a protective shell, they do
have other means of protection. Most cephalopods can move
quickly, either by swimming or crawling. They can also move
by using a form of jet propulsion. The cephalopods draw water
into their mantle cavities and then force that water out through
use their shells to drill their way slowly through pieces of wood
in the water, are sometimes described as the termites of the
sea, They settle on wood in large numbers and can reduce a
good-sized log to a pile of wet sawdust over the course of a few
years. Shipworms cause millions of dollars worth of damage to
wooden boats and docks every year. Another problem with
mollusks is associated with their use as food. Clams and oys¬
ters, which are among the few marine animals that are farmed
in the sea, are filter feeders and thus gather and concentrate
particles floating in the water—including bacteria, viruses, and
the toxic protists that cause red tides. Eating bivalves that con¬
tain high concentrations of pathogens (things that cause dis¬
ease), toxins, or pollutants can result in sickness or even death.
the tubelike siphon. By pointing the siphon in different direc¬
tions, they can shoot out a jet of water that propels them back¬
ward, away from danger. In addition, many cephalopods can
release large amounts of dark-colored, foul-tasting ink when
they are frightened. After squirting out a large cloud of ink,
they make a hasty retreat. Perhaps most fascinating of all, oc¬
topi can quickly change color to match the colors of their sur¬
roundings. The match is often close enough that the octopi are
nearly invisible.
Figure 27-11 The luminescent squid (top), chambered nautilus
(center), and extremely venomous blue-ringed octopus (bottom)
are examples of cephalopods.
f >: | SECTION
Cr'-'i REVIEW
1. What are mollusks? List the three major classes of
mollusks and give an example of each.
2. Describe some of the ways mollusks affect humans.
3. What are some different ways mollusks use a radula?
is dwindling rapidly. What effect would this have on the
Figure 27-12 One trend in
cephalopod evolution has been a
reduction in the size of the shell.
Most modem species have a small
shell or no shell at all. The "shell" of
the paper nautilus is actually an egg
surrounding area? On the human community?
cose.
4. How do mollusks protect themselves?
5. Connection—Ecology The number of oyster beds in
Chesapeake Bay, which is an arm of the Atlantic Ocean,
593
Guide For Reading
27-2 Annelids
: What are annelids, and how do they
perform essential life functions?
¦ What are the three classes of
annelids?
¦ How do annelids fit into the world?
Have you ever dug in a garden? If so, you have probably
made the acquaintance of the long, thin, pink earthworm. The
soft-bodied earthworm is the most common terrestrial, or landdwelling, segmented worm. But this species is only one of ap¬
Esophagus
proximately 9000 species of segmented worms that live in
moist soil, in fresh water, and in the sea. Segmented worms, or
annelids, live just about everywhere in the world. But because
most segmented worms live in the sea, and many others spend
their lives underground, only a few species are familiar to us.
What Is an Annelid?
Members of the phylum Annelida are known as annelids,
or segmented worms. An annelid is a round, wormiike animal
that has a long, segmented body. The name Annelida is de¬
rived from the Latin word annellus, which means little ring, and
refers to the ringlike appearance of the body segments.
Annelids range in size from tiny aquatic worms less than
half a millimeter long to giant earthworms more than 3 meters
long. Although they also vary greatly in color, patterning,
number of bristles, and other superficial features, most anne¬
lids are quite wormlike in appearance.
Form and Function in Annelids
The many segments of an annelid's body are separated by
internal walls called septa (singular: septum). Most of the body
Figure 27-13 Many polychaete
annelids, such as the sandworm
Nereis, use hooklike jaws to
capture prey or nibble on algae.
segments are virtually identical to one another. However, some
segments are modified to perform special functions. For exam¬
ple, the first few segments may carry one or more pairs of eyes,
several pairs of antennae, and other sense organs.
Intestine
Anus
it. When the pharynx returns to its normal position, it carries
these food particles back into the gut. In other detritus feeders,
such as earthworms, the pharynx acts like a pump. It sucks a
mixture of soil and detritus through the mouth and forces it
down into the gut. In parasites, such as leeches, the pharynx is
used to suck blood and tissue fluids from the host.
Annelids have a number of other structures that are used in
feeding. For example, some annelids filter-feed by fanning
water through their tubelike burrows and catching passing
food particles in a mucus bag. In other filter-feeding annelids,
such as the plume worm shown in Figure 27-15, the first seg¬
ment forms featherlike structures that sift detritus and plank¬
ton from the surrounding water. These feeding structures are
also used as gills for respiration.
RESPIRATION Aquatic annelids often breathe through
gills. In some of these annelids, such as feather-duster worms,
the large brightly colored feathery gills protrude from the
opening of the worm's burrow or tube. In other annelids, small
FEEDING The digestive tract, or gut, is a long tube within
Figure 27-14 The digestive system
of an earthworm is shown here. The
pharynx pumps a mixture of food
and soil into a tube called the
esophagus. The food then moves
through the crop, where it con be
stored, and through the gizzard,
where it is ground into smaller
pieces. The food is digested in the
intestine. Undigested materials pass
through the intestine and are
eliminated through the anus.
Figure 27-15 The spaghetti worm
(left) uses long tentacles to pluck
bits of detritus from the ocean floor.
In plume worms (right), a brushshaped structure on the head is used
in filter feeding and in respiration.
the body cavity of the worm that extends from the mouth to the
anus (in the tip of the "tail"). Food enters through the mouth
and travels through the gut, where it is digested. Like mollusks,
annelids have evolved structures and behaviors that allow
them to use a wide variety of foods.
One feeding organ that has evolved many different forms in
different groups of annelids is the pharynx, or the muscular
front end of the digestive tube. Many annelids can extend the
pharynx through the mouth. In carnivorous annelids, this type
of pharynx usually has two or more sharp jaws attached to it.
When a suitable animal approaches, the worm lunges forward,
rapidly extends the pharynx, and grabs the prey with its jaws.
Jaws are also present in herbivores, which use them to tear off
bits of algae. In some detritus feeders, the pharynx is covered
with sticky mucus. When these worms extend the pharynx and
press it against the sea-floor sediments, food particles stick to
595
"Hearts"
Ring
(pumping ring
vessel
Dorsal blood vessel
vessels)
Clitellum
il :
RESPONSE Many annelids are active animals with welldeveloped nervous systems. The brain sits on top of the gut at
the front end of the body. Two large nerves pass around the gut
and connect the brain with a pair of ganglia below. From these
ganglia, a ventral nerve cord runs the entire length of the
worm. Nerves from each segment of the worm enter and leave
the nerve cord at a pair of small ganglia. These nerves help
carry messages from sense organs and coordinate the move¬
ments of muscles.
Ventral'
nerve
cord
Male
reproductive
organs
[Circulatory
System
Female
reproductive
organs
Excretory
System
Ventral blood vessel
Nephridia
Nervous
System
Reproductive
System
Figure 27-16 The circulatory, excretory, nervous, and
ri
reproductive systems of an earthworm are shown here. How
many "hearts" does an earthworm have?
delicate gills are located on the sides of the body. The tube-
dwelling annelids with this type of gill breathe by fanning
water through their tubes. Many annelids take in oxygen and
give off carbon dioxide through their skin. Because the skin
must stay moist to make gas exchange possible, the worms die
if the skin dries out. To help guard against this, terrestrial an¬
nelids, such as earthworms, secrete a thin protective coating
called a cuticle to hold moisture around them.
i'l'i
I, i
'!
INTERNAL TRANSPORT Annelids typically have closed
circulatory systems organized around two blood vessels that
run the length of their bodies. Blood moves toward the head of
the worm in the dorsal (top side) vessel and toward the back of
the worm in the ventral (bottom side) vessel. In each body
segment is a pair of smaller vessels called ring vessels that
connect the dorsal and ventral vessels and supply blood to the
internal organs. In annelids such as earthworms, several of the
ring vessels near the anterior (front) end of the worm are
larger than the other ring vessels and have muscle tissue in
their walls. These vessels are often called hearts because they
contract rhythmically and help pump blood through the sys¬
tem. In other annelids, blood is moved through the body by
muscle contractions when the worm moves.
EXCRETION Like other animals, annelids produce two
kinds of wastes. Solid wastes pass out through the anus at the
end of the gut. Wastes resulting from cellular metabolism are
eliminated by nephridia (simple tube-shaped excretory
organs). A pair of nephridia in each body segment removes
waste products from the body fluids and carries them to the
outside.
596
Sense organs are best developed in the free-living marine
species of annelids. Many of these annelids have sensory ten¬
tacles, statocysts, chemical receptors, and two or more pairs of
eyes. Although the eyes are usually simple light detectors, in a
few species the eyes can actually perceive objects. Most tubedwelling species have light-sensitive cells either on their gills
or near their mouths. These cells allow the animals to detect
the shadows of predators passing overhead. When a shadow is
detected, the worm pulls back into the shelter of its tube with
amazing speed. In addition to specialized sense organs, these
free-living marine worms also have various types of isolated
sensory cells scattered along their epidermis. These cells re¬
spond to light, chemicals, and vibration.
Many other annelids have much simpler sensory systems.
For example, earthworms have no specialized sense organs.
They rely on simple sensory cells in the skin that are similar to
those found in the skin of marine annelids.
Most free-living annelids do not have body structures that
protect them from predators. Many depend on rapidly burrow¬
ing or swimming away from danger. Some, like earthworms,
grab onto the walls of their burrows to make it harder to pull
Figure 27-17 Sense organs are best
developed in free-swimming
annelids such as the paddleworm,
which has a pair of beady eyes and
a number of sensory tentacles on its
them out, Others, such as the marine fanworms, secrete pro¬
head.
tective tubes of calcium carbonate into which they withdraw if
frightened. But some annelids do fight back. Several carnivo¬
rous annelids use their sharp jaws to attack animals that try to
eat them. And the marine fireworms have tufts of poisonous
bristles that easily break off and penetrate skin, causing painful
sores and a burning sensation.
MOVEMENT Annelids have two major groups of muscles
in their body walls. One group, called longitudinal muscles,
runs from the front of the worm to the rear. When these mus¬
cles contract, they make the worm shorter. Another group of
muscles runs in circles around the body of the worm. When
these muscles contract, they make the worm skinnier. Marine
annelids can swim by using these muscles to wriggle through
the water. Burrowing annelids use their muscles to force their
way through heavy sediment—not an easy thing for a soft-
bodied animal to do!
REPRODUCTION Although a few annelids are able to
reproduce asexually by budding, most annelids reproduce
sexually. Some species have separate sexes and external
597
,1
dlelike appendages on their body segments. These appendages
are tipped with the bristles that give this class its name. In the
sea mouse, shown in Figure 27-19, the bristles are so long that
they extend over the back of the worm and look like hair or fur.
Polychaetes live in cracks and crevices in coral reefs, in
sand, mud, and piles of rocks, and even out in the open water.
Some burrow through or crawl over sediments. Others live al¬
most entirely in tubes they build for themselves. Some poly¬
chaetes are dull in color and rather uninteresting; some are
brightly colored, iridescent, or even luminescent.
Earthworms and Their Relatives
Figure 27-18 Although they look
very different from each other, both
the fanworm (left) and the fire worm
(right) are polychaetes. The fanworm
is a filter feeder that retreats into its
tube when threatened. The fireworm
defends itself with poisonous bristles
that break off and penetrate skin at
the slightest touch. The pain caused
by these bristles gives the fireworm
its name.
fertilization. This means that females and males release eggs
and sperm, respectively, into the open water where fertilization
takes place. Of course, the chances of fertilization taking place
are enhanced if many worms in an area release their eggs and
sperm at the same time. This is exactly what happens in some
species. In the South Pacific, islanders eagerly await the au¬
tumn spawning season of the annelids called palolo worms. At
a particular phase of the moon, hundreds of thousands of male
and female palolo worms swarm at the surface of the water to
release their eggs and sperm. Just before sunrise, the sea is lit¬
erally covered with these worms. The islanders, who consider
these worms a great delicacy, join sea birds and fishes that
gather to feast on the spawning worms.
Some annelids, such as earthworms and leeches, are her¬
maphrodites that undergo internal fertilization. Although an in¬
dividual worm produces both sperm and eggs, it rarely
fertilizes its own eggs. Instead, worms pair up, attach them¬
selves to each other, and exchange sperm. Each worm stores
the sperm it has received in special sacs. When eggs are ready
for fertilization, a band of thickened, specialized segments
called the clitellum (cligh-TEHL-um) secretes a mucus ring into
which eggs and sperm are released. The ring then slips off the
worm's body and forms a cocoon that shelters the eggs.
Sandworms, Bloodworms, and Their
Relatives
The class Polychaeta (poly- means many; chaeta refers to
bristles) contains many common and important marine worms.
Polychaetes (PAHL-ee-keets) are characterized by paired pad¬
The class Oligochaeta contains earthworms and related
species. Two oligochaetes (AHL-ih-goh-keets) that you might
be familiar with are earthworms and tubifex worms. Earth¬
worms are long pink worms that often show up on the surfaces
of lawns and sidewalks after it rains, are dug up in gardens, or
are sold as fishing bait. Tubifex worms are red threadlike
aquatic worms that are sold as tropical-fish food in pet stores.
Most oligochaetes live in soil or fresh water, although some
species live in the ocean. As the name of the class indicates
(oligo- means few), oligochates have fewer bristles than poly¬
chaetes. These bristles, which can be felt as a roughness on the
ventral (bottom) side of an earthworm, help anchor it in its
Figure 27-19 The long bristles of
the sea mouse look like iridescent
fur.
burrow.
Although earthworms spend most of their lives hidden
under ground, an observant person may find evidence of their
presence above ground in the form of squiggles of mud known
as castings. Recall that an earthworm (which swallows just
about anything it can get into its mouth) uses its pharynx to
suck a mixture of detritus and soil particles into its mouth. As
the mixture of food and soil passes through the intestine, part
of it is digested. Sand grains, clay particles, and indigestible or¬
ganic matter pass out through the anus in large quantities, pro¬
ducing castings. Some tropical earthworms produce enormous
castings—as large as 18 centimeters long and 2 centimeters in
diameter!
Figure 27-20 Earthworms are
hermaphrodites that undergo
internal fertilization. The sperm
from one worm fertilizes the eggs of
its partner, and vice versa.
Leeches
The class Hirudinea contains the leeches, most of which
live in moist tropical countries. Leeches are typically no more
than 6 centimeters long, but there are some tropical species
that are as long as 30 centimeters. Most leeches are freshwater
organisms that exist as external parasites, drinking the blood
and body fluids of their host. However, there are some marine
and terrestrial leeches. And roughly one fourth of all leeches
are carnivores rather than parasites. Carnivorous leeches,
which feed on soft-bodied invertebrates such as snails, worms,
and insect larva, either swallow their prey whole or suck all the
soft parts from its body.
599
SCIENCE,
All leeches have powerful suckers at both ends of their
bodies. These suckers—especially the anterior one, which
usually surrounds the mouth—are used to attach a leech to its
host. The posterior sucker is also used to anchor a leech to
rocks, leaves, and other objects as it waits for a host to come
by. Leeches penetrate the skin of their host in one of two ways
Some leeches use a muscular proboscis (proh-BAHS-ihs), or tu¬
bular organ, that they force into the tissue of their host. Others
slice into the skin of their hosts with a razor-sharp pair of jaws
Once the wound has been made, the leech uses its muscular
pharynx to suck blood from the area. Both types of leeches re¬
lease a special secretion from their salivary glands to prevent
the blood from clotting as they drink it. Some leeches also pro¬
duce a substance that anesthetizes the wound—thus keeping
the host from knowing it has been bitten!
During feeding, a leech can swallow as much as 10 times its
weight in blood. Such a huge meal can take the leech up to 200
Figure 27-21 A leech attaches to
its host with suckers on its anterior
and posterior ends (top). As it feeds,
the body of the leech swells to
accommodate as much as ten times
its mass in blood (bottom).
days to digest, with the help of symbiotic bacteria that live in
its gut. A leech can live for a year before it must feed again.
AND SOCIETY
ciiiiiiiic i null
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Leeches: Modern Applications of Ancient Medicine
There are few medical techniques as
ancient as leeching, or applying leeches to a pa¬
tient. The earliest known reference to leeching
was written by a Greek physician more than
2200 years ago. And experts believe that
#•
leeching is much older than that!
Many people once believed that diseases
could be cured by using leeches to remove
blood from the patient. However, when people
began to better understand the nature of dis¬
ease, leeches ceased to be popular medical
tools. After all, it seemed senseless to remove
blood from a patient when it was clear that mi¬
•
%
How Annelids Fit into the World
Annelids are important in many habitats. Small polychaetes and their larvae are members of the ocean plankton,
where they are food for many fishes, crabs, and lobsters. Bot¬
tom-dwelling polychaetes are important items in the diets of
food fishes such as flounder.
Oligochaetes, particularly earthworms, perform an essen¬
tial task in conditioning soil, as Charles Darwin noted in a
lengthy and detailed study. By constantly burrowing through
the ground, earthworms help aerate the soil. And by grinding
and partially digesting the incredible amount of soil and de¬
tritus that passes through their guts, earthworms speed the re¬
turn of nitrogen and other important nutrients from dead
organisms to forms that can be used by plants. Without the
continual efforts of these annelids, the structure and fertility of
farm soils would degenerate quickly, lowering crop yields.
1. What is an annelid? List and give examples of three
classes of annelids.
2. Discuss three adaptations for feeding in annelids.
3. Describe the structure of the digestive tract in an
earthworm.
4. Critical Thinking—Making Inferences Explain why
it is advantageous for an earthworm to have more lightsensitive cells in its anterior and posterior segments
than in other parts of its body.
ease. But interestingly enough, leeches are
once again in the medical spotlight.
One modern medical problem faced by
surgeons is that blood tends to collect in body
parts reattached by microsurgery. Here-is
where leeches come in handy. They are used
to remove the excess blood until the blood
vessels in the reattached part have healed.
The chemicals in leech saliva make it pos¬
sible to use leeches for a variety of other medi¬
cal purposes. These chemicals prevent blood
from clotting, dissolve existing blood clots, ex¬
pand blood vessels (to keep blood flowing),
loosen the connections between cells (to help
disperse the other chemicals), and anesthetize
the area of the bite. Researchers are currently
developing medicines based on chemicals ex¬
tracted from leech saliva. These new medi¬
cines may soon be used to clear blocked blood
vessels and to treat a variety of circulatorysystem diseases.
Leeches also produce chemicals that harm
bacteria—chemicals that they may inject into
the host as they feed. Symbiotic bacteria in¬
side the leech's gut produce an antibiotic that
keeps stored blood fresh by killing bacteria.
And the chemical in leech saliva that loosens
or dissolves connections between host cells
may also dissolve the protective coating on
bacteria, thus making them vulnerable to an
attack by the immune system.
4
/,
I.
V
croorganisms—not "bad blood"—caused dis¬
SECTION
REVIEW
600
TECHNOLOGY,
->
r,
V
•
1
(
¦
•
The leech shown here is being "milked" for its
saliva. Researchers are currently developing
medicines based on the chemicals found in
leech saliva.
As medical researchers discover new uses
for leeches, they are reminded that the ancient
practice of leeching may not have been quite
as senseless as it seemed. They are also re¬
minded of the role evolution plays in shaping
and refining the relationships between organ¬
isms (such as leeches and vertebrates). Of
course, the leeches themselves didn't decide
to perform a useful function for humans. But
recall that parasites must evolve along with
their hosts. Under pressure from natural se¬
lection, leeches have evolved adaptations that
enable them to feed effectively on vertebrate
hosts, including humans. Although leeches ap¬
peared on Earth long before humans, the
chemicals they produce still affect us. Why?
Due to common descent, our body chemistry
is similar to that of other vertebrates—includ¬
ing the leech's original vertebrate host.
601
SUMMARIZING THE CONCEPTS
PROBLEM
How do live earthworms respond to moisture and light?
The key concepts in each section of this chapter are listed below to help you
review the chapter content. Make sure you understand each concept and its
relationship to other concepts and to the theme of this chapter.
MATERIALS (per group)
2 live earthworms
piece of cardboard
in a storage container desk lamp
tray
medicine dropper
paper towels
PROCEDURE i II
1. Open the storage container and examine the
earthworms. Record your observations of
their physical characteristics. Fill the medi¬
cine dropper with water and use it to give
your earthworms a "bath." Note: Make sure
you keep your earthworms moist by giving
them frequent baths. If an earthworm's skin
dries out, it dies.
2. Fold a dry paper towel and place it on one
side of your tray, as shown in the accompany¬
ing figure. Fold a dampened paper towel and
place it on the other side of the tray.
3. Place the earthworms in the center of the
tray, between the dry paper towel and the
moist paper towel. Cover the tray with the
piece of cardboard.
4. After 5 minutes, remove the cardboard and
observe the location of the earthworms.
Record your observations.
5. Return the earthworms to their storage con¬
tainer. Using the dropper, moisten the earth¬
tray.
8. Cover one half of the tray with the piece of
cardboard. Position the lamp above the open
side of the tray.
9. After 5 minutes, observe the location of the
earthworms. Record your observations.
10. Return the earthworms to their storage con-
602
27-2 Annelids
• Mollusks are soft-bodied animals such as
snails, clams, and squids that usually have
an internal or external shell. The body plan
of mollusks consists of four basic parts: foot,
mantle, shell, and visceral mass.
• Annelids, which are also known as seg¬
mented worms, are round, wormlike animals
with long, segmented bodies. An annelid's
segments are very similar to one another
and are separated by internal partitions.
• Most gastropods move by means of a broad,
muscular ventral foot. Many gastropods have
• Polychaetes have a pair of paddlelike appen¬
dages on each segment. Most polychaetes
are free-living marine worms.
a one-piece shell.
• Bivalves have a hinged two-part shell. Al¬
though larvae are motile, most adult bivalves
are sessile.
• Cephalopods have a well-developed nervous
system, relatively advanced sense organs,
and a closed circulatory system. A cephalopod's foot is divided into eight or more
tainer. Using the dropper, moisten the earth¬
worms with water. Cover the container.
tentacles.
• Mollusks play many roles in the natural
world. Many products that are important or
• Oligochaetes have few bristles and lack ap¬
pendages. Many are burrowing worms. Most
live in fresh water or in soil.
• Leeches are typically blood-sucking external
parasites that live in fresh water,
• Annelids interact in many different ways
with other parts of the living world. Burrow¬
ing annelids such as earthworms are impor¬
tant in aerating soil.
OBSERVATIONS
1. Which kind of surface did the earthworms
prefer—moist or dry?
2. Do earthworms prefer light or darkness?
REVIEWING KEY TERMS
3. Describe the earthworms' color, texture, ex¬
ternal features, and other physical character¬
istics.
worms with water.
6. Cover the entire bottom of the tray with a
damp paper towel.
7. Place the earthworms in the center of the
27-1 Mollusks
ANALYSIS AND CONCLUSIONS
1. How does an earthworm's response to mois¬
ture help it survive?
2. Does an earthworm's response to light have
any protective value? Explain.
3. How is an earthworm's body adapted for
movement into and through soil?
4. Would you expect to find earthworms in hard
soil? Explain.
Vocabulary terms are important to your understanding of biology. The key terms
listed below are those you should be especially familiar with. Review these terms
and their meanings. Then use each term in a complete sentence. If you are not
sure of a term's meaning, return to the appropriate section and review its definition.
27-1 WSoIlosks gill 27-2 Annelids
m0l]usk open circulatory system annelid
foot closed circulatory system polychaete
nephridium oligochaete
she,! gastropod
visceral mass bivalve
leech
radula cephalopod
603
CONTENT REVIEW
B. Replace the underlined definition with the correct vocabulary word.
Multiple Choice
Choose the letter of the answer that best completes each statement
1. Which characteristic do many mollusks and
annelids have in common?
a. segmented body
b. one- or two-part shell
c. open circulatory system
d. trochophore larvae
2. One major class of mollusks is
a. Cephalopoda. c. Oligochaeta.
5. A bristly marine worm that has paired ap¬
pendages on each segment belongs to Class
a. Annelida. c. Oligochaeta
b. Polychaeta. d. Cephalopoda.
6. An oligochaete probably
a. is a parasite.
b. has paired appendages.
c. has septa.
b. Hirudinea. d. Polychaeta.
3. A mollusk that swims by flapping its broad
muscular foot is probably a
d. has a mantle, foot, and visceral mass.
7. A scraping organ used for feeding is a
a. nephridium. c. mantle.
b. pharynx. d. radula.
a. bivalve. c. cephalopod.
b. gastropod. d. polychaete.
8.
4. Which organ is used for both respiration
and filter feeding in some animals?
a. nephridium c. gill
b. radula d. ganglion
In earthworms, the clitellum
a. is involved in asexual reproduction.
b. secretes a cocoon for the eggs.
c. often has a pair of jaws.
d. grinds food particles into smaller pieces.
True or False
Determine whether each statement is true or false. If it is true, write "true." If it
is false, change the underlined word or words to make
the statement true.
!• A pharynx is an organ used in excretion
2. Softbodied animals that typically have a
shell are known as oligochaetes.
3. Segmented worms belong to phylum
Hirudinea.
4. Pearls and the shells of mollusks are
formed by secretions from the radula.
5.
6.
7.
8.
Many leeches are blood-sucking parasites.
Cephalopods are characterized by a onepart shell and a broad, muscular foot.
Gastropods are usually sessile as adults.
Hermaphrodites usually undergo external
fertilization. "
5. Octopi have a circulatory system in which the blood is always contained
in blood vessels.
6. Earthworms are members of the segmented worms phylum.
7. The part of a mollusk that contains the mouth and is often used in
locomotion in cephalopods is divided into tentacles.
CON CERT M AST E RY
Use your understanding of the concepts developed in the chapter to answer each
of the following in a brief paragraph.
nf
oarasranh.
1. How are mollusks adapted to different
modes of feeding?
2. Compare the ways in which polychaetes
and oligochaetes perform their essential
functions.
CRITICAL AND CREATIVE THINKING
Discuss each of the following in a brief paragraph.
1. Assessing concepts Although a number
of animals are hermaphrodites, they rarely
fertilize their own eggs. Explain why crossfertilization is usually better than selffertilization. Under what circumstances
might self-fertilization be better than
cross-fertilization?
2. Making inferences Some oligochaetes can
survive in areas that have little oxygen and
can even tolerate a complete lack of
oxygen for short periods of time. Some of
these oligochaetes die when exposed to
normal oxygen levels for a long period of
time. What is probably the natural habitat
of these oligochaetes? Explain.
3. Developing a hypothesis Female octopi
die after brooding their eggs (tending and
Word Relationships
A. An analogy is a relationship between two pairs of words or phrases eenerallv
— the wnner: abed The symbol: is read "is to."and the
symbol ,s read as. For example. cot:animal,rose:planl is read "cat is to
animal as rose is to plant." e u tut is 10
In the analogies that follow, a word or phrase is missing. Complete each
analogy by providing the missing word or phrase.
1. one-part shell:gastropod::two-part shell2. cocoon:clitellum:;shell:
3. shelI;snaiI::nematocyst and chemicals:
4. light detection:ocelli::balance:
3. How are clams adapted for burrowing in
mud and sand?
4. How do mollusks fit into the world?
5. Explain why a person might purchase
earthworms to put in a garden.
Although many nudibranchs are simultaneously
male and female, they do not fertilize their own
eggs.
protecting eggs until they hatch). However,
if certain glands near the brooding
octopus's eyes are surgically removed, the
octopus stops brooding, resumes feeding,
and has a life span longer than the normal
three to four years. Develop a hypothesis
to explain this phenomenon. How might
you go about testing your hypothesis?
4. Using the writing process Suppose that
the topic of a debate is, "Resolved: It is
better to be a free-swimming polychaete
than a sessile one." Take either the
affirmative or the negative stance and
prepare a persuasive argument for your
position.
J
605