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Chapter 26: Sponges, Cnidarians, and Unsegmented Worms
Section 1: Introduction to the Animal Kingdom
Introduction to the Animal Kingdom
What Is an Animal?
All animals share certain basic characteristics
Animals are heterotrophs (they do NOT make their own food)
Instead, they obtain the nutrients and energy they need by feeding on organic
compounds that have been made by other organisms
What Is an Animal?
Animals are multicellular, which means that their bodies are composed of more
than one cell
Animal cells are also eukaryotic – they contain a nucleus and membraneenclosed organelles
An animal is a multicellular eukaryotic heterotroph whose cells lack cell walls
Cell Specialization and Division of Labor
The bodies of animals contain many types of specialized cells
Each specialized cell has a shape, physical structure, and chemical composition that
make it uniquely suited to perform a particular function within a multicellular
organism
For this reason, groups of specialized cells carry out different tasks for the organism
– division of labor
What Animals Must Do to Survive
In order to survive, animals must be able to perform a number of essential
functions
For each animal group we study in the next several chapters, we will examine
these functions and describe the cells, tissues, organs, and organ systems that
perform them
Feeding
Animals have evolved a variety of ways to feed
Herbivores eat plants
Carnivores eat animals
Parasites live and feed either inside or attached to outer surfaces of other organisms, causing
harm to the host
Filter feeders strain tiny floating plants and animals from the water around them
Detritus feeders feed on tiny bits of decaying plants and animals
Respiration
Living cells consume oxygen and give off carbon dioxide in the process
of cellular respiration
Entire animals must respire, or breathe, in order to take in and give off
these gases
Small animals that live in water or in moist soil may respire through
their skin
Respiration
For large active animals, however, respiration through the skin is not efficient
The respiratory systems these animals have evolved take many different forms
in adaptations suited to different habitats
Internal Transport
Some aquatic animals can function without an internal transport system
But once an animal reaches a certain size, it must somehow carry oxygen,
nutrients, and waste products to and from cells deep within its body
Many multicellular animals have evolved a circulatory system in which a pumping
organ called a heart forces a fluid called blood through a series of blood vessels
Excretion
Cellular metabolism produces chemical wastes such as ammonia that are harmful
and must be eliminated
Small aquatic animals depend on diffusion to carry wastes from their tissues into the
surrounding water
But larger animals, both in water and on land, must work to remove poisonous
metabolic wastes
Response
Animals must keep watch on their surroundings to find food, spot predators, and
identify others of their own kind
To do this, animals use specialized cells called nerve cells, which hook up together
to form a nervous system
Response
Sense organs, such as eyes and ears, gather information from the environment by
responding to light, sound, temperature, and other stimuli
The brain, which is the nervous system’s control center, processes the information and
regulates how the animal responds
The complexity of the nervous system varies greatly in animals
Movement
Some animals are sessile, which means that they live their entire adult lives
attached to one spot
But many animals are motile, which means that they move around
To move, most animals use tissues called muscles that generate force by
contracting
In the most successful groups of animals, muscles work together with a skeleton,
or the system of solid support in the body
Movement
Insects and their relatives wear their skeletons on the outside of their bodies
exoskeletons
Reptiles, birds, and mammals have their skeletons inside their bodies
endoskeletons
We call the combination of an animal’s muscles and skeleton its musculo –
skeletal system
Reproduction
Animals must reproduce or their species will not survive
Some animals switch back and forth between asexual and sexual reproduction
Many animals that reproduce sexually bear their young alive
Reproduction
Others lay eggs
The eggs of some species hatch into baby animals that look just like miniature
adults
These baby animals increase in size but do not change their overall form
Direct development
Reproduction
In other species, eggs hatch into larvae, which are immature stages that look and
act nothing like the adults
As larvae grow, they undergo a process called metamorphosis in which they
change shape dramatically
Indirect development
Trends in Animal Evolution
The levels of organization become higher as animals become more complex in form
The essential functions of less complex animals are carried out on the cell or tissue level of
organization
As you move on to more complex animals, you will observe a steady increase in the
number of specialized tissues
You will also see those tissues joining together to form more and more specialized organs
and organ systems
Trends in Animal Evolution
Some of the simplest animals have radial symmetry; most complex animals have bilateral
symmetry
Some of the simplest animals have body parts that repeat around an imaginary line drawn
through the center of their body
Radial symmetry
Animals with radial symmetry never have any kind of real “head”
Many of them are sessile, although some drift or move in a random pattern
Trends in Animal Evolution
Most complex invertebrates and all vertebrates have body parts that repeat on
either side of an imaginary line drawn down the middle of their body
One side of the body is a mirror image of the other
These animals are said to have bilateral symmetry
Trends in Animal Evolution
Animals with bilateral symmetry have specialized front and back ends as well as upper
and lower sides
Anterior = front end
Posterior = back end
Dorsal = upper side
Ventral = lower side
Trends in Animal Evolution
More complex animals tend to have a concentration of sense organs
and nerve cells in their anterior (head) end
This gathering of sense organs and nerve cells into the head region is
called cephalization
Nerve cells in the head gather into clusters that process the information
gathered by the nervous system and control responses to stimuli
Trends in Animal Evolution
Small clusters of nerve cells are called ganglia
In the most complex animals, large numbers of nerve cells gather together to form
larger structures called brains
Chapter 26: Sponges, Cnidarians, and Unsegmented Worms
Section 2: Sponges
Sponges
Sponges are among the most ancient of all animals that
are alive today
Most sponges live in the sea, although a few live in
freshwater lakes and streams
Sponges inhabit almost all areas of the sea – from the
polar regions to the tropics and from the low-tide line
down into water several hundred meters deep
Sponges
Sponges belong to the phylum Porifera
Literally means pore-bearers
Tiny openings all over their body
Sponges were once thought to be plants
Sponges are sessile and show little detectable movement
Sponges are heterotrophic, have no cell walls, and contain several specialized cell types that live together
Sponges
Sponges are very different from other animals
Sponges have nothing that even vaguely resembles a mouth or gut, and they have no
specialized tissues or organ systems
Most biologists believe that sponges evolved from single-celled ancestors separately from
other multicellular animals
The evolutionary line that gave rise to sponges was a dead end that produced no other
groups of animals
Form and Function in Sponges
Very simple body plan
The body of a sponge forms a wall around a central cavity
In this wall are thousands of pores
A steady current of water moves through these pores into the central cavity
This current is powered by the flagella of cells called collar cells
Form and Function in Sponges
The water that gathers in the central cavity exits through a large hole called the
osculum
The current of water that flows through the body of a sponge delivers food and
oxygen to the cells and carries away cellular waste products
The water also transports gametes or larvae out of the sponge’s body
Form and Function in Sponges
Many sponges manufacture thin, spiny spicules that form the skeleton of the sponge
A special kind of cell called an amebocyte builds the spicules from either calcium
carbonate or silica
These spicules interlock to form beautiful and delicate skeletons
The softer but stronger sponge skeletons that we know as natural bath sponges
consist of fibers of a protein called spongin
Form and Function in Sponges
Sponges are filter feeders that sift microscopic particles of food from the water that
passes through them
All digestion in sponges is intracellular; it takes place inside cells
The water flowing through a sponge simultaneously serves as its respiratory, excretory,
and internal transport system
As water passes through the body wall, sponge cells remove oxygen from it and give
off carbon dioxide to it
Form and Function in Sponges
The water that flows through the body of a sponge also plays a role in sexual
reproduction
Although eggs are kept inside the body wall of a sponge, sperm are released
into the water flowing through the sponge and are thus carried out into the open
water
If those sperm are taken in by another sponge, they are picked up by
amebocytes and carried to that sponge’s eggs, where fertilization occurs
Form and Function in Sponges
The zygote that results develops into a larva that swims and can be carried by currents for a long distance
before it settles down and grows into a new sponge
Sponges can also reproduce asexually
Faced with cold winters, some freshwater sponges produce structures called gemmules
Sphere-shaped collections of amebocytes surrounded by a tough layer of spicules
Can survive long periods of freezing temperatures and drought
When conditions become favorable, gemmules grow into new sponges
Form and Function in Sponges
Sponges can also reproduce asexually by budding
In this process, part of a sponge simply falls off the parent and grows into a new
sponge
Remarkable powers of regeneration
How Sponges Fit into the World
Sponges provide housing for many other marine animals
Sponges are also involved in symbiotic relationships with other organisms
Humans have used the dried and cleaned bodies of some sponges in bathing
Some chemicals that sponges secrete are being used as powerful antibiotics
that are used to treat bacteria and fungi
Chapter 26: Sponges, Cnidarians, and Unsegmented
Worms
Section 3: Cnidarians
Cnidarians
The phylum Cnidaria includes many animals with brilliant colors and unusual
shapes
Jellyfish, sea anemones, etc.
These beautiful and fascinating animals are found all over the world, but most
species live only in the sea
What is a Cnidarian?
Cnidarians are soft-bodied animals with stinging tentacles arranged in circles around their
mouth
Some cnidarians live as single individuals
Others live as groups of dozens or even thousands of individuals connected into a colony
All cnidarians exhibit radial symmetry and have specialized cells and tissues
Many cnidarians have life cycles that include two different-looking stages, the sessile
flowerlike polyp and the motile bell-shaped medusa
Some cnidarians, such as sea nettles and sea anemones, are solitary. Others, such as gorgonian coral
polyps, are colonial.
Both polyps and medusa have a body wall that surrounds an internal space called the gastrovascular cavity
This is where digestion takes place
The body wall consists of three layers:
Epidermis
Layer of cells that covers the outer surface of the cnidarian’s body
Mesoglea
Located between the epidermis and the gastroderm
Gastroderm
Layer of cells that covers the inner surface, lining the gastrovascular cavity
Form and Function in Cnidarians
Almost all cnidarians capture and eat small animals by using stinging structures called
nematocysts, which are located on their tentacles
Poison-filled sac containing a tightly coiled spring loaded dart
When an animal touches a nematocyst, the dart uncoils and buries itself into the skin of the
animal
Paralyzes or kills the prey
Form and Function in Cnidarians
From here, the cnidarian’s tentacles push the food through the mouth and into the
gastrovascular cavity
There the food is gradually broken up into tiny pieces
These food fragments are taken up by special cells in the gastroderm that digests them
further
The nutrients are then transported throughout the body by diffusion
Any materials that cannot be digested are passed back out through the mouth, which is the
only opening in the gastrovascular cavity
Form and Function in Cnidarians
Because most cnidarians are only a few cell layers thick, they have not had
to evolve many complicated body systems in order to survive
There is no organized internal transport network or excretory system in
cnidarians
Cnidarians also lack a central nervous system and anything that could be
called a brain
They have simple nervous systems called nerve nets
Concentrated around the mouth
Form and Function in Cnidarians
Cnidarians lack muscle cells that most other animals use to move about
Many of the epidermal cells in cnidarians can change shape when stimulated by the
nervous system
Cnidarian polyps can expand, shrink, and move their tentacles by relaxing or
contracting these epidermal cells
Form and Function in Cnidarians
Most cnidarians can reproduce both sexually and asexually
Polyps can produce new polyps asexually by budding
When medusae mature, they reproduce sexually by releasing gametes into the water
Fertilization occurs either in open water or inside an egg-carrying medusa
The zygote grows into a ciliated larva that swims around for some time
Later, the larva settles down, attaches to a hard surface, and changes into a polyp that begins the cycle
again
Hydras and Their Relatives
Class Hydrozoa is made up of cnidarians that spend most of their lives as polyps, although they
usually have a short medusa stage
Most hydrozoan polyps grow in branching sessile colonies
Range in length from a few centimeters to more than a meter
Specialized polyps perform particular functions
Feeding
Reproduction
Defense
Hydras and Their Relatives
Most common are the hydras
Hydras can reproduce either asexually by budding or sexually by producing
eggs and sperm in their body walls
In most species of hydras, the sexes are separate
However, a few species are hermaphrodites
An individual that has both male and female reproductive organs and
produces both sperm and eggs
Hydras and Their Relatives
One unusual hydrozoan in the Portuguese man-of-war
Form floating colonies that contain several polyps
One polyp forms a balloon-like float that keeps the colony on the surface
Some of the polyps produce long stinging tentacles that paralyze and capture prey
Some polyps digest the food held by tentacles
Jellyfish
Class Scyphozoa
Go through the same life-cycle stages as hydrozoans
Some jellyfish, such as the lion’s mane, often grow up to 2 meters in diameter
The largest jellyfish ever found was more than 3.6 meters in diameter and had tentacles more than
30 meters long
The nematocysts of most jellyfish are harmless to humans, but a few can cause painful stings
One tiny Australian jellyfish has a toxin powerful enough to cause death in 3 – to 20 minutes
Sea Anemones and Corals
Class Anthozoa
Most beautiful and ecologically important invertebrates
Have only the polyp stage in their life cycle
Adult polyps reproduce sexually by producing eggs and sperm that are released into the water
The zygote grows into a ciliated larva that settles to the ocean bottom and becomes a new polyp
Many anthozoans also reproduce asexually by budding
Sea Anemones and Corals
Sea anemones are solitary polyps that live in the sea from the low-tide line to great
depths
Although they can catch food with the nematocysts on their tentacles, many shallowwater species depend heavily on their photosynthetic symbionts
Some sea anemones can grow up to a meter in diameter
Sea Anemones and Corals
Corals grow in shallow tropical water around the world
Corals produce skeletons of calcium carbonate or limestone
Most corals are colonial
As a coral colony grows, new polyps are produced by budding
Sea Anemones and Corals
Coral colonies grow very slowly, but they may live for hundreds, or even thousands, of
years
Together, countless coral colonies produce huge structures called coral reefs
Some of these reefs are enormous and contain more rock and living tissue than even
the largest human cities
The Great Barrier Reef off the coast of Australia is more than 2000 km long and some
80 km wide
How Cnidarians Fit into the World
Certain fish, shrimp, and other small animals live among the tentacles of large
sea anemones
Corals and the reefs provide shelter for thousands of species of marine life
Reefs protect the land from erosion
Jewelry and decorations
Medical research
Chapter 26: Sponges, Cnidarians, and Unsegmented
Worms
Section 4: Unsegmented Worms
Unsegmented Worms
Unsegmented worms have bodies that are not divided into special segments
Phylum Platyhelminthes
Consists of simple animals called flatworms
Phylum Nematoda
Consists of long, thin worms called roundworms
Flatworms
The members of the phylum Platyhelminthes are the simplest animals with bilateral symmetry
Most members of this phylum exhibit enough cephalization, or development of the anterior end,
to have what we call a head
Many flatworms are no more than a few millimeters thick, although they may be up to 20 meters
long
Flatworms have more developed organ systems than either sponges or cnidarians
Form and Function in Flatworms
Flatworms feed in either of two very different ways
Worms may be carnivores that feed on tiny aquatic animals
Free-living flatworms have a gastrovascular cavity with one opening at the end of a muscular tube
called a pharynx
They use the pharynx to suck food into the gastrovascular cavity
The gastrovascular cavity forms an intestine with many branches along the entire length of the
worm
Form and Function in Flatworms
In the intestines, enzymes help break down the food into small particles
These particles are taken inside the cells of the intestinal wall, where digestion
is completed
Like Cnidarians, flatworms expel undigested material through the mouth
Form and Function in Flatworms
Many other flatworms are parasites that feed on blood, tissue fluids, or pieces of cells
inside the body of their host
In many parasitic flatworms, the digestive tract is simpler than in free-living forms
Tapeworms, which live within the intestines of their host, do not have any digestive tract at
all
They have hooks and/or suckers with which they latch onto the intestinal wall of the host
Form and Function in Flatworms
From this position, they can simply absorb the food that passes by – food that has
already been broken down by the host’s digestive enzymes
Flatworms lack any kind of specialized circulatory or respiratory system
Freshwater flatworms such as planarians have structures called flame cells that help
them get rid of extra water
Form and Function in Flatworms
Free-living flatworms have nervous systems that are much more developed than those of
cnidarians and sponges
They have a definite head in which a simple brain is located
One or more long nerve cords run from the brain down the length of the body on either
side
Many flatworms have one or more pairs of light-sensitive organs called ocelli, or eyespots
Form and Function in Flatworms
The nervous system of free-living flatworms allows them to gather
information from their environment – information that they use to locate
food and to find dark hiding places
Parasitic flatworms often do not have much of a nervous system
Form and Function in Flatworms
Free-living flatworms usually use two means of locomotion at once
Cilia on their epidermal cells help them glide through the water
Muscle cells controlled by the nervous system allow them to twist and turn so
that they are able to react to environmental conditions
Form and Function in Flatworms
Reproduction in free-living flatworms can be either sexual or
asexual
Most free-living flatworms are hermaphrodites
The eggs hatch within a few weeks
Planarians
The free-living flatworms belong to the class Turbellaria
Most familiar members of this class are planarians
Turbellarians vary greatly in color, form, and size
Although most Turbellarians are less that 1 cm in length, some giant land
planarians, which are found in moist tropical areas, can attain lengths of more
than 60 cm
Flukes
Class Trematoda contains parasitic flatworms known as flukes
Most flukes are internal parasites that infect the blood and organs
These flukes have complicated life cycles that involve at least two different host
animals
Blood flukes are found primarily in Southeast Asia, North Africa, and other tropical
areas
Humans are the primary hosts of blood flukes
Flukes
Most flukes are hermaphrodites and undergo sexual reproduction in a manner similar
to that of free-living flatworms
Flukes produce many more eggs than free-living flatworms
Blood flukes lay so many eggs that the tiny blood vessels of the host’s intestine break
open
The broken blood vessels leak both blood and eggs into the intestine
Flukes
The eggs are not digested by the host and thus become part of the feces
In developed countries, where there are toilets and proper sewage systems, these
eggs are usually destroyed in the sewage treatment process
But in many undeveloped parts of the world, human wastes are simply tossed into
streams or even used as fertilizer
Flukes
Once the fluke eggs get into the water, they hatch into swimming larvae
When these larvae find a snail of the correct species, they burrow inside it and digest
its tissues
The snail is an intermediate host for the fluke
In the intermediate host, the flukes reproduce asexually
Flukes
The resulting new worms break out of the snail and swim around in the water
If they find a human, the worms bore through the skin and eat their way to the
blood vessels
In the blood, the get carried around through the heart and lungs to the
intestine, where they live as adults
Flukes
People infected with blood flukes get terribly sick
They become weak and often die – either as a direct result of the
fluke infection or because they cannot recover from other diseases
in their weakened condition
Tapeworms
Members of the class Cestoda are long, flat parasitic worms that live a very simple
life
They have a head called a scolex on which there are several suckers and a ring of
hooks
These structures attach to the intestinal walls of humans and other animals
Adult human tapeworms can be up to 18 meters long
Tapeworms almost never kill their host
Tapeworms
Behind the scolex of the tapeworm is a narrow neck region that is constantly dividing
to form many proglottids, or sections, that make up most of the body of the tapeworm
The youngest and smallest proglottids are at the anterior end of the tapeworm
Male and female reproductive organs are contained in the proglottids
Tapeworms
If food or water contaminated with tapeworm eggs is consumed by cows, pigs, fish,
or other intermediate hosts, the eggs enter the intermediate host and hatch into
larvae
These larvae grow for a time and then burrow into the muscle tissue of the
intermediate host and form a dormant protective stage called a cyst
Tapeworms
If a human eats raw or incompletely cooked meat containing these
cysts, the larvae become active within the human host
Once inside the intestine of a new host, they latch onto the intestinal
wall and grow into adult worms
Roundworms
Members of the phylum Nematoda, which are known as roundworms, are among the
simplest animals to have a digestive system with two openings – a mouth and an anus
Food enters through the mouth, and undigested food leaves through the anus
Roundworms may be the most numerous of all multicellular animals
A single rotting apple can contain as many as 90,000 roundworms
Form and Function in Roundworms
Most roundworms are free-living
All roundworms have a long tube-shaped digestive tract with openings at both ends
Any material in the food that cannot be digested leaves through an opening called the
anus
Roundworms breathe and excrete their metabolic wastes through their body walls
They have no internal transport system
Form and Function in Roundworms
Roundworms have simple nervous systems
They have several ganglia in the head region but no definite brain
Roundworms reproduce sexually
Fertilization takes place inside the body of the female
How Unsegmented Worms Fit into the World
Do not have a lot of positive influence on humans
Responsible for some of the most painful and horrific diseases
known
Hookworm
Eye worm
Trichinosis