Chapter 26: Sponges, Cnidarians, and Unsegmented Worms
Section 3: Cnidarians
Cnidarians
 The phylum _____________________________ includes many animals with brilliant
colors and unusual shapes
o Jellyfish, sea anemones, etc.
o These beautiful and fascinating animals are found all over the world, but most
species live only in the ____________________
What is a Cnidarian?
 Cnidarians are ________________________________________________ with
_________________________________________________ arranged in circles around
their _______________________
 Some cnidarians live as single individuals
 Others live as groups of dozens or even thousands of individuals connected into a colony
 All cnidarians exhibit ___________________________________________________ and
have specialized _________________________________________
 Many cnidarians have life cycles that include two different-looking stages, the sessile
flowerlike ______________________ and the motile bell-shaped ____________________
 Both polyps and medusa have a body wall that surrounds an internal space called the
________________________________________________________
o This is where _______________________________ takes place
 The body wall consists of three layers:
o _________________________________

Layer of cells that covers the ___________________________________
of the cnidarian’s body
o _________________________________

Located between the ____________________________ and the
_______________________________
o _________________________________

Layer of cells that covers the ___________________________________,
lining the gastrovascular cavity
Form and Function in Cnidarians
 Almost all cnidarians capture and eat small animals by using stinging structures called
_____________________________________, which are located on their tentacles
o ____________________________________________ containing a tightly coiled
spring loaded dart
o When an animal touches a nematocyst, the dart uncoils and buries itself into the
skin of the animal
o _________________________________________________________________
 From here, the cnidarian’s tentacles push the food through the mouth and into the
gastrovascular cavity
 There the food is ______________________________________________________ into
tiny pieces
 These food fragments are taken up by _________________________________________
in the gastroderm that digests them further
 The nutrients are then transported throughout the body by _________________________
 Any materials that cannot be digested are passed back out through the
_____________________, which is the only opening in the gastrovascular cavity
 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 ____________________________________
o _________________________________________________________________
 Cnidarians lack _______________________________________ 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
 Most cnidarians can reproduce both sexually and asexually
 Polyps can produce new polyps asexually by ___________________________________
 When medusae mature, they reproduce sexually by releasing ______________________
into the water
 Fertilization occurs either in open water or inside an egg-carrying medusa
 The zygote grows into a _________________________________________ 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 ___________________________________ 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 _______________________________________________
_________________________
o Range in length from a few centimeters to more than a meter
o Specialized polyps perform particular functions

____________________________________

____________________________________

____________________________________
 Most common are the _______________________________
 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 _________________________________________
o An individual that has both male and female reproductive organs and produces
both sperm and eggs
 One unusual hydrozoan in the _______________________________________________
o Form floating colonies that contain several polyps
o One polyp forms a balloon-like float that keeps the colony on the surface
o Some of the polyps produce long stinging tentacles that paralyze and capture prey
o Some polyps digest the food held by tentacles
Jellyfish
 Class __________________________________
 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
o One tiny Australian jellyfish has a toxin powerful enough to cause death in 3 – to
20 minutes
Sea Anemones and Corals
 Class _______________________________
 Most beautiful and ecologically important invertebrates
 Have only the ________________________________ 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 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 shallow-water
species depend heavily on their photosynthetic symbionts
 Some sea anemones can grow up to a meter in diameter
 Corals grow in shallow tropical water around the world
 Corals produce skeletons of _________________________________________________
or __________________________________
 Most corals are ___________________________________
 As a coral colony grows, new polyps are produced by budding
 Coral colonies grow very slowly, but they may live for hundreds, or even thousands, of
years
 Together, countless coral colonies produce huge structures called ___________________
_____________________
 Some of these reefs are enormous and contain more rock and living tissue than even the
largest human cities
 The ______________________________________________________________ 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 _____________________________
 ____________________________________________________________________
 _________________________________________________