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CHAPTER 13 – The Cnidarians
(Radiate Animals)
Phylum Cnidaria
• Hydra, jellyfish, coral, & sea
anemones
Jellyfish can be funny….
General Characteristics
 Radial symmetry
 Their body is arranged around an oral-aboral axis
 Oral end terminates in a mouth that is surrounded by
tentacles
 Good for sessile; sedentary lifestyle
 Reach the tissue level of organization
 Cells are organized into groups of tissues that work
together
 Contains 2 tissue layers (diploblastic) including:
epidermis (gives rise to the outer body wall) and
gastrodermis(gut)
General Characteristics
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Acoelomated organism- they contain NO body cavity
All are carnivores
Over 9,000 species are in the phylum
No system for circulation, respiration or excretion-> occur
by diffusion
 Some cnidarians can regenerate lost parts or even a
complete body
 Most cnidarians are dioecious (2 forms: male and female)
 All named for the presence of cells called Cnidocytes: cells
that contain the stinging organelle called nematocysts
 Found only in cnidarians
General Characteristics
 Diploblastic: contain 2 well-defined germ layers:
ectoderm (epidermis) and endoderm(gastrodermis)
 Examples include: Hydras, jellyfish, sea anemone,
corals, Portuguese man of war, box jelly fish
 Movement: mostly sessile, some move or “swim”
slowly
 All contain some type of Gastrovascular cavity with
one opening for food intake and elimination of waste
General Characteristics
 Both the outer (epidermis) and inner ( gastrodermis)
contains nerve cells that are arranged in a loose
network called a Nerve Net
 Innervates their primitive muscles that extend form the
epidermal and gastrodermal cells
 Stimulus in one part will spread across the whole body
by the network
General Characteristics
 Location
 Found mostly in marine habitats (some freshwater)
 Most abundant in warm (tropical), shallow, marine
waters
 Colonial organisms can be found attached to rocks,
animals, or wharves
Ecological Role
 Predator/Prey Relationships
 Neurotoxins in medical research
 Coral- jewelry, reef building
 Symbiosis with other organisms
 Coral reefs- habitat for many
 Great biodiversity
 Protect the coastline
Symbiotic Relationships
 Mutualism
 Hydroids and sea anemones can live on hermit crab
shells
 Algal cells live in the tissue of cnidarians
 Clown fish and sea anemone
 Portuguese Man of War and the Nomeus fish
Cnidarian Body Plans
 Dimorphism
 Existence of 2 morphological types within the same
species
 Allows the organism to obtain food from different
locations
All cnidarians fit into one of the following types:
*polyp
*medusa
Cnidarian Body Plans
 Polyp (hydroid):
 Fits a sessile lifestyle
 Can be found singly or in colonies
 Structure: tubular body with a mouth at one end
(directed upward) surrounded by tentacles
 The aboral end is attached to a substrate
Polyp Structure
Cnidarian Body Plans
 Medusa
 Jellyfish form
 Fits the swimming/floating lifestyle
 Mobile, move by weak contractions of body
 Bell or umbrella shape
 Mouth is usually directed downward and centered on
the concave side and tentacles extend from the rim
 “mouth down” version of the polyp
 Contains more mesoglea (middle jelly-like layer) than
polyp stage
Polyp vs. Medusa
Cnidocyte/Nematocyst
Cnidarians are named after the presence of cnidocytes: the
stinging cell of cnidarians that produces the nematocyst
 Equivalent to the container(gun) that contains the stinging
organelle (bullet)
Nematocyst: the stinging organelle
contained within the cnidocyte
• Most characteristic structure
• Helps with taxonomic classification
• Over 20 different types of nematocysts
Structure of the Nematocyst
 Enclosed within the cnidocyte (made of chitin)
 All cnidocytes (except for Anthozoa class) have a
cnidocil- modified cilium that triggers the nematocyst
to eject
 Tactile or chemical stimulation causes the nematocyst
to discharge(prey swimming)
 After discharge the cnidocyte is absorbed and a new
one replaces it
Structure of the Nematocyst
 Operculum: the covering (lid) that encloses the
nematocyst inside the cnidocyte
 Triggered to open through a change in pressure
 Effects (types) of the nematocysts
 Barbs: (not found in all)
 Penetrants: inject poisons
 Volvents: Long tentacle (string like); entangles prey
 Glutinants: secretes an adhesive
Steps of release for a nematocyst
 Stimulation of the cnidocil
 The operculum opens
 When the operculum opens there is a change in
pressure that forces the nematocyst out
 The nematocyst is ejected inside out
 Poisons may be injected when it penetrates the prey
Cnidocyte: Before and After
Discharge
Nerve Net
 The simple nervous system of cnidarians
 Variation of the nerve net is found between the classes
 Cnidarians have neurotransmitters on both sides of
their synapses (junction between 2 nerve cells) this
functions to transmits impulses in both directions
 Located in the gastrodermis and epidermis
 Forms 2 interconnected nerve nets
Nerve Net
 They do form a simple neuromuscular system- their
sensory nerve net plus they contain contractile fibers
in their body wall to coordinate muscular contractions
 They do not have:
 Myelin covering their axons
 Central nervous system
Nerve Net
4 Classes of Cnidarians
 Class Hydrozoa (Hydra, PMOW, Obelia)
 Class Scyphozoa (common jellyfish)
 Class Cubozoa (box jelly)
 Class Anthozoa (sea anemone and coral)
Class Hydrozoa
 General Characteristics:
 Mostly marine (except freshwater hydra)
 Most are colonial
 Most have a sexual (medusa) and asexual (polyp)
lifecycle
 Examples we will discuss include:
 Freshwater hydra- only has a polyp stage
 Obelia
 PMOW
Freshwater Hydra
 Belong to class Hydrozoa
 Are solitary animals (not colonial)
 Found in freshwater environments attached to
anything…rocks, aquatic leaves…
 Only found in the polyp stage (no medusa stage)
 Feed mainly on plankton
Body Plan of the Hydra
 Cylindrical tube with a point of attachment at the
bottom and mouth at the top of the tube surrounded
by tentacles
 Contains the following parts
 Basal disc
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Located at the bottom of the tube body
Serves as the site for attachment
Secretes substances for adhesion
Can produce gas bubble at the end of the basal disc for
floating
Body Plan of the Hydra
 Hypostome
 Cone shaped elevation
 Mouth is at the center
 Surrounded by tentacles
 Mouth
 Surrounded by tentacles
 Ingests food
 Gastrovascular cavity
 Open cavity that is surrounded by the body wall
 Site of extracellular digestion
Body plan of the Hydra
Body wall of the Hydra
 Surrounds the GVC
 Consists of 3 layers
 Epidermis (outer layer)
 Gastrodermis (inner layer)
 Mesoglea (middle layer)
Epidermis of the Hydra
 Outer layer that serves as protection
 Composed of several cell types
 Epitheliomuscular cells
 Interstitial cells
 Gland cells
 Nerve cells
 Sensory cells
 Cnidocytes
Epitheliomuscular Cells
 Composes the majority of the epidermis
 Functions to protect the hydra and to produce
muscular contractions
Interstitial Cells
 Located at the base of the epitheliomuscular cells
 Functions as stem cells to turn into almost any other
cell type (cnidoblasts, sex cells, buds, nerve cells)
***What type of sponge cell does this sound like?
Gland Cells
 Located on the basal disc and around the mouth
 Functions to secrete adhesive or lubrication
Cnidocytes
 Contain nematocysts
 Function to protect the organism and to trap and kill
food
Sensory Cells
 Located around the mouth, tentacles, or basal disc
 Allows the organism to be stimulated by the
environment and to be aware of its surroundings
 Structure: one end had flagella for stimulation and the
other end goes to the nerve cells
Nerve Cells
 Connects with sensory cells, other nerve cells,
cnidocytes, or muscular cells
 Coordinates all of the activities
Body wall of the Hydra
Gastrodermis of the Hydra
 Lines and directly touches the GVC
 The following cell types are found in the gastrodermis:
 Nutritive-muscular cells
 Interstitial cells (performs the same function as in the
epidermis- they act as stem cells)
 Gland cells
Nutritive-muscular cells
 Contain cilia to create a flow of food, water and
nutrients inside the GVC
 Tall cells that contain food vacuoles
 Absorbs, digests and circulates food and fluids
 Site of intracellular digestion
Gland cells
 Gland cells in the gastrodermis are located around the
hypostome and inside the column
 Function:
 Hypostome gland cells secrete lubricant and digestive
enzymes into the GVC
 Inside the GVC the gland cells secrete digestive enzymes
Gastro-vascular cavity
 Open space inside the cnidarian
 Filled with water to act as a hydrostatic skeleton
(provides support)
 Lined with gland cells to help with digestion
 Extracellular digestion occurs in the GVC
Mesoglea
 Location: in between the epidermis and gastrodermis
 Structure: non cellular, gelatinous material composed
mainly of water
 Function: support and flexibility
 Mesoglea is thickest in the stalk (strength)
 Mesoglea is thinnest in the tentacles (flexibility)
Locomotion of Hydra
 The Hydra can move in 3 ways:
 Glide on basal disc (bottom of the organism)
 Bend over and attach tentacles to the surface
 Gas bubble
Feeding and Digestion: Hydra
 Hydras eat: crustaceans, insect larvae, annelid worms
 Feeding process:
 Tentacles trap prey with the help of the nematocyst
(encased in the cnidocyte)
 Mouth widens (with the help of gland cells) to engulf
prey
 Extracellular digestion takes place in the GVC with the
help of gland cells secreting digestive enzymes
 Food particles go into nutritive-muscular cells for
further intracellular digestion
Glutathione
 A chemical activator that triggers the feeding process
of certain cnidarians
 Cause the tentacles to move more
 Causes the hydra to be more alert
 Causes the gland cells to release lubricant and widen the
mouth
 Prepare for food intake
Hydra Reproduction
 Reproduce both sexually and asexually
 Sexual Reproduction
 Form temporary gonads in the fall
 Eggs and sperm and are shed into the water and form
fertilized eggs
 Hydra will hatch in the spring (favorable environment)
 Asexual Reproduction
 Budding: starts as a growth on the side of the “parent”
hydra
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Eventually will detach from parent and live on its own
Hydra Budding
Obelia- Hydroid colony
 2nd example of Hydrozoan class
 Has a polyp and medusa stage in its life cycle
 All polyps in the colony are usually interconnected
 Contains a base, stalk and terminal zooids
 Zooid: general term for an individual polyp animal in
a colonial cnidaria
Obelia Structure
 Hydrorhiza: (root-like) base that attaches the
colonial polyp to a substrate
 Hydrocauli: (stem) part that extends from the
hydrorhiza and the individual zooids
 Zooid: attached to the hydrocaulus; individual polyp
animals
Obelia Structure
Types of tissue found in the Obelia
 Coenosarc: the inner, living, cellular part of the
Obelia
 Perisarc: the outer, protective, non-living, covering of
the Obelia
 Made of chitin
Obelia Structure
Types of Zooids
 Gastrozooids
 Also called hydranths
 Feeding animals of the colony
 All have tentacles with cnidocytes
 Feed on crustaceans, worms, and larvae
 If one polyp eats it provides nutrients for the entire
colony
 Digestion and circulation of food is aided by ciliated
nutritive muscular cells and body wall contractions
Types of Zooids
 Gonozooids
 Reproductive polyp that release medusa buds
Obelia reproduces both sexually and asexually.
Sexual Reproduction: (most common pathway)
* young medusa leaves the colony
* medusa buds mature and shed gametes
(sperm and egg)
* fertilization occurs
* Zygote created turns into a Planula (free swimming
larvae stage) that swims and attaches to form new colony
Obelia Reproduction
 Asexual Reproduction:
 Occurs by budding-> increases the size of the colony
 Occurs but outgrowth of the body wall
 Creates new gastro- and gono- zooids
Obelia Reproduction
Medusa structure of Obelia
 “Jellyfish” form
 Contains:
 Velum: membrane under the surface if the umbrella of
the medusa that projects inward
 Contains tentacles with cnidocytes
 Manubrium: tissue projecting from the oral side of the
medusa; surrounds the mouth
Internal medusa structure of
Obelia
 Entire system is lined by the gastrodermis
 Gastrovascular cavity is continuous from the mouth to
the tentacles
 Pathway of food flow: (disperses food w/o circulatory
system)
 Mouth-> opens to the manubrium projecting form the
oral side of the mouth-> leads to the stomach and the 4
radial canals-> connects to the ring around the margin
of the bell-> connects the tentacles
Movement of Obelia medusa
 Aboral side first
 Characterized by weak jet propulsion
 Muscular pulsations fill bell with water and empty water
to propel medusa
Obelia: Medusa Stage
Nerve Net of Obelia Medusa
 Concentrated in 2 nerve rings at the base of velum
 Composed of
 Statocysts: functions in equilibrium
 Ocelli: light sensitivity
Nerve Net
Physalia physalis
 Portuguese Man of War (PMOW)
 3rd example of Hydrozoan class
 Located in warmer, tropical waters
 Considered to be a polymorphic swimming
community that is composed of several types of
animals that swim and act as 1 colony
 Nematocysts are located all over the tentacles
 Some are very dangerous-> secrete a neurotoxin
Structure of PMOW
 Pneumatophore
 Rainbow colored float
 Filled with gas
 Carries future generations and acts as a nursing center
 Can briefly deflate for protection
Types of individuals in a colony
 Gastrozooids: feeding zooids that ingest the food
 Dactylzooids: long fishing tentacles that sting,
capture the prey and bring it to the mouth
 On average are 30 feet long (can be as long as 165 feet
underwater)
 Covered in nematocysts
 Gonozooids: sacs with ovaries and testes
 Function in reproduction
Portuguese Man 0’ War
Tentacles of Physalia physalis