This document summarizes Chapter 7 of the textbook BIOL 201: Invertebrate Zoology, which covers the phylum Cnidaria. Cnidaria includes jellyfish, sea anemones, corals, and hydroids. They have radial symmetry, two tissue layers, and a gastrovascular cavity. Their defining feature is cnidocytes, which are stinging cells used for defense and prey capture. Cnidarians can be colonial or solitary and have either a polyp or medusa body form. The document then describes key characteristics of major cnidarian classes including Anthozoa, Scyphozoa, and Hydrozoa. It provides examples of orders within each class and details their anatomy, life

1. BIOL 201:
Invertebrate Zoology
Chapter 7:
Cnidaria
Rob Swatski
Asst. Prof. Biology
HACC-York

2. Phylum
Cnidaria
Hydra, anemones,
stony & soft corals,
hydroids, & jellyfishes
Most marine (10,000
sp); few FW (20 sp);
no terrestrial sp
Colonial or solitary
Corals build reefs (#2
biodiversity)
2

3. Cnidaria
Structure &
Function
Cnidocytes!
Body plan resembles
gastrula; mouth
surrounded by
tentacles
Coelenteron
(Gastrovascular
cavity, GVC)
Radial symmetry
around oral-aboral
axis: advantage?
3

4. Body Forms
Polyp
Resembles
flower &
stem
Pedal disc
& oral
disc
Sessile &
benthic;
“mouth-
up”
Medusa
Umbrella
or bell-
shaped
Manubrium:
“elephant
trunk”
“Mouth-
down”
4

5. 5

6. 6

7. 7

8. 3 Tissue
Layers
Epidermis: epithelium
Gastrodermis:
epithelium
Mesoglea (gelatinous
ECM): CT
Diploblastic
8

9. Colonial
Cnidarians
Reproduce via
budding, but buds
don’t separate
Produce zooids that
resemble juveniles
Small zooids have
large SA:V
Fillter-feeding:
many mouths &
tentacles, broad
distribution, small
size
9

10. Types of
Colonies
Stolonate: have
stolons (strawberry
runners)
Coenosarc: have
coenosarc & solenia
(hollow tubes)
Fruticose: upright &
branching; plantlike
or feathery
Budding: fixed-length
(Obelia) & axial-polyp
10

11. 11

12. Cnidarian
Skeletons
More diverse than
sponges
Exoskeletons:
chitinous periderm
(hydrozoans); calcium
carbonate (stony
corals); shell
fragments (anemones)
Endoskeletons: fibers
& spicules (soft corals)
or cell columns with
turgid vacuoles
(hydrozoans)
Hydrostatic skeletons:
Hydra, anemones
12

13. Musculature
Antagonistic sheets
of muscle
Circular smooth
muscle in
gastrodermis
Longitudinal smooth
muscle in epidermis
Coronal muscles
around medusa
subumbrella
13

14. Diversity of
Movement
Polyps: shorten,
extend, & bend
Medusae:
constrict bells for
swimming
Some polyps inch-
worm &
somersault!
Tentacles move
for prey
manipulation
Subumbrella
retracts in polyps
& medusae 14

15. 15

16. Nervous
Systems
Two nerve nets:
base of epidermis &
gastrodermis; joined
by nerve bridges
across mesoglea
Nerve impulses can
travel any direction
(radial sensory
system)
Medusae: nerve
rings, musculature,
ganglia, & sense
organs around bell
Statocysts, ocelli,
chemoreceptors, &
mechanoreceptors 16

17. Cnidocytes
Used for prey capture
& defense
Cnidocytes (cells)
contain cnida (fluid-
filled capsule with
tubule)
Nematocytes contain
nematocyst that stings
& releases toxins
Spirocytes contain
spirocyst that contains
sticky threads for
adhesion
17

18. Cnidocytes,
cont.
Cnidocytes: abundant
in epidermis, on
tentacles, in
gastrodermis
Chemical &
mechanical cues
from prey trigger
firing of nematocyst
Toxins interfere with
Na+/K+ pumps or
degrade cell
membranes
Hydra discharges 25%
of nematocysts eating
1 brine shrimp
(replaced in 24hr) 18

19. 19

20. Nutrition &
Internal
Transport
Coelenteron (GVC):
blind gut
Septa increase SA for
digestion, absorption,
gas exchange,
excretion, repro,
hydrostatic skeleton
Radial & ring canals in
medusae radiate out
from central stomach
Coelenteron often
branches into each
tentacle
20

21. 21

22. Feeding
Prey is caught, mouth
opens, tentacles stuff
prey inside, &
enzymes digest
extracellulary
Gastrodermis absorbs
nutrients & larger
particles are digested
intracellularly
Slow process, taking
several days
Wastes ejected out
through mouth
22

23. Nutrition &
Circulation
Many species have
PSN endosymbionts
May account for
90% of nutrition in
some species
Often defined
patterns of fluid
circulation around
coelenteron
Ciliated
gastrodermis;
muscular
contractions 23

24. Gas &
Waste
Exchange
Gas exchange occurs
across general body
surfaces: tentacles,
body wall
Waste exchange
(excretion) occurs
across body wall
Ammonia is primary
waste product
Terrestrial inverts are
adapted for
conserving water
during excretion
24

25. General
Reproduction
Amazing
regenerators!
Can lose oral end &
regrow it
Anemones can fully
recover from
dissections
Clonal reproduction
is common in
polyps, but less
common in
medusae
25

26. 26

27. Sexual
Reproduction
Sexual repro in most:
some monoecious
(hermaphroditic); most
dioecious
Germ cells develop in
gastrodermis &
gametes released into
coelenteron
Most have external
fertilization; some
internal fertilization
Zygote develops into
planula larva, settles
with aboral end down,
& develops into
juvenile polyp 27

28. Class
Anthozoa
“Flower animals”
Sea anemones, corals,
sea fans, & sea pens
Largest class: 6000 sp;
solitary or colonial
No medusa phase in
lifecycle
28

29. Anthozoan
Body Form
Long pharynx
attached to
coelenteron
Many septa, 6-192
depending on
species size
Some have acontia:
stringy filaments
attached near septa
base
Heavily armed: spew
from mouth as
anemone deflates
29

30. Musculature
Mostly epidermal &
gastrodermal
epitheliomuscular cells
Epidermal musculature:
controls tentacles & oral
disc
Gastrodermal
musculature: controls
body column
30

31. Retraction
Controlled by
retractors
(longitudinal septal
muscles)
Tentacles & oral disc
deflate & pulled into
body
Mesogleal sphincter
muscle closes
opening (draw-string)
Must remove pump
water out to retract;
siphonoglyph cilia
beat inwards to
pump water back in31

32. Diversity of Class
Anthozoa
Subclass
Zoantharia
(Hexacorallia)
Order
Actiniaria:
anemones
Order
Scleractinia:
stony corals
Subclass Alcyonaria
(Octocorallia)
Order
Stolonifera:
organ pipe
corals
Order
Gorgonacea:
sea fans,
whips, &
plumes
Order
Pennatulacea:
sea pens,
feathers, &
pansies
32

33. Subclass
Zoantharia:
Order Actiniaria
Hexamarous symmetry:
septa & tentacles in
multiples of 6
Sea anemones: 1350 sp;
brightly colored; most
<10 cm
Some have unique
methods of locomotion
Burrowing via
peristalsis; walking on
tentacles; swimming by
thrashing tentacles
33

34. Order
Actiniaria:
Traits
Carnivorous suspension
feeders: mucus traps
particles on tentacles,
moved to mouth
Photosynthate: may
have 2 sets of tentacles:
false for PSN (exposed
during day) & true
(exposed during night)
Clonal repro via pedal
laceration
Gonads located in septa
34

35. Order
Actiniaria:
Ecology
Hermit crabs wear
anemone on shell &
will transfer to new
shells
Anemone gets
substrate, food,
protection, access to
mates; Crab gets
camouflage &
protection
Clownfish has
protective surface
mucus
Anemone gets food,
removal of sediment
& necrotic tissue;
Clownfish gets
protection & food
scraps 35

36. Subclass
Zoantharia:
Order
Scleractinia
Stony corals (3600
sp); close relation to
anemones
Secrete CaCO3
exoskeleton that can
weigh tons
Produce cups
(corallites) that they
retract into
Most colonial
polyps, 1-3mm
diameter 36

37. 37

38. 38

39. Subclass
Zoantharia:
Order
Scleractinia
Corals are often very
colorful due to PSN
endosymbionts
Algae often released
in conjunction with
gametes
Coral bleaching can
occur under stressful
environmental
conditions
Incorrect light/UV
intensity, salinity,
temp (even 1°C)
39

40. Subclass
Alcyonaria:
Order
Stolonifera
Octomerous
symmetry:
septa/tentacles in
multiples of 8
Soft corals: don’t
produce CaCO3;
organ pipe corals
Most lack
nematocysts;
secrete noxious
chemicals to deter
predators
More tolerant of
environmental
fluctuations 40

41. 41

42. Subclass
Alcyonaria:
Order
Gorgonacea
Plant-like sea
whips, fans, &
plumes
Highly branched
Endoskeleton: axial
rod made of
gorgonin (highly
cross-linked
collagen)
42

43. Subclass
Alcyonaria:
Order
Pennatulacea
Sea pens, feathers, &
pansies
43

44. Medusozoa
Medusa phase in
life cycle: planula 
polyp  medusa
Tetramerous radial
symmetry:
multiples of 4
Cnidae are all
nematocysts
2 major classes:
Scyphozoa (large
jellies) & Hydrozoa
(small jellies - Hydra,
& hydroids) 44

45. Class
Scyphozoa
200 sp of large jellies
Polyps: small &
funnel-shaped
(scyphistomae)
Coelenteron divided
by 4 septa
Have 4 septal funnels
that circulate water to
gonads in adults
45

46. 46

47. Class
Scyphozoa:
Body Form
Medusa bells: 2-40
cm diameter
Manubrium divided
into 4 oral arms
Tentacles located
around periphery of
bell
47

48. 48

49. Class
Scyphozoa:
Body Form
Coelenteron divided
by septa into 4 gastric
pockets
4 pairs of gonads in
septa & 4 septal
funnels
Many have radial
canals & marginal
canals
Gastrodermal cilia
circulate water
49

50. Class
Scyphozoa:
Body Form,
cont.
Lappets: rounded lobes
on umbrella margin
Rhopalia: sensory
organs in grooves b/w
lappets
Statocysts:
mechanoreceptors
(also chemoreceptors
& photoreceptors)
Nerve net & nerve ring
50

51. Scyphozoan
Reproduction
Clonal: differentiate
into strobila
Stacked mini
medusae separate
via transverse fission
(strobilation)
Juvenile medusae are
called ephyra
Adult medusae
reproduce sexually
51

52. 52

53. Scyphozoan
Diversity: 5 Orders
Semaeostomeae
Aurelia
Rhizostomeae
Stomolophus
Coronatae
Linuche
Cubomedusae
Chironex
Stauromedusae
Haliclystus
53

54. Class
Hydrozoa
Hydra & hydroids, fire
corals, & Portuguese
man-of-war; 3000 sp
Most are colonies of
polyp & medusa
zooids
Medusa often form as
buds, but are not
released from colony
Hydra life cycle does
not include medusa
phase
54

55. Hydrozoan
Body Plans
Two types of polyps:
Athecate (A-form) &
Thecate (L-form)
Zooids 1mm or
smaller in length
Large SA:Vol
No gastrodermal
septa
55

56. Hydrozoan
Body Plans
Medusae arise
from colony as
lateral buds (not
via strobilation)
Velum: iris
diaphragm on
subumbrellar
margin for
swimming
Hydroid colonies
are sessile &
benthic (seaweed-
like)
3 colony forms:
stolonate,
coenosarcal, or
fruticose 56

57. Hydrozoan
A-form
Colonies
Athecate: lack theca
(protective cup)
Periderm ends at
attachment point of
zooids
Typically grow via axial-
polyp budding
57

58. Hydrozoan
L-form
Colonies
Periderm forms a
wine-glass shape
theca
Hydranth can retract
into theca
Theca may have a
hinged lid
(operculum)
Grow via fixed-length
budding
58

59. Zooid
Terminology
Monomorphic
colonies: 1 type of
zooid
Gastrozooids: feeding
hydranths
Polymorphic colonies:
several types of zooids
Gonozooids:
reproductive polyps
Dactylozooids: have
nematocysts for
protection & food
capture
59

60. 60

61. Class
Hydrozoa:
Order
Anthoathecatae
Hydra: both clonal &
sexual repro, but no
medusa stage
Fire corals: reef builders
with nasty sting
Velella: by-the-wind
sailor with a float & sail;
extreme polymorphism,
with zooids suspended
mouth-down61 Hydra

62. 62
Fire coral

63. 63 Vellela: By-the-wind sailor

64. Class Hydrozoa:
Order
Siphonophora
Physalia: Portuguese
man-of-war
Pedal end of polyp bears
a gas-filled float
(pneumatophore)
Buds arise from a column
of polyp sets called
cormidia (oldest near
float)
Possess diversity of zooids
64

65. 65
Physalia

66. Class
Hydrozoa:
Order
Leptothecatae
L-form hydroids:
Obelia
Feather-shaped
colonies
66

67. Class
Hydrozoa:
Order
Limnomedusae
L-form hydroids with
both polyp & medusa
phase
Gonionemus
67

68. 68
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