Phylum Cnidaria

1
Phylum Cnidaria
2

• General Characteristics
• They are radially symmetrical oral end
  terminates in a mouth surrounded by tentacles.
• They have 2 tissue layers
• Outer layer of cells - the epidermis
• Inner gastrodermis, which lines the gut cavity
  or gastrovascular cavity (gastrodermis secretes
  digestive juices into the gastrovascular cavity)
• In between these tissue layers is a noncellular
  jelly-like material called mesoglea

3

• Cnidarian Body Plans
• Polyp form
• Tubular body, with the mouth directed upward.
• Around the mouth are a whorl of feeding
  tentacles.
• Only have a small amount of mesoglea
• Sessile
• Medusa form
• Bell-shaped or umbrella shaped body, with the
  mouth is directed downward.

• Small tentacles, directed downward.
• Possess a large amount of mesoglea Motile, move
  by weak contractions of body

4

• Movement
• The cnidarian body is capable of some kind of
  coordinated movement
• Both the epidermis and the gastrodermis possess
  nerve cells arranged in a loose network - nerve
  net (plexus), which innervate primitively
  developed muscle fibers that extend from the
  epidermal and gastrodermal cells
• Stimulus in one part will spread across the
  whole body via the network

5

• Nutrition
• Cnidarians are carnivores with hydras and corals
  consuming plankton and some of the sea anenomes
  consuming small fishes
• They use they tentacles to capture prey and
  direct it toward the mouth so that it can be
  digested in the gastrovascular cavity via
  secretions from gland cells (extracellular
  digestion) some food is phagocytized by special
  cells and digestion occurs intracellularly

6

• Nutrition
• The gastrovascular cavity exists as 1 opening for
  food intake and the elimination of waste
• There is no system of internal transport, gas
  exchange or excretion all these processes take
  place via diffusion

7

• Stinging Organelles
• Prey capture is enhanced by use of specialized
  stinging cells called cnidocytes located in the
  outer epidermis.
• Each cnidocyte has a modified cilium - cnidocil,
  and is armed with a stinging structure called a
  nematocyst.

8

• Stinging Organelles
• The undischarged nematocyst is composed of a
  long coiled thread
• When triggered to release, either by touch or
  chemosensation, the nematocyst is released from
  the cnidocyte and the coiled thread is everted
• Some nematocysts function to entangle the prey
  others harpoon prey and inject a paralyzing toxin

9

• Reproduction
• One of the most amazing adaptations is the
  ability of some cnidarians to regenerate lost
  parts or even a complete body
• Asexual reproduction is common with new
  individuals being produced by budding

Planula larva
10

• Reproduction
• Sea anenomes engage in a form of asexual
  reproduction called pedal laceration
• Cnidariand are dioecious
• Fertilization is external, with the zygote
  becoming a elongated, ciliated, radially
  symmetrical larva - planula larva

Planula larva
11
Cnidarian Taxonomy
12

• Class Hydrozoa
• Includes the solitary freshwater hydra most are
  colonial and marine
• Typical life cycle includes both asexual polyps
  and sexual medusa stages however, freshwater
  hydras and some marine hydroids do not have a
  medusa stage

13

• Solitary Hydras
• Freshwater hydras are found in ponds and streams
  occurring on the underside of vegetation
• Most possess a pedal disc, mouth, hypostome
  surrounded by 6-10 tenetacles

14

• Solitary Hydras
• Mouth opens to the gastrovascular cavity
• The life cycle is simple eggs and sperm are
  shed into the water and form fertilized eggs
  planula is by passed with eggs hatching into
  young hydras
• Asexual reproduction via budding

15

• Colonial Hydrozoans - e.g., Obelia
• Possess a skeleton of chiton that is secreted by
  the epidermis
• All polyps in the colony are usually
  interconnected
• Two different kinds of individuals that comprise
  the colony feeding polyps or gastrozooids (C)
  and reproductive polyps or gonozooids (B)

16

• Life Cycle of Obelia
• Gonozooids release free swimming medusae
• Zygotes become planula larvae, which eventually
  settle to become polyp colonies

• The medusae of hydroids are smaller than those
  of jellyfishes (C. Scyphozoa)
• Also, the margin of the bell projects inward
  forming a shelf-like velum

17
(No Transcript)
18
Class Hydrozoa cont. Other Hydrozoans
Portuguese man-of-war Single gas-filled float
with tentacles Tentacles house the polyps and
modified medusae of the colony
19

• Class Scyphozoa
• Jellyfish
• The medusae are large and contain massive
  amounts of mesoglea
• The differ from the hydrozoan medusa in that the
  lack a velum

20

• Class Scyphozoa
• Jellyfish
• Possess four gastric pouches lined with
  nematocysts these are connected with the mouth
  an the gastrovascular system

21
Scyphozoan Life Cycle - Aurelia

• Gametes develop in gastrodermis of gastric
  pouches eggs and sperm are shed through mouth
• Fertilized eggs develop into a planula larva
  settles on substrate and develops into a polyp -
  scyphistoma

22
Scyphozoan Life Cycle - Aurelia

• Scyphistoma produces a series of polyps by
  budding - strobila
• The polyps undergo differentiation and are
  released from the strobila as free swimming
  ephyra
• Ephyra matures into an adult jellyfish

23

• Class Anthozoa
• Exclusively marine there is no medusa stage
• At one or both ends of the mouth is a ciliated
  groove called the siphonoglyph generates a water
  current and brings food to the gastrovascular
  cavity
• Possess a well developed pharynx

24

• The gastrovascular cavity is large and
  petitioned by septa or mesenteries increase
  surface area for digestion or support
• Edges of the septa usually have threadlike
  acontia threads, equipped with nematocysts and
  gland cells

25

• Class Anthozoa cont.
• Solitary anthozoans include sea anemones
• Most anthozoans are colonial (e.g. corals) and
  secrete external skeletons composed of calcium
  carbonate.

26

• Class Anthozoa cont.
• Corals obtain much of their energy from
  microscopic photosynthetic green algae
  (zooxanthellae) or dinoflagellates that live
  symbiotically inside the cells of the coral