1. BODY WALL OF
CNIDARIANS
BY
CHANG PEI XIAN D20101037455

3. Body Wall
In a cnidarian
polyp, such as hydra, the
body wall surrounding
the gastrovascular cavity
consists of an outer
epidermis (ectordermal)
and an inner
gastrodermis
(endodermal) with
mesoglea between them.

4. Mesoglea
 Mesoglea lies between the epidermis and
the gastrodermis and adheres to both
layers.
 It is gelatinous, or jelly like, and has no
fibers or cellular elements in hydrozoan
polyps.
 It is thicker in medusae and has elastic
fibers; in scyphozoan medusae, it has
ameboid cells and epitheliomuscular
cells.

6. Gastrodermis
 The gastrodermis, a layer of cells lining the
gastrovascular cavity, consists of
large, ciliated, columnar epithelial cells
with irregular flat bases.
 Cells of the gastrodermis include nutritive-
muscular, interstitial, and gland cells
and, in classes other than
Hydrozoa, cnidocytes also included.
Gonads are gastrodermal in most
cnidarians.

8.  Nutritive muscular cells are usually tall
columnar cells that have laterally extended
bases containing myofibrils.
 Water is brought into the cavity through the
mouth by the beating of the cilia on the
nutritive-muscular cells in hydrozoans or by
ciliated cells in the pharynx of anthozoans
and serves as a hydrostatic skeleton.
 Interstitial cells scattered among the bases
of the nutritive cells can transform into
other cell types.
 Gland cells are tall cells that secrete
digestive enzymes.

10.  Epitheliomuscular cells
Form most of the epidermis and serve both for
covering and for muscular contraction.
The bases of most of these cells are extended
parallel to the tentacle or body axis and contain
myofibrils, contractions of these fibrils shorten the
tentacles or body.
 Interstitial cells
Undifferentiated stem cells found among the
bases of the epitheliomuscular cells.
Differentiation of interstitial cells produce
cnidoblasts, sex cells, buds, nerve cells, and
others, but generally not epitheliomuscular cells
(which reproduce themselves).

11.  Gland cells
Abundant around the mouth and in the pedal
disc of hydra.
Secrete mucus or adhesive material.
 Nerve cells
Often multipolar, although in more highly
organized cnidarians, the cells may be bipolar.
Their processes (axons) form synapses with
sensory cells and other nerve cells, and
junctions with epitheliomuscular cells and
cnidocytes.
Both one-way and two-way synapses with other
nerve cells are present.

12.  Cnidocytes
Occur throughout the epidermis.
They may be between the epitheliomuscular
cells or housed in invaginations of these cells.
Most abundant on the tentacles.
 Sensory cells
Scattered among the other epidermal
cells, especially around the tentacles and
mouth.
The free end of each sensory cell bear a
flagellum, which is the sensory receptor for
chemical and tactile stimuli.

13. CNIDAE
 Secreted within cells called cnidoblasts
 Abundant on the feeding tentacles of all species and
within the digestive cavity of some species
 Cnidal morphology is crucial in making species
identification
 Functions of cnidae:
-food collection
-defense
-locomotion

14. CNIDAE
 3 major types
-Nematocysts
occur in all cnidarian classes
can be stinging or tangling depending on type of
cnidarian
-Spirocysts
only found in Anthazoa
contribute to the adhesive stickiness of the
tentacles
-Ptychocysts
only occur in the anthazoan order Ceriantharia
used for building protective tubes

15. CNIDAE OF
CNIDARIA
-Nematocysts
( Blue )
-Spirocysts
( Yellow )

16. Nematocysts Discharged
 Can only be discharged once
 Both mechanical and chemical stimuli may act to
trigger firing – triggered by cnidocil
 The discharged is initiated by the opening of
capsule operculum
 Then, the tubules will begin to evert out with a
twisting motion
 The discharge may involve the increase of osmotic
pressure within the capsule or because of release of
tension within capsule wall

18. NERVE NET
 Simple nervous system
 In the centre of nerve net is the nerve ring which
controls swimming and where to go
 The nerve cells help to coordinate the work of
other body cells that are linked in the net- if
touched, whole body will react
 Nerve net consist of sensory neuron and motor
neuron which are connected by the intermediate
neuron
 The communication of nerve cells are via chemical
synapses