5 The radiate animals — Phylum Coelenterata
( Cnidaria )
The Phylum Coelenterata includes the polyps, jellyfishes, sea
anemones, and corals. All of these animals have a body wall consisting of two
layers of cells, between which is a jellylike substance, the mesoglea. Within the body is a single gastrovascular
cavity, or coelenteron. Because of the presence of
two cellular layers, Coelenterates are side to have a tissue-level organization.
They are also acoelomates; that is, they don’t
possess a second body cavity, the coelom .
5. 1 Position
in animal kingdom
Phylum Cnidaria are characterized by primary radial or biradial symmetry . Radial symmetry , in which the body parts are arranged
concentrically around the oral-aboral axis , is
particularly suitable for sessile or sedentary animals .
The phylum has not advanced generally beyond the
tissue level of organization, although a few organs occur .
a. The phylum has developed two well-defined germ layers , ectoderm and endoderm ; a third , or mesodermal ,layer , which is derived embryologically
from the ectoderm , is present in some . The body plan is saclike
, and the body wall is composed of two distinct layers , epidermis and gastrodermis , derived from the ectoderm and endoderm ,
respectively . The gelatinous matrix , mesoglea , between these layers may be structureless
, may contain a few cells and fibers , or may be composed largely of mesodermal connective tissue and muscle fibers .
b. An internal body cavity, the gastrovascular
cavity , is lined by the gastrodermis
and has a single opening , the mouth , which also serves as the anus .
c. Extracellular digestion
occurs in the gastrovascular cavity
, as does intracellular digestion in the gastrodermal
Fig.Hydra capturing a water flea,
having already swallowed another
d. Most radiates have tentacles ,
or extensible projections around the oral end , that aid in food capture .
e. The first true nerve cells ( protoneurons ) occur in the radiates , but the
nerves are arranged as a nerve net , with no central nervous system .
f. Sense organs appear first in the radiates and
include well-developed statocysts (organs of equilibrium ) and ocelli.
Fig. The nerves are arranged as a nerve net
g. Locomotion in the free-moving forms is achieved
either by muscular contractions . However the groups
are still better adapted to floating or being carried by currents than to
strong swimming .
h. Polymorphism (polyp stage and medusa stage )
in the cnidarians has widened their ecologic possibilities .
Some unique features are found in this phylum , such
as nematocysts ( stinging organoids ) in Cnidarians .
a. Entirely aquatic , some in fresh water but
b. Radial symmetry or biradial
symmetry around a longitudinal axis with oral and aboral
ends ; no definite head
c. Two basic types of individuals: polyps and medusae
d. Exoskeleton or endoskeleton of chitinous,
calcareous , or protein components in some
g. Special stinging cell organoids
called nematocysts in either or both epidermis and gastrodermis ; nematocysts
abundant on tentacles , where they may form batteries or rings
h. Nerve net with symmetric and asymmetric synapses;
with some sensory organs; diffuse conduction
e. Body with two layers, epidermis and gastrodermis, with mesoglea ( diploblastic ) ; mesoglea with cells and connective tissue ( ectomesoderm ) in some ( triploblastic
f. Gastrovascular cavity (often
branched or divided with septa ) with a single opening
that serves as both mouth and anus; extensible tentacles usually encircling the
mouth or oral region
Reproduction by asexual budding (
in polyps ) or sexual reproduction by gametes ( in all medusae
and some polyps ) ; sexual forms monoecious or dioecious ; planula larva ; holoblastic cleavage .
j. No excretory or respiratory systems
k. No coelomic cavity
a. Class Hydrozoa
This class includes the freshwater polyps
, the small jellyfishes , the hydroid zoophytes, and a few stony corals
b. Class Scyphozoa
Most of the large jellyfishes are placed in this class .
c. Class Anthozoa
In this class are included the sea anemones , and most of the stony and horny corals .
5.4.1 Class Hydrozoa
a. solitary or colonial ;
b. asexual polyps and sexual medusae
, although one type may be suppressed ;
with no mesenteries.
d. medusae ( when present ) with a velum;
fresh-water and marine .
(2) Typical animal
a. The structure of hydra ;
b. The physiological function of hydra ;
c. The life cycle of hydra .
Fig. Life cycle of Obelia,
showing structure of the hydroid colony
5.4.2 Class Scyphozoa
Characteristics of Class Scyphozoa
a. solitary ;
b. polyp stage
reduced or absent ;
c. bell-shaped medusae without velum ;
d. gelatinous mesoglea
much enlarged ;
e. margin of bell or umbrella typically with eight
notches that are provided with sense organs ;
f. all marine
a. The structure of Aurelia aurita
b. The life cycle of Aurelia aurita
Fig. The structure of Aurelia aurita
female ovum zygote
5.4.3 Class Anthozoa
all polyps, no medusae ;
solitary or colonial ;
c. enteron subdivided by at least
8 mesenteries or septa bearing nematocysts;
d. gonads endodermal ;
e. all marine .
Generally, after swimming
for a few hours to many days the planula attaches and
develops into a polyp or polypoid form
, which in colonial species subsequently gives rise to the colony .
In some hydroids the planula
remains in the gonophore, developing into the tentaculate
actinula larva which is liberated and creeps about .
After attachment, it develops into a polyp . In many
hydrozoans with no polypoid phase the planula develops into an actinula
and then a medusa .
In most scyphozoans, after attachment, the planula develops into a polypoid scyphistoma, with a stalked trumpet-shaped body . At maturity, the scyphistoma
produces a free-swimming medusa stage, the ephyra
larva, by transverse fission or strobilization. Ephyrae may be produced singly or several at a time, and
develop to adult medusae .
In zoantharian anthozoans, the planula does not
attach but develops into an anemone-like Edwardsia
larva, then the Halcampoides larva. After attachment,
tentacles develop and the adult polyp form is attained.
There are both dioecious and
hermaphroditic species .
5.6.1 Sexual reproduction
reproduction is common, it may occur by :
pedal laceration, e.g. in sea anemones ;
transverse fission, e.g. in the production of ephyrae
longitudinal fission , e.g. in many sea anemones .
In many cnidarians the
life-cycle contains two morphologically dissimilar individuals
, the pulp and the medusa . In colonial species, each of these types may
occur in a number of different morphological forms ,
specialized to perform a particular function .
The main types of modified polyp are
a. the gastrozooid ---- feeding
b. the gonozooid ---- reproductive
c. the dactylozooid ---- protective
polyp , or tentaculozooid
a, b and c are found in hydrozoan
the autozooid ---- feeding and reproductive polyp
e. the siphonozooid ---- current producing polyp
d, and e are found in some authozoan
A colony may also bear medusoid
forms in different stages of formation or degeneration, which may or may not be
freed . Medusae may become
modified as swimming bells, floats , protective bracts
or phyllozooids, or gonophores which serve only for
About 100 species known . Ovoid forms measure up to about 5 cm , flattened forms may be up to 1 meter or more in length .
symmetry biradial ; arrangement of internal canals
and the position of the paired tentacles change the radial symmetry into a
combination of the two ( radial + bilateral )
usually ellipsoidal or spherical in shape , with radially
arranged rows of comb plates for swimming
c. Ectoderm , endoderm , and
a mesoglea ( ectomesoderm )
with scattered cells and muscle fibers ; may be considered tripoblastic
d. Nematocysts absent ( except
in one species ) , but adhesive cells ( colloblasts )
e. Digestive system consisting of mouth, pharynx , stomach , and a serious of canals
f. Nervous system consisting of a subepidermal plexus concentrated around the
mouth and beneath the comb plate rows; an aboral sense organ ( statocyst )
g. No polymorphism or attached stages
h. Reproduction monoecious,
gonads ( endodermal origin )
on the walls of the digestive canals , which are under the rows of comb plates
; determinate cleavage ; cydippid larva
i. Luminescence common
(2) Comparison with Cnidaria
A. Ctenophores resemble the cnidarians
in the following ways:
Form of radial symmetry ; together with the cnidarians , they form the group Radiata
b. Aboral - oral axis
around which the parts are arranged
c. Well - developed gelatinous ectomesoderm
d. No coelomic cavity
e. Diffuse nerve plexus
f. Lack of organ systems
They differ from the cnidarians in the following ways:
a. No nematocysts except in Euchlora
b. Development of muscle cells from mesenchyme
c. Presence of comb plates and colloblasts
d. Mosaic, or determinate type of development
e. Presence of pharynx generally
f. No polymorphism
With tentacles. Tentacles may or may not have sheaths into which
they retract . Some types flattened for creeping ; other compressed to a bandlike
form . In some the comb plates may be confined to the larval form.
conical form; wide mouth and pharynx; gastrovascular
canals much branched .
1.What characteristics of phylum
Cnidaria are most important in distinguishing it from
other phyla ?
2. Name and distinguish the classes in phylum Cnidaria.
3. Distinguish between polyp and medusa forms.
4. What is an un unusual feature
of the nervous system of cnidarians?
5. Diagram a hydra and label the main body parts.
6. Name and give functions of the main cell types in the
epidermis and in the gastrodermis of hydra.
7. Define the following with regard to hydroids: hydrocaulus, hydrorhiza, perisarc, hydranth, gonangium, manubrium, statocyst, ocellus, polymorphism, alternation
of asexual and sexual stages.
8. Distinguish the following from each other: statocyst and rhopalium; scyphomedusae and hydromedusae; scyphistoma,
strobila, and ephyrae;
velum, velarium, and pedalium.
9. Define the following with regard to sea anemones: siphonoglyph; primary septa or mesenteries; incomplete
septa; septal filaments; acontia
threads; pedal laceration.
10. What characteristics of Ctenophora
are most important in distinguishing it from other phyla?
11. Compare cnidarians and ctenophores, giving five ways
in which they resemble each other and five ways in which they differ.
12. What is a widely held hypothesis on the origin of