Phylum Cnidaria - Prof. L. D. Amarasinghe

Phylum Cnidaria
(a stinging thread)
Also known as Phylum Coelenterata (-hollow gut)
This phylum contains about 10,000 species worldwide.
They are mostly marine with a few freshwater species.
• Cnidarians are regarded as originating close to the
basal stock of the metazoan line.
• They have a 700 million back history based on fossil
records.
Defining characteristics are:
• 1. nematocysts (cnidae); complex intracellular
organelles that secreted by the animal
• 2. Planula larva in the life cycle.
Larva is bilaterally symmetrical
Cnidarians include;
1. corals
2. sea anemones
3. Fresh water hydroids (Hydra),
4. Jellyfish
5. Portuguese man-of-war
Portuguese war ship
• Physalia
Cnidarians have a basic radial symmetry.
They possess only 2 layers of living tissue;
- ectoderm ; the epidermis*
- endoderm; the gastrodermis*
*Muscle fibers in epithelia
They have a Gelatinous layer called mesoglea (non living) in
between epidermis and gastrodermis.
Amoebocytes (living cells) in the mesoglea involve in food
digestion, nutrient transport, storage and defense.
• The body is sac-like with an internal gastro vascular cavity
called “coelenteron” and an opening called the “mouth”.
• (Open-ended tubular gut; no anus)
• The mouth surrounds a ring of tentacles.
• No head
• Body axis is oral- aboral
• Cnidarians are characterized by having stinging cells called
cnidoblast (nematoblasts).
• These cells contain cnidae (nematocysts), a fluid-filled
proteinaceous capsule with an opening in one end. Inside
capsule is a long, spirally coiled hollow thread/tube.
Cnidocil is a cluster of modified cilia
in the cnidoblast through which
chemical and tactile stimulation perceive to
act on cnida
Upon stimulation, the hollow thread/tube shoots
out explosively from the capsule, and eject from
the lid turning inside out as it goes.
It contains toxins which are used in food capture
and defense.
The entire process requires only about 3 milli seconds
• Cnidarians are primarily carnivorous; but
• some soft coral feed on phytoplanktons
• Some obtain nutrients through photosynthesizing
symbiotic algae
• They show tissue level of organization. Muscle fibres
occur at the base of the epidermal and gastrodermal cells.
•
• They do not have circulatory, respiratory and excretory
systems.
• A nerve net consisting of multipolar neurones are
located below the epidermis near the mesoglea.
• Some of them have sense organs such as statocysts and
oceli.
(Simplest animals to possess true nerve cells and sense
organs)
• The body form of a cnidarian is constructed following two
basic body plans, the polyp form and the medusa form.
(Polymorphism)
• Both forms have tentacles arranged around the mouth.
Polyp/Polypoid form
Medusa/Medusoid form
Usually sessile
Vase/cylindrical shape
Mesoglea thin
Mouth at the upper surface
Tentacles occur around the
mouth
Solitary or colonial
Sometimes they build hard
or soft corals
Eg. Corals,
Hydras,
Sea anemones
Usually mobile
Bell/umbrella shape
Mesoglea thick
Mouth at the lower surface
Tentacles occur along the edge
of the umbrella
Always solitary
Never build skeletons
Eg. Jelly fishes
• In some species only the polyp form is seen in the life
cycle while in some others only the medusa form is
seen.
• Still, in the life cycle of some other species
alternation of generation is seen between the
medusa and the polyp forms.
Only polyp - Hydra
Only medusa –
Physalia
Aurelia (Jellyfish)
Typical cnidarian life cycle
Cnidarians are divided into four taxonomic classes.
• Class Hydrozoa – includes- hydras
portuguese-man-of war
Obelia
• Class Schyphozoa – includes-jellyfishes
• Class Anthozoa - includes- sea Anemones
corals
• Class Cubozoa
Class Schyphozoa –
Cup animals
• Include few hundreds (about 200) species of cnidarians
• All are marine.
• Eg. Aurelia
Scyphozoans are characterized by the predominant
medusoid stage (scyphomedusae)
It looks an inverted cup.
The polyp stage exists only as a short-lived larval stage.
(Scyphistoma larva)
Scyphistoma larva
• Scyphozoan medusae are big; 2 cm - 2 m in
diameter (relative to the hydrozoan medusae)
• Most scyphozoans swim actively by contraction of muscle
fibres and mechanical properties of mesoglea.
• Contraction of muscle fibre results the reduction of the
volume of space under the bell and excess water expel
out. This effects animal to propel opposite direction.
Mesoglea is thick that supports the body structure like a skeleton.
It is firm gelatin.
Hence, scyphozoans are commonly known as Jellyfish.
Mouth locates at the end of a muscular cylinder-known as
manubrium.
• They lack a velum(a shelf of tissue projecting inward from
the margin of the bell in hydromedusae - Hydrozoan)
• The medusae bell is circular in cross section and the
tentacles are distributed around the circumference.
• Sensory organs distributed around the edge of the bell,
called the rhopalia, contain the ocelli (simple eyes) and
statocysts used for determining spatial orientation.
Scyphomedusae always have 4 oral arms surrounding the mouth.
• Gastrovascular cavity of scyphozoans is developed as fluid
filled gastrovascular canals.
• Gastrovascular canals connect to the mouth through the
manubrium.
• Food particles captured by nematocysts on tentacles and
oral arm-----ingested at the mouth--- through manubrium--- to stomach -----distributed among four gastric pouches.
• Wall of the gastrovascular pouch cansists short
tentacles with nematocysts that secrete digestive
enzymes.
• Partially digested food particles are phagocytized
and digestion completes intracellularly.
• Circulation of fluid in the gut take place by
movement of cilia lining the gastrovascular canals.
The life cycle of a typical scyphozoan (Aurelia)
The sexes are separated.
The life cycle of Aurelia includes alternating generations of medusae and polyps
Notes
The lifecycle of a typical scyphozoan
(Aurelia)
• In medusae, gametes are released from the gastrodermal
gonads into the gastric pouches.
• Ova remain in the coelenteron but spermatozoa exit via
the mouth into the sea. They enter the mouth of a
female and make their way to the gonad where they
fertilize the eggs.
Notes
• Cleavage begins in the gastric pouches. Embryos are
released from the mouth and brooded on the oral
arms. A hollow, ciliated blastula develops and
invaginates to become a gastrula.
• The gastrula, known as the planula larva, is capable of
independent existence and leaves the mother to swim
in the plankton.
• The planula locates a suitable hard substratum,
attaches, and grows into a scyphistoma larva a solitary
polyp form.
• The scyphistoma can feed and may survive more than
one reproductive season.
Notes
• The sessile minute scyphistoma, reproduce asexually by
budding or by formation of cysts (podocysts).
• Under suitable environmental conditions, scyphistomae
undergo transverse segmentation and segment
metamorphosis (strobilation).
• The segmented parts of the strobilating polyp (strobila)
develop into medusae that eventually break loose and
become free swimming young jellyfishes (ephyrae).
• The ephyra grows rapidly into an adult medusa, completing
the life cycle.
Class Hydrozoa
(Water animals)
• 2,700 species of cnidarians include in Class Hydrozoa
• Include marine and freshwater species
Eg. Hydras, Portuguese-man-of war,
• Most hydrozoans represent a polyp stage than a medusa
stage in the life cycle.
• Eg. Hydra - do not have a medusa
• But, some spend part of their lives as medusa and part as
polyp stage.
• Eg. Obelia
gonozoid
gastrozoid
• Hydrozoan medusae are also known as ‘’jellyfishes’’ but,
much smaller (few cms or even less)
• Gastrodermal tissues of hydrozoans do not have
nematocysts. They are restricted to epidermis.
• Mesoglea do not contain cells.
• A shelf of tissues called velum extend inward from the
edge of the swimming bell towards manubrium
• This help to eject water in a grater pressure from under the
bell through a narrow opening when contraction of
musculature
Class Anthozoa –
(Flower animals)
corals, sea anemones
~6,600
species.
Marine.
The anthozoans are a key feature of all marine
environments, most notably in the form of coral reefs.
They are sessile and either solitary or colonial.
Lack a medusa stage entirely (Only polyp stage that
look like flowers).
• The oral disc is encircled by numerous tentacles (in
multiples of 6 or 8) placed at top of a cylindrical column .
• The body cavity (coelenteron) is divided by mesenteries
(septa) also in multiples of 6 or 8.
• Anthozoans have a muscular pharynx leading from the
mouth to the gut.
Coral polyp secrete a prototheca by its epidermal layer;
first as a basal plate and then develop as radial folds
and a rim as a thecal wall in between mesenteries.
• There are two subclasses within the Anthozoa:
Hexacorallia
Octocorallia
Hexacorallia (Zoantharia)
• The hexacorals have;
- simple tentacles around the mouth in multiples of six
- and six pairs of internal mesenteries.
• Eg. - common sea anemones, Fungia;(They are solitary) and
- most are reef forming true stony corals (hard corals) and
thorny corals; (They are colonial)
• Fungia
Sea anemony
Coral
Octacorallia (Alcyonaria)
• Octocorals have eight branching (pinate) tentacles
and eight mesenteries (septa). Colonial & soft corals
• Eg. Gorgonia (Sea fan), sea pens, sea pancies (Renilla).
Many show polymorphism.
Class Cubozoa - box jellyfish
• ~20 species.
• Marine.
A cubozoan medusa.
• The cubozoans are similar to the Scyphozoa but the bell is
square in cross section, with a velum-like structure called
the velarium.
• The velarium restricts the size of the opening through
which water is expelled when the bell contracts, thus
increasing thrust and making them stronger swimmers
than the Scyphozoa.
• There are four clusters of tentacles, one at each corner of
the bell.
• The rhopalia of the Cubozoa differ from those of the
Scyphozoa in possessing very complex eyes with lenses,
corneas and retinas.
• The lens is capable of producing very sharp images, as
good as human eyes but the focal length is longer than
the distance between the lens and the retina making box
jellyfish strangely far-sighted.
• Development is also different in the cubozoans.
• Each scyphistoma forms a single medusa via complete
metamorphosis.
• Cubozoa includes the highly toxic box jellyfish, found in
tropical regions and often in swarms which can drift into
bays, disrupting human activities.