Phylum Cnidaria
This phylum includes branching plant-like hydroids, flowerlike sea anemones, jellyfishes and stony corals which
produce great reefs and coral islands. The phylum gets its name from cells called cnidocytes, which contain
organelles (cnidae). The most common type of cnidae is the nematocyst. Only cnidarians produce cnidocytes. They
are widespread in marine habitats, especially in shallow warm areas. They sometimes live symbiotically with snail
shells of crabs proving the crab with protection from predators. Although many cnidarians have little positive
economic importance, reef-building corals are an exception.
Form and Function:
Characteristics of phylum Cnidaria:
Cnidocytes present, typically with stinging organelles called nematocysts
All aquatic
Radial or biradial symmetry
Polyps and medusae
Diploblastic with epidermis and gastrodermis derived from embryonic ectoderm and endoderm
Mesoglea, an extracellular matrix lies between body layers
Incomplete gut called gastrovascular cavity, often branched or divided with septa
Extracellular digestion in gastrovascular cavity and intracellular digestion in gastrodermal cells
Extensible tentacles usually encircle the mouth or oral region
Muscular contractions via epitheliomuscular cells
Sense organs include well developed statocysts (organs of balance) and ocelli (photosensitive organs)
Nerve net with symmetrical and asymmetrical synapses
Asexual reproduction by budding in polyps to form clones and colonies
Sexual reproduction by gametes in all medusae and some polyps
Monoecious or dioecious
No excretory or respiratory system
No coelomic cavity
Dimorphism and Polymorphism:
- All cnidaria forms fit into one of 2 categories (dimorphism), a polyp/hydroid form adapted to sessile life or a
medusa/jellyfish form adapted for a free-swimming existence.
Polyps:
o Mostly have tubular bodies
o Mouth surrounded by tentacles. The mouth leads to a blind gut or gastrovascular cavity
o Aboral end of the polyp is usually attached to a substratum by a pedal disc or other device
o Reproduce asexually by budding, fission or pedal laceration. In budding a knob of tissue forms on the side of an
existing polyp and develops a functional mouth and tentacles. If a bud detaches from the polyp that made it, a
colony will form and food maybe shared through a common gastrovascular cavity. Polyps that do not bud are
solitary. Polyps that do bud are clonal or colonial. In fission, a polyp divides in half as one side of the polyp pulls
away. In pedal laceration, tissue torn from the pedal disc develops into tiny new polyps
o Polymorphism occurs when a single genotype can express more than one body form
Medusae:
o Usually free-swimming
o Have bell or umbrella shaped bodies
o Tetramerous symmetry where body parts are arranged in fours
o Mouth is usually centred on the concave side
o Tentacles extend out the rim of the umbrella
o Sensory structures for orientation (statocysts) and light reception (ocelli)
o Sensory information is integrated with motor response by a nerve ring or 2 at base of the bell
Life cycles:
- A zygote develops into a planula larva
This phylum includes branching plant-like hydroids, flowerlike sea anemones, jellyfishes and stony corals which
produce great reefs and coral islands. The phylum gets its name from cells called cnidocytes, which contain
organelles (cnidae). The most common type of cnidae is the nematocyst. Only cnidarians produce cnidocytes. They
are widespread in marine habitats, especially in shallow warm areas. They sometimes live symbiotically with snail
shells of crabs proving the crab with protection from predators. Although many cnidarians have little positive
economic importance, reef-building corals are an exception.
Form and Function:
Characteristics of phylum Cnidaria:
Cnidocytes present, typically with stinging organelles called nematocysts
All aquatic
Radial or biradial symmetry
Polyps and medusae
Diploblastic with epidermis and gastrodermis derived from embryonic ectoderm and endoderm
Mesoglea, an extracellular matrix lies between body layers
Incomplete gut called gastrovascular cavity, often branched or divided with septa
Extracellular digestion in gastrovascular cavity and intracellular digestion in gastrodermal cells
Extensible tentacles usually encircle the mouth or oral region
Muscular contractions via epitheliomuscular cells
Sense organs include well developed statocysts (organs of balance) and ocelli (photosensitive organs)
Nerve net with symmetrical and asymmetrical synapses
Asexual reproduction by budding in polyps to form clones and colonies
Sexual reproduction by gametes in all medusae and some polyps
Monoecious or dioecious
No excretory or respiratory system
No coelomic cavity
Dimorphism and Polymorphism:
- All cnidaria forms fit into one of 2 categories (dimorphism), a polyp/hydroid form adapted to sessile life or a
medusa/jellyfish form adapted for a free-swimming existence.
Polyps:
o Mostly have tubular bodies
o Mouth surrounded by tentacles. The mouth leads to a blind gut or gastrovascular cavity
o Aboral end of the polyp is usually attached to a substratum by a pedal disc or other device
o Reproduce asexually by budding, fission or pedal laceration. In budding a knob of tissue forms on the side of an
existing polyp and develops a functional mouth and tentacles. If a bud detaches from the polyp that made it, a
colony will form and food maybe shared through a common gastrovascular cavity. Polyps that do not bud are
solitary. Polyps that do bud are clonal or colonial. In fission, a polyp divides in half as one side of the polyp pulls
away. In pedal laceration, tissue torn from the pedal disc develops into tiny new polyps
o Polymorphism occurs when a single genotype can express more than one body form
Medusae:
o Usually free-swimming
o Have bell or umbrella shaped bodies
o Tetramerous symmetry where body parts are arranged in fours
o Mouth is usually centred on the concave side
o Tentacles extend out the rim of the umbrella
o Sensory structures for orientation (statocysts) and light reception (ocelli)
o Sensory information is integrated with motor response by a nerve ring or 2 at base of the bell
Life cycles:
- A zygote develops into a planula larva
