Radiate Animals: Phyla Cnidaria & Ctenophora
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Important Characteristics of Members of the Phyla Cnidaria and Ctenophora
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two embryological primary germ layers (ectoderm and endoderm) that are
homologous to those of more complex metazoans
internal space for digestion, the gastrovascular cavity, which lies along the polar axis
and opens to the outside by a mouth
some cnidarians have a skeleton (eg. coral), but in most radiates, fluid in the
gastrovascular cavity serves as a simple form of hydrostatic skeleton.
Although both cnidarians and ctenophores are grouped together as radiate phyla,
they differ in important ways:
o cnidarians have characteristic stinging organelles called nematocysts,
usually absent in ctenophores.
o polymorphism- the presence in a species of more than one morphological
kind of individual- is common in cnidarians but absent in ctenophores.
o ctenophores have distinctive adhesive cells called colloblasts on their
tentacles and unique rows of ciliated comb plates not found in other phyla
There are two main types of body form in cnidarians:
1. polyp (hydroid) form, often sessile.
2. medusa (“jellyfish”) form, which is free-swimming.
In some groups of cnidarians, both polyp and medusa stages are found in their life cycle.
These animals are therefore polymorphic. In others, such as sea anemones and corals, there
is no medusa; in still others, such as the scyphozoans, or "true" jellyfish, the polyp stage is
reduced or absent. In life cycles having both polyps and medusae, the juvenile polyp stage
gives rise asexually to a medusa, which reproduces sexually. Both polyp and medusa have the
diploid number of chromosomes, but the gametes are haploid.
Phylum Cnidaria: Radiate Animals
Average Sizes
¾ millimeters to meters (colonial corals)
Life Span
¾ highly variable – years?
Symmetry & Body Plan
¾ radial symmetry
¾ no cephalization
¾ oral and aboreal ends
Development & Coelom Formation
¾ true tissues
¾ no coelom
¾ free swimming ciliated larval forms
¾ polymorphism with polyp and medusa stages; polyp stage reduced in some species;
medusa stage reduced in some species
¾ diploblastic body plan with endoderm and ectoderm; a few have simplified mesoderm
Radiate Animals: Phyla Cnidaria & Ctenophora
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Type of Skeleton
¾ hydrostatic skeleton
Appendage Types
¾ none really
¾ tentacles, although these are not typically considered appendages
Basic
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Form & Function
diploblastic body plan with an epidermal and gastrodermal layer
cnidocytes with nematocysts (stinging cells)
gastrovascular cavity with single opening that is both mouth and anus
extracellular digestion occurs in the gastrovascular cavity with digestion of food
products contuining in the gastrodermis.
Most have tentacles or extensible projections around the oral end for food capture
nerve net with no centralized nervous system
have statocysts sense organs for balance and ocelli for detecting light
mesoglea which provides for support and acts as a type of elastic support skeleton
between the epidermis and the gastrodermis
first neuromuscular system developed
Specialized Cell Types
¾ epitheliomuscular cells =
¾ cnidocytes with nematocysts
¾ gland cells that secrete digestive enxymes
¾ interstitial cells which undifferentiated cells scattered among other cells that can
become other cells as necessary
¾ sensory cells for tactile or chemical stimuli
“Organ Systems”
Integumentary System (skin)
¾ epidermis that can contain epitheliomuscular, neurosensory cells and cnidocytes
Muscular System
¾ epitheliomuscular cells - not true skeletal muscle yet developed
Circulatory System
¾ none – nutritent/waste removal by diffusion/osmosis/cell transport
(phagocytosis/pinocytosis)
Respiratory System (gas exchange)
¾ none – gas exchange by simple diffusion
Nervous System
¾ nerve net with protoneurons (most are multipolar = look like a stereotypical neuron)
Sensory Organs
¾ mechanoreceptors that respond to tactile stimulation
¾ chemoreceptors that respond to chemical/molecular stimulation
Digestive System (food/water supply)
¾ intracellular by diffusion/osmosis/cell transport (phagocytosis & pinocytosis)
¾ mostly carnivorous
Radiate Animals: Phyla Cnidaria & Ctenophora
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Immune System
¾ none (?)
Excretory System (waste removal)
¾ none – waste removal by diffusion/osmosis/cell transport
Reproduction
¾ asexual reproduction regeneration by budding (in polyps)
¾ sexual reproduction by eggs and sperm (gametes produced by medusae)
¾ monoecious or dioecious reproduction depending on species and portion of
lifecycle
¾ ciliated planula (flattened) larval form
Mechanism/Mode of Locomotion
¾ free swimming ciliated larval forms
¾ polyps are sessile
¾ medusae are free-swimming
Specialized Defenses/Toxin/Poisons
¾ organic or protein based toxins released from nematocytst on tentacles
Ecology & Adaptive Radiation
¾ marine and freshwater
Social Organization
¾ none
Communication
¾ none(?)
Thermoregulation
¾ none
Phylum Ctenophora: Radiate Animals
Average Sizes
¾ millimeters to centimeters
Life Span
¾ months to years?
Symmetry & Body Plan
¾ biradial symmetry
¾ no cephalization
¾ ellipsoidal or spherical in shape
Development & Coelom Formation
¾ true tissues
¾ no coelom
¾ free swimming ciliated larval forms
¾ NO polymorphism
¾ diploblastic body plan with endoderm and ectoderm with mesoderm; some consider
these to be triploblastic
Radiate Animals: Phyla Cnidaria & Ctenophora
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Type of Skeleton
¾ hydrostatic skeleton
Appendage Types
¾ none really
¾ two tentacles in some (class Tenaculata), although these are not typically considered
appendages
Basic Form & Function
¾ diploblastic body plan with an epidermal and gastrodermal layer
¾ mesoglea which provides for support and acts as a type of elastic support skeleton
between the epidermis and the gastrodermis
¾ radially arranged rows of comb plates for swimming
¾ NO nematocysts (stinging cells)
¾ colloblasts (adhesive cells) present
¾ never colonial
¾ gastrovascular cavity with both mouth and anal opening
¾ extracellular digestion occurs in the gastrovascular cavity with digestion of food
products contuining in the gastrodermis.
¾ tentacles near the oral end for food capture
¾ nerve net with no centralized nervous system
¾ rows of comb plates for sensory detection
¾ have statocysts sense organs for balance
Specialized Cell Types
¾ epitheliomuscular cells =
¾ cnidocytes with nematocysts
¾ gland cells that secrete digestive enxymes
¾ interstitial cells which undifferentiated cells scattered among other cells that can
become other cells as necessary
¾ sensory cells for tactile or chemical stimuli
¾ luminescence is common in ctenophores
“Organ Systems”
Integumentary System (skin)
¾ epidermis that can contain epitheliomuscular and neurosensory cells
Muscular System
¾ epitheliomuscular cells - not true skeletal muscle yet developed
Circulatory System
¾ none – nutritent/waste removal by diffusion/osmosis/cell transport
(phagocytosis/pinocytosis)
Respiratory System (gas exchange)
¾ none – gas exchange by simple diffusion
Nervous System
¾ nerve net with protoneurons (most are multipolar = look like a stereotypical neuron)
Radiate Animals: Phyla Cnidaria & Ctenophora
Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Sensory Organs
¾ mechanoreceptors that respond to tactile stimulation
¾ chemoreceptors that respond to chemical/molecular stimulation
Digestive System (food/water supply)
¾ intracellular by diffusion/osmosis/cell transport (phagocytosis & pinocytosis)
¾ mostly carnivorous
Immune System
¾ none (?)
Excretory System (waste removal)
¾ none – waste removal by diffusion/osmosis/cell transport
Reproduction
¾ monoecious reproduction
¾ cydippid larval form
Mechanism/Mode of Locomotion
¾ free swimming ciliated larval forms
¾ Adults are free swimming
Specialized Defenses/Toxin/Poisons
¾ none?
Ecology & Adaptive Radiation
¾ all are marine
Social Organization
¾ none
Communication
¾ none(?)
Thermoregulation
¾ none
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Radiate Animals: Phyla Cnidaria & Ctenophora
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Classification of the Radiate Animals
Phylum Cnidaria
Class Hydrozoa (hy-dro-zo'a) (Gr. hydra, water serpent, + zoon, animal). Both polyp and
medusa stages represented, although one type may be suppressed; medusa with a velum;
found in fresh and marine water. The hydroids. Examples: Hydra, Obelia, Gonionemus,
Tubularia, Physalia.
Class Scyphozoa (sy-fo-zo'a) (Gr. skyphos, cup, + zoon, animal). Solitary; medusa stage
emphasized; polyp reduced or absent; enlarged mesoglea; medusa without a velum. The true
jellyfish. Examples: Aurelia, Rhizostoma, Cassiopeia.
Class Cubozoa (ku'bo-zo'a) (Gr. kybos, a cube, + zQon, animal). Solitary; polyp stage reduced;
bellshaped medusae square in cross section, with a tentacle or group of tentacles at each
corner; margin without velum but with velarium; all marine. Examples: Carybdea, Chironex.
Class Anthozoa (an-tho-zo'a) (Gr. anthos, flower, + zQon, animal). All polyps, no medusae;
gastrovascular cavity subdivided by mesenteries (septa).
Subclass Hexacorallia (hek-sa-ko-ral'e-a) (Gr. hex, six, + korallion, coral) (Zoantharia).
Polyp with simple, unbranched tentacles; septal arrangement hexamerous; skeleton, when
present, external. Sea anemones and stony corals. Examples: Metridium, Tealia, Astrangia.
Subclass Ceriantipatharia (se-re-an-tip' a-tha' ri-a) (N. 1. combination of Ceriantharia and
Antipatharia, from type genera). With simple, unbranched tentacles; mesenteries unpaired.
Tube anemones and black or thorny corals. Examples: Cerianthus, Antipathes.
Subclass Octocorallia (ok'to-ko-ral'e-a) (1. octo, + Gr. korallion, coral) (Alcyonaria). Polyp
with eight pinnate tentacles; septal arrangement octamerous. Soft and horny corals.
Examples: Gorgonia, Renilla, Alcyonium.
Phylum Ctenophora
Class Tentaculata (ten-tak'yu-la'ta) (1. tentaculum, feeler, + ata, group suffix). With tentacles;
tentacles may have sheaths into which they retract; some types flattened in oral-aboral axis for
creeping; others compressed in tentacular plane to a band-like form; in some the comb plates
may be confined to the larva. Examples: Pleurobrachia, Cestum.
Class Nuda (nu-da) (1. nudus, naked). Without tentacles, but flattened in tentacular plane;
wide mouth and pharynx; gastrovascular canals much branched. Example: Beroe.