Reading: Cnidarians - Biology

This laboratory exercise covers the following animals. You should learn this classification scheme and be able to classify the animals into these categories.

  • Phylum: Cnidaria
    • Class: Hydrozoa (Hydra and relatives)
    • Class: Anthozoa (Sea Anemones and Corals)
    • Class: Scyphozoa (Jellyfishes)

Some examples of Cnidarians are hydra, jellyfishes, corals, sea anemones, and Portuguese man-of-wars.


Radial Symmetry

The body parts of a radially symmetrical animal are arranged around a central axis so that each part extends from the center. The animal can be cut along the axis in more than one plane to produce identical halves. Animals that exhibit radial symmetry tend to be sessile (immobile). Radial symmetry allows them to reach out in all directions.

Cnidarians have two tissue layers. The outer layer is the epidermis. It is formed from ectoderm. The inner layer, the gastrodermis, secretes digestive juices into the inner space called the gastrovascular cavity. The gastrodermis is formed from endoderm.

Cnidarians do not have mesoderm and therefore do not have organs.

A nonliving gelatinous material called mesoglea separates the two tissue layers. A nerve net is located between the epidermis and mesoglea. The body contains long structures called tentacles that can be moved to capture prey. The tentacles contain stinging cells called cnidocytes and within each one is a capsule called a nematocyst, which discharges to either trap or sting the prey. Contractile (muscle-like) fibers are found in both the epidermis and the gastrodermis. Their movements are not complex because they do not have a brain.

Cnidarians have a hydrostatic skeleton. The contractile fibers act against the fluid-filled gastrovascular cavity. The movements are like a balloon; the animal can be short and thick or long and thin. Cnidarians have a saclike gut and extracellular digestion.

Two body forms are found among the Cnidarians, a polyp and a medusa. A polyp is attached and has the tentacles and mouth directed upward. A medusa is free-floating and has the mouth and tentacles on the ventral surface. It resembles an upside-down polyp. Some species have both a polyp and a medusa in their life cycle, others have one or the other form dominant.



  1. Use a dropper to place a live Hydra on a slide. Examine the Hydra using a dissection microscope.

    Figure 1. Hydra (live)

  2. Hydra reproduce both sexually and asexually by budding. Try to find a live Hydra with buds. If you cannot find a live Hydra budding, look for budding in a prepared slide of Hydra.

    Figure 2, Hydra budding. This is a form of asexual reproduction.

  3. Add a drop of vinegar to the slide containing Hydra. Describe what happened to the cnidocytes.

    Figure 3. Left: Hydra (Live) Exposed to 5% Vinegar Solution X 100. Right: Hydra (Live) Exposed to 5% Vinegar Solution X 200

  4. Examine microscope slides of hydra l.s. and hydra c.s. Look for the presence of two tissue layers. Identify stinging cells (Cnidocytes) in a slide of the whole animal.

    Figure 4. Left: Hydra l.s. X 100. Middle: Hydra c.s. Right: Hydra c.s. X 200

    Figure 5. Portion of a Hydra tentacle showing cnidocytes

    Figure 5. Left: Hydra l.s. X 40. Right: Hydra l.s. with ingested food X 40

Other Hydrozoans

Examine preserved specimens of Gonionemius, Polyorchis, and Physalia.

Figure 6. Left: Gonionemus, preserved. Middle: Polyorchis, preserved. Right: Portuguese Man-Of-War

Sea Anemones and Coral (Class Anthozoa)

Examine a sea anemone and coral.

Figure 7. Sea anemone, preserved

Figure 8. Left: Astrangia (Northern Coral) Skeleton. Middle: Coral Skeleton. Right: Coral Skeleton.


Cnidaria ( / n ɪ ˈ d ɛər i ə , n aɪ -/ ) [5] is a phylum under kingdom Animalia containing over 11,000 species [6] of aquatic animals found both in freshwater and marine environments, predominantly the latter.

  • A jellyfish Chrysaora melanaster
  • A gorgonian Annella mollis
  • A rocky coral Acropora cervicornis
  • A sea anemone Nemanthus annamensis
  • Subphylum Anthozoa
    • Class Octocorallia
    • Class Hexacorallia
    • Class Ceriantharia
    • Class Cubozoa—box jellyfish, sea wasps
    • Class Hydrozoa—hydroids, hydra-like animals
    • Class Polypodiozoa—parasites
    • Class Scyphozoa—true jellyfish
    • Class Staurozoa—stalked jellyfish

    Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick.

    Cnidarians mostly have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are trimorphic). Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and sexual medusae. Some, however, omit either the polyp or the medusa stage, and the parasitic classes evolved to have neither form.

    Cnidarians were formerly grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. [7] Cnidarians are classified into four main groups: the almost wholly sessile Anthozoa (sea anemones, corals, sea pens) swimming Scyphozoa (jellyfish) Cubozoa (box jellies) and Hydrozoa (a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members, such as Hydra, and colonial swimmers, such as the Portuguese Man o' War). Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the highly derived parasitic Myxozoa and Polypodiozoa were firmly recognized as cnidarians in 2007. [8]

    Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites. Many are preyed on by other animals including starfish, sea slugs, fish, turtles, and even other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs—possess polyps that are filled with symbiotic photo-synthetic zooxanthellae. While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions, and in freshwater.

    Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the sister group of bilaterians. [9] Fossil cnidarians have been found in rocks formed about 580 million years ago , and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. However, molecular clock analysis of mitochondrial genes suggests a much older age for the crown group of cnidarians, estimated around 741 million years ago , almost 200 million years before the Cambrian period as well as any fossils. [10]

    Two reasons why coral reefs are being threatened worldwide are because of tourists, who cause sewage to pollute the ocean and give nutrients to algae, step o.

    Increased ocean acidification isn’t the only thing affecting the ocean, but it is very problematic because it erodes aragonite which is a mineral of calcium .

    Sedimentation limits the light ability to the corals inhibiting their ability to feed and reproduce. Sewage and untreated wastewater carry a deadly bacteria .

    The struggle for balance between man and nature is as old as time, while precious treasures could cease to exist as a sacrifice to find that balance. Corals .

    High temperature has the ability to kill off reef ecosystems that are sensitive to change (Veron, 2008). Reefs provide habitat and food, and they are one of .

    Endangered species such as sharks, often harvested for their fins, are known as a keystone species, a species of plant or animal that produces a major impact.

    New research suggests that a disease now killing many sea turtles -fibropapillomas-may be linked to pollution in the oceans and in near shore waters. When po.

    While we have already lost 27% of coral reefs, why do we care if more die too? The great barrier reef in Australia was pronounced “dead” which opened people’.

    These projects adds a stress upon the dolphin population, forcing them to move farther away, possibly towards less suitable habitats. In addition, these recl.

    The deep-sea jellyfish has an especially unique way to survive in the ocean “in depths up to 12,000 feet” (Deep-Sea). Unlike the giant squid, deep-sea jellyf.

    Link to Learning

    Click here to follow an Obelia life cycle animation and quiz.

    All cnidarians are diploblastic and thus have two “epithelial” layers in the body that are derived from the endoderm and ectoderm of the embryo. The outer layer (from ectoderm) is called the epidermis and lines the outside of the animal, whereas the inner layer (from endoderm) is called the gastrodermis and lines the digestive cavity. In the planula larva, a layer of ectoderm surrounds a solid mass of endoderm, but as the polyp develops, the digestive or gastrovascular cavity opens within the endoderm. A non-living, jelly-like mesoglea lies between these two epithelial layers. In terms of cellular complexity, cnidarians show the presence of differentiated cell types in each tissue layer, such as nerve cells, contractile epithelial cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as the presence of intercellular connections. However, with a few notable exceptions such as statocysts and rhopalia (see below), the development of organs or organ systems is not advanced in this phylum.

    The nervous system is rudimentary, with nerve cells organized in a network scattered across the body. This nerve net may show the presence of groups of cells that form nerve plexi (singular: plexus) or nerve cords. Organization of the nervous system in the motile medusa is more complex than that of the sessile polyp, with a nerve ring around the edge of the medusa bell that controls the action of the tentacles. Cnidarian nerve cells show mixed characteristics of motor and sensory neurons. The predominant signaling molecules in these primitive nervous systems are peptides, which perform both excitatory and inhibitory functions. Despite the simplicity of the nervous system, it is remarkable that it coordinates the complicated movement of the tentacles, the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.

    The gastrovascular cavity has only one opening that serves as both a mouth and an anus this arrangement is called an incomplete digestive system. In the gastrovascular cavity, extracellular digestion occurs as food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb nutrients. However, some intracellular digestion also occurs. The gastrovascular cavity distributes nutrients throughout the body of the animal, with nutrients passing from the digestive cavity across the mesoglea to the epidermal cells. Thus, this cavity serves both digestive and circulatory functions.

    Cnidarian cells exchange oxygen and carbon dioxide by diffusion between cells in the epidermis and water in the environment, and between cells in the gastrodermis and water in the gastrovascular cavity. The lack of a circulatory system to move dissolved gases limits the thickness of the body wall and necessitates a non-living mesoglea between the layers. In the cnidarians with a thicker mesoglea, a number of canals help to distribute both nutrients and gases. There is neither an excretory system nor organs, and nitrogenous wastes simply diffuse from the cells into the water outside the animal or into the gastrovascular cavity.

    The phylum Cnidaria contains about 10,000 described species divided into two monophyletic clades: the Anthozoa and the Medusozoa. The Anthozoa include the corals, sea fans, sea whips, and the sea anemones. The Medusozoa include several classes of Cnidaria in two clades: The Hydrozoa include sessile forms, some medusoid forms, and swimming colonial forms like the Portuguese man-of-war. The other clade contains various types of jellies including both Scyphozoa and Cubozoa. The Anthozoa contain only sessile polyp forms, while the Medusozoa include species with both polyp and medusa forms in their life cycle.

    General features

    Cnidarians are radially symmetrical (i.e., similar parts are arranged symmetrically around a central axis). They lack cephalization (concentration of sensory organs in a head), their bodies have two cell layers rather than the three of so-called higher animals, and the saclike coelenteron has one opening (the mouth). They are the most primitive of animals whose cells are organized into distinct tissues, but they lack organs. Cnidarians have two body forms— polyp and medusa—which often occur within the life cycle of a single cnidarian.

    The body of a medusa, commonly called a jellyfish, usually has the shape of a bell or an umbrella, with tentacles hanging downward at the margin. The tubelike manubrium hangs from the centre of the bell, connecting the mouth at the lower end of the manubrium to the coelenteron within the bell. Most medusae are slow-swimming, planktonic animals. In contrast, the mouth and surrounding tentacles of polyps face upward, and the cylindrical body is generally attached by its opposite end to a firm substratum. The mouth is at the end of a manubrium in many hydrozoan polyps. Anthozoan polyps have an internal pharynx, or stomodaeum, connecting the mouth to the coelenteron.

    Most species of cubozoans, hydrozoans, and scyphozoans pass through the medusoid and polypoid body forms, with medusae giving rise sexually to larvae that metamorphose into polyps, while polyps produce medusae asexually. Thus, the polyp is essentially a juvenile form, while the medusa is the adult form. In contrast, anthozoans are polypoid cnidarians and do not have a medusa stage. Commonly polyps, and in some species medusae too, can produce more of their own kind asexually.

    One body form may be more conspicuous than the other. For example, scyphozoans are commonly known as true jellyfishes, for the medusa form is larger and better known than the polyp form. In hydrozoans, the polyp phase is more conspicuous than the medusa phase in groups such as hydroids and hydrocorals. Hydromedusae are smaller and more delicate than scyphomedusae or cubomedusae they may be completely absent from the life cycle of some hydrozoan species. Some other species produce medusae, but the medusae never separate themselves from the polyps. Cubozoans have medusae commonly known as box jellyfish, from their shape. Some of these are responsible for human fatalities, mostly in tropical Australia and Southeast Asia, and include the so-called sea wasps. The polyp is tiny and inconspicuous.

    Many cnidarian polyps are individually no more than a millimetre or so across. Polyps of most hydroids, hydrocorals, and soft and hard corals, however, proliferate asexually into colonies, which can attain much greater size and longevity than their component polyps. Certain tropical sea anemones (class Anthozoa) may be a metre in diameter, and some temperate ones are nearly that tall. Anthozoans are long-lived, both individually and as colonies some sea anemones are centuries old. All medusae and sea anemones occur only as solitary individuals. Scyphomedusae can weigh more than a ton, whereas hydromedusae are, at most, a few centimetres across. Tentacles of medusae, however, may be numerous and extensible, which allows the animals to influence a considerably greater range than their body size might suggest. Large populations of hydroids can build up on docks, boats, and rocks. Similarly, some medusae attain remarkable densities—up to thousands per litre of water—but only for relatively brief periods.


    The exact relationships between the different cnidarian groups are unknown. Among theories proposed on the evolution of the phylum Cnidaria, most treat the radial symmetry and tissue level of organization as evidence that the group is primitive (that is, it evolved before the evolution of bilateral symmetry) and hold that the medusa is the original body form, being the sexually reproductive phase of the life cycle. Another theory is that the original cnidarian was a planula-like organism that preceded both polyp and medusa. In either case, Hydrozoa is considered to be the most ancient of cnidarian classes, and Trachylina is thought to be the most primitive extant order of that group. An alternative view is that anthozoans are the stem of the phylum, which evolved from bilateral flatworms and is secondarily simplified. A corollary to this theory is that the polyp is the ancestral body form.

    Speculations about the origin of the phylum are not easily resolved, for preservable skeletal structures developed relatively late in cnidarian evolution. The oldest fossilized cnidarians were soft-bodied. Representatives of all four modern classes have been identified in Ediacaran fauna of the Precambrian Period (that is, those appearing between about 635 million and 541 million years ago) known from more than 20 sites worldwide. As much as 70 percent of Ediacaran species have been considered to be cnidarians. Curiously, there are few fossil cnidarians of the Cambrian Period (541 million to 485.4 million years ago). The Conulariida, which existed from the Cambrian Period to the Triassic Period (251.9 million to 201.3 million years ago), are considered by some scientists to be skeletal remains of scyphopolyps, either ancestral to the coronates or without modern derivatives. Presumed fossil sea anemones are found in the lower Cambrian System. Colonies of Stromatoporoidea, considered to be an order of the class Hydrozoa that extended from the mid-Cambrian Period to the Cretaceous Period (about 145 million to 66 million years ago), produced massive skeletons. Although there were two groups of Paleozoic corals, neither of which has modern descendants, they were not great reef-builders during that era. Scleractinians arose in the mid-Triassic Period blue corals, gorgonians, millepores, and hydrocorals have records from the Jurassic Period (201.3 million to 145.0 million years ago) or the Cretaceous Period to the present. Most other cnidarians are known only from the Holocene Epoch (within the last 11,700 years).


    Cnidarians form a phylum of animal that are more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all other animals. Both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like basement membranes muscles nervous systems and some have sensory organs. Cnidarians are distinguished from all other animals by having cnidocytes that fire harpoon like structures and are usually used mainly to capture prey. In some species, cnidocytes can also be used as anchors. ⎗] Cnidarians are also distinguished by the fact that they have only one opening in their body for ingestion and excretion i.e. they don't have a separate mouth and anus.

    Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians more complex animals have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled diploblastic, along with sponges. ⎗] ⎘] However, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer. ⎙] As a result, some recent text books classify ctenophores as triploblastic, ⎚] and it has been suggested that cnidarians evolved from triploblastic ancestors. ⎙]

      Sponges ⎛] ⎜] Cnidarians ⎗] ⎘] Ctenophores ⎗] ⎚] Bilateria ⎗]
    Cnidocytes No Yes No
    Colloblasts No Yes No
    Digestive and circulatory organs No Yes
    Number of main cell layers Two, with jelly-like layer between them Three ⎝] Two ⎗] or Three ⎙] ⎚] Three
    Cells in each layer bound together cell-adhesion molecules, but no basement membranes except Homoscleromorpha. ⎞] inter-cell connections basement membranes
    Sensory organs No Yes
    Number of cells in middle "jelly" layer Many Few (Not applicable)
    Cells in outer layers can move inwards and change functions Yes No (Not applicable)
    Nervous system No Yes, simple Simple to complex
    Muscles None Mostly epitheliomuscular Mostly myoepithelial Mostly myocytes

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    Viewed from space, Earth (Figure 1.1) offers few clues about the diversity of life forms that reside there. The first forms of life on Earth are thought to have been microorganisms that existed for billions of years before plants and animals appeared. The mammals, birds, and flowers so familiar to us are all relatively recent, originating 130 to 200 million years ago. Humans have inhabited this planet for only the last 2.5 million years, and only in the last 300,000 years have humans started looking like we do today.

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