phylum cnidaria l.
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Phylum Cnidaria. General Characteristics They are radially symmetrical; oral end terminates in a mouth surrounded by tentacles. They have 2 tissue layers Outer layer of cells - the epidermis

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General Characteristics

  • They are radially symmetrical; oral end terminates in a mouth surrounded by tentacles.
  • They have 2 tissue layers
    • Outer layer of cells - the epidermis
    • Inner gastrodermis, which lines the gut cavity or gastrovascular cavity (gastrodermis secretes digestive juices into the gastrovascular cavity)
  • In between these tissue layers is a noncellular jelly-like material called mesoglea

Cnidarian Body Plans

  • Polyp form
  • Tubular body, with the mouth directed upward.
  • Around the mouth are a whorl of feeding tentacles.
  • Only have a small amount of mesoglea
  • Sessile
  • Medusa form
  • Bell-shaped or umbrella shaped body, with the mouth is directed downward.
  • Small tentacles, directed downward.
  • Possess a large amount of mesoglea
  • Motile, move by weak contractions of body


  • The cnidarian body is capable of some kind of coordinated movement
  • Both the epidermis and the gastrodermis possess nerve cells arranged in a loose network - nerve net (plexus), which innervate primitively developed muscle fibers that extend from the epidermal and gastrodermal cells
  • Stimulus in one part will spread across the whole body via the network


  • Cnidarians are carnivores with hydras and corals consuming plankton and some of the sea anenomes consuming small fishes
  • They use they tentacles to capture prey and direct it toward the mouth so that it can be digested in the gastrovascular cavity via secretions from gland cells (extracellular digestion); some food is phagocytized by special cells and digestion occurs intracellularly
  • The gastrovascular cavity exists as 1 opening for food intake and the elimination of waste
  • There is no system of internal transport, gas exchange or excretion; all these processes take place via diffusion

Stinging Organelles

  • Prey capture is enhanced by use of specialized stinging cells called cnidocyteslocated in the outer epidermis.
  • Each cnidocyte has a modified cilium - cnidocil, and is armed with a stinging structure called a nematocyst.
  • The undischarged nematocyst is composed of a long coiled thread
  • When triggered to release, either by touch or chemosensation, the nematocyst is released from the cnidocyte and the coiled thread is everted
  • Some nematocysts function to entangle the prey; others harpoon prey and inject a paralyzing toxin


  • One of the most amazing adaptations is the ability of some cnidarians to regenerate lost parts or even a complete body
  • Asexualreproduction is common with new individuals being produced by budding
  • Sea anenomes engage in a form of asexual reproduction called pedal laceration
  • Cnidariand are dioecious
  • Fertilization is external, with the zygote becoming a elongated, ciliated, radially symmetrical larva - planula larva

Planula larva


Class Hydrozoa

  • Includes the solitary freshwater hydra; most are colonial and marine
  • Typical life cycle includes both asexual polyps and sexual medusa stages; however, freshwater hydras and some marine hydroids do not have a medusa stage
  • Solitary Hydras
  • Freshwater hydras are found in ponds and streams occurring on the underside of vegetation
  • Most possess a pedal disc, mouth, hypostome surrounded by 6-10 tenetacles
  • Mouth opens to the gastrovascular cavity
  • The life cycle is simple: eggs and sperm are shed into the water and form fertilized eggs; planula is by passed with eggs hatching into young hydras
  • Asexual reproduction via budding

Class Hydrozoa cont.

  • Colonial Hydrozoans - e.g., Obelia
  • Possess a skeleton of chiton that is secreted by the epidermis
  • All polyps in the colony are usually interconnected
  • Two different kinds of individuals that comprise the colony: feeding polyps or gastrozooids(C) and reproductive polyps or gonozooids (B)

Class Hydrozoa cont.

  • Life Cycle of Obelia
  • Gonozooids release free swimming medusae
  • Zygotes become planula larvae, which eventually settle to become polyp colonies
  • The medusae of hydroids are smaller than those of jellyfishes (C. Scyphozoa)
  • Also, the margin of the bell projects inward forming a shelf-like velum

Class Hydrozoa cont.

Other Hydrozoans

Portuguese man-of-war:

Single gas-filled float with tentacles

Tentacles house the polyps and modified medusae of the colony


Class Scyphozoa

  • Jellyfish
  • The medusae are large and contain massive amounts of mesoglea
  • The differ from the hydrozoan medusa in that the lack a velum
  • Possess four gastric pouches lined with nematocysts; these are connected with the mouth an the gastrovascular system

Scyphozoan Life Cycle - Aurelia

  • Gametes develop in gastrodermis of gastric pouches; eggs and sperm are shed through mouth
  • Fertilized eggs develop into a planula larva; settles on substrate and develops into a polyp - scyphistoma
  • Scyphistoma produces a series of polyps by budding - strobila
  • The polyps undergo differentiation and are released from the strobila as free swimming ephyra
  • Ephyra matures into an adult jellyfish

Class Anthozoa

  • Exclusively marine; there is no medusa stage
  • At one or both ends of the mouth is a ciliated groove called the siphonoglyph; generates a water current and brings food to the gastrovascular cavity
  • Possess a well developed pharynx
  • The gastrovascular cavity is large and petitioned by septa or mesenteries; increase surface area for digestion or support
  • Edges of the septa usually have threadlike acontia threads, equipped with nematocysts and gland cells

Class Anthozoa cont.

  • Solitary anthozoans include sea anemones
  • Most anthozoans are colonial (e.g. corals) and secrete external skeletons composed of calcium carbonate.
  • Corals obtain much of their energy from microscopic photosynthetic green algae (zooxanthellae) or dinoflagellates that live symbiotically inside the cells of the coral