Cnidarians

• Cnidaria


  Animals with stinging nettle ("cnidos"), also known as coelenterates, are the second least developed phylum of the kingdom Animalia
• Cnidaria


  • Are the first group, according to level of advancement, to have true tissues
  • Have blind-ending cavity called gastrovascular cavity, which function in digestion, exchange of gases and metabolic wastes as well as for discharge of gametes
  • Have radial or biradial symmetry
  • Nervous system is made up of a nerve net
  • Usually have alternation of generations between polyp and medusa
  • Are uncephalized animals with one body opening, the mouth

  
• Cnidocyte


  Stinging cell
• Cnidarians are generally believed to be passive predators, depending on organisms that stray into their tentacles by accident, because polyps are immobile and not all medusae have sensory structures

  
• Feeding


  1. Mouth surrounded with tentacles used in capturing food

  2. Mucus secreted from the mouth helps in trapping food

  3. Cnida or cnidos (plural: cnidae) in tentacles assist in prey paralyzing and capturing

  4. Captured food drawn through mouth into gastrovascular cavity

  5. Gastrodermal gland cells produce proteolytic enzymes that digest the food

  6. Nutritive-muscular cells take up partially digested food by phagocytosis and form food vacuoles in which digestion is completed

  7. Nutritive-muscular cells move material into and out of the gastrovascular cavity by peristalsis

  8. Some nutritive-muscular cells in symbiotic association with zooxanthellae absorb dissolved already made food (sugars) and provide symbionts with waste materials like CO2

  
• Respiration


  Oxygen diffuses across the body surface, O2 can also be obtained from symbionts
• Excretion


  Digestive and metabolic waste products are eliminated through the mouth and diffusion across body surface
• Nervous system and sensory structures

  • Have a nerve net made up of sensory neurons
  • Some have ocelli (perception of light), statocysts (for balancing and telling the animal whether it is upside down or not) and statoliths (contained in the statocyst sac, for recognition of up and down displacement)
• Asexual reproduction


  1. Regeneration

  2. Budding

  3. Binary fission where the animal splits along the lateral axis
• Sexual reproduction


  • Most cnidarians are gonochoristic (dioecious- having separate sexes)
  • Gametes are released (by polyps or medusa-the two body forms) into the gastrovascular cavity, where the eggs may be retained until after fertilization
• Sexual life cycle


  1. Early development results in formation of an embryo called blastula

  2. The embryo elongates to form a ciliated, free-swimming larva, called a planula

  3. The planula attaches itself to a substrate and develops further to form a polyp (but not medusa)

  4. An adult polyp produces both polyps and medusae by budding from its body wall

  5. Depending on the species, buds may break away to form separate individuals or remain on the parent to form a colony

  
• Metagenesis in scyphozoans and cubozoans

  1. The larva develops into a polyp called scyphistoma

  2. The scyphistoma absorbs its tentacles and splits horizontally into a series of disks that develop into young medusa (ephyra) in a process called strobilation

  3. The young medusae swim off and develop into adults, while the polyp re-grows and may continue strobilating periodically

  
• Cnidarian groups


  • Anthozoans (about 6,100 species)
  • Hydrozoans (about 3,600 species)
  • Scyphozoans (about 228 species)
  • Cubozoans (about 42 species)
• Cnidarians


  • Sea anemones
  • Corals
  • Jellyfishes

  
• Cnidarian body forms


  • Polyp - tube with a mouth surrounded by tentacles, sessile and attached to substrate at the aboral end
  • Medusa - umbrella-shaped, free-swimming
• Colonial polyps


  • Individual members connected by tubelike hydrocauli (singular: hydrocualus)
  • Root-like stolon anchors various segments of the colony to the substrate
  • Specialized polyps (zooids) such as gastrozooid for feeding, gonozooid for reproduction, and dactylozooid with tentacles studded with nematocysts for defense
• Cnidarians are diploblastic and, like poriferans, have body made up of two cell layers and a jellylike middle matter
• The entire body and tentacles of cnidarians consist of two layers of cells, the inner gastrodermis (also known as endoderm) and the epidermis (ectoderm)

  
• Mesoglea


  The jellylike middle matter between the two cell layers, which may range from little more than a glue to bind the layers (as in Hydra) to a very thick, gelatinous, matter accounting for most of the mass of the animal (as in jellyfishes)
• Gastrovascular cavity


  The blind-ending cavity that functions in digestion, exchange of gases and metabolic wastes, as well as for discharge of gametes
• Nematocyst


  The capsule contained in the cnidocyte (nettle cell) that can eject a thread with poisonous stinging (harpoon-like) structures, found in the tentacles and assist in prey paralyzing and capturing

  
• Nematocyst


  • Like-structures found in the tentacles and assist in prey paralyzing and capturing

  
• Polyp


  A tube with a mouth surrounded by tentacles, sessile and attached to substrate at the aboral end, mesoglea is thin
• Medusa (jellyfish)


  Shaped like an inverted bowl that has tentacles hanging from its brim, free-floating and swims by pulsating contractions, mesoglea forms bulk of the organism giving it the characteristic jellylike nature

  
• Specialized polyps in colonies

  • Gastrozooid for feeding
  • Gonozooid for reproduction
  • Dactylozooid, tentacles studded with nematocysts for defense

  
• Colonial forms (Siphonophores)


  • Comprise both polyps and medusae, medusa serve as floats and propel the colony, polyp represented by gastrozooids, gonozooids and dactylozooid
• Class Hydrozoa


  • Most species have both polypoid and medusoid stages in life cycle, but polypoid stage dominates in most
  • Some have only one stage, e.g. Hydra has no medusoid stage
  • Medusae have separate sexes but difficult to differentiate
  • Reproduce by budding
  • Nematocysts mostly on tentacles
  • Water-filled gastrovascular cavity acts as hydrostatic skeleton
  • Mostly marine but with freshwater species

  
• What accounts for the green colour in Hydra and what is its significance?
• Examples of Hydrozoa


  • Hydra
  • Portugese man of war (Physalia)
• Class Scyphozoa (jellyfishes)


  • Medusoid generation is large and noticeable, polypoid is small and inconspicuous
  • Medusa lack the velum
  • Medusa have separate sexes
  • Mesoglea is thick, gelatinous, and accounts for most of the mass
  • Mesoglea contains amoeboid cells
  • Cnidocytes concentrated on tentacles
  • Gastrodermal cells have cilia for continuous circulation of seawater
  • Movement by rhythmic muscular contraction of the bell and elastic recoil of the mesoglea, also drift with currents
  • All marine, about 200 species

  
• Example of Scyphozoa


  • Aurelia
• Class Anthozoa ("flower animals")

  • All sedentary polyps, no medusoid stage
  • Sexes usually separate, some oviparous, others viviparous
  • Reproduce asexually by budding
  • Cnidocytes in epidermis and gastrodermis
  • Mouth leads into tubular pharynx that opens into gastrovascular cavity
  • Mesoglea contains amoeboid cells
  • Gastrovascular cavity divided into radial compartments by mesenteries
  • Solitary forms (sea anemones) and colonial forms (stony and soft corals)
  • All marine, more than 6,000 species
• Examples of Anthozoa


  • Sea anemones (Metridium)
  • Corals (Corallium)
  • Sea fans (Gorgonia)
  • Sea pens (Pennatula)
  • Sea pansies (Renilla)

  
• Class Cubozoa ("scyphozoa cubed")


  • Medusa is cuboidal swimming bell with four tentacles
  • Polyps very small where known
  • Active swimmers
  • Toxic and possess dangerous nematocysts

  
• Examples of Cubozoa


  • Sea wasps
  • Box jellyfish

  
• Anti-viral and anti-cancer drugs were developed from sponges, e.g. the HIV drug azidothymidine (AZT), anti-leukemia drug
• Sponges were used, in the past, for padding and packing, to paint with and to bathe with
• Some sponges are still used today as luxury bath items, especially those that produce only sponging (no spicule)- Spongia officinalis
• Sponges and cnidarians serve as home and protection for some crabs, shrimps and brittle stars
• Corals are used in live aquarium trade