Cnidaria

• Phylum Cnidaria


  Most primitive Eumetazoa
• Cnidarians


  • Some are polymorphic with 2 stages: Polyp & Medusa
  • Have the same opening for the mouth and anus
  • Have a gelatinous mesoglea between epidermal and gastrodermal tissue layers
  • Some have a true skeleton, some hydrostatic
  • Have a Gastrovascular Cavity
  • Have a Nerve Net
  • Have Cnidocytes as specialized cells for defense, feeding, and attachment
  • Are dioecious
  • Have a planula larva as a characteristic larval stage

  
• Form, Layers, & Cell Types

  1. Nerve Cells interconnect to form a nerve net

  2. Nerve net conducts nerve impulses to respond to localized stimulus, spreads over the body depends on the stimulus strength

  3. Sensory structures distributed throughout the body and receptors for perceiving touch and other certain chemicals

  4. All cells are short distance from the body surface so O and CO2 and nitrogenous wastes are exchanged via Diffusion

  5. Epidermis is the external layer, ectoderm

  6. Mesoglea is the middle jelly-like layer

  7. Gastrodermis is the internal layer, endoderm
• Nematocytes


  • Stinger, type of cnida used in food gathering and defense, may discharge a long tube armed with spines that penetrate the prey to deliver paralyzing toxins
• Cnidocytes


  • Stinging cells, produce structures called Cnidae, usually has a modified cilium called Cnidocil which triggers the discharge of the coiled tube
• Ptychocytes


  • In the tentacles, release a sticky substance to aid in adhesion and prey capture
• Spirocytes


  • In the tentacles, not used for stinging, instead function in entangling or immobilizing prey
• Amoebocytes


  • In the mesoglea, not specialized, deliver nutrients to cells and differentiate into other cell types, for reproduction, secreting skeletal elements, transporting and storing food, may carry undigested particles to the gastrovascular cavity
• Gastrovascular Cavity


  • For digestion, exchange of respiratory gases and metabolic wastes and discharge of gametes, food, digestive wastes, and reproductive stages enter and leave through the mouth because it has an incomplete digestive system
• Alternation of Generations


  1. Polyp: asexual and sessile, attached to a substrate, has a cylindrical body, mouth surrounded by tentacles for food gathering, may reproduce asexually by budding, fission, or laceration

  2. Medusa: inverted bowl shape with tentacles dangling from its margins, mouth opening is centrally located, swims by gentle pulsations of the body wall
• Gastrozooid


  • Feeding polyp
• Gonozooid


  • Reproductive polyp that produces medusa by budding
• Reproduction and Development


  1. All cnidarians are dioecious, sperm and eggs may be released into the gastrovascular cavity or outside of the body, larval stage is the planula larva, when the medusa bud detaches it becomes an ephyra, buds may detach from polyp or remain attached to contribute to the colony

  2. Polyp -> Blastula -> Gastrodermis (formed by Blastula) -> Planula (attaches to a substrate) -> Young Polyp -> budding -> Medusa

  
• Feeding & Digestion


  Some cnidarians feed on small fish using nematocysts to entangle and paralyze prey, food enters and gastrodermal gland cells secrete lubricating mucus and enzymes, digestion starts in the gastrovascular cavity, nutritive-muscular cells phagocytize partially digested food, undigested material is expelled through the mouth by peristalsis
• Respiration, Circulation & Excretion

  • Cnidarians rely on diffusion to exchange gases, have a decentralized circulatory system of canals and chambers to circulate nutrients and waste, excrete metabolic waste through specialized cells in the gastrodermis
• Nervous System


  • Primitive, nerve cells scattered across the body function as sensory and motor neurons, detect physical, chemical, and light stimuli, have statocysts for balance and orientation
• Classes under Phylum Cnidaria

  • Anthozoa: includes sea anemones, sea pens, and corals, can reproduce asexually or sexually
  • Hydrozoa: most are marine, exhibit both polyp and medusa forms, many form colonies
  • Scyphozoa: dioecious, nerve cells scattered, planula larvae settle and form scyphistoma polyps, polymorphic
  • Cubozoa: includes box-shaped jellies, most venomous group

  
• Colonial hydroids


  Usually attached to mollusk shells, rocks, and other animals, some mollusks and flatworms eat hydroids bearing nematocysts and use it for their own defense, sometimes live symbiotically with other animals as commensals
• Economic Importance of Cnidarians

  • Aquaculture, Medical Research, Tourism, Biotechnology, Food
  • Reef-building corals
  • Precious coral used for jewelry and ornaments
  • Coral rock serves for building purposes
• Characteristics of Cnidarians


  • Symmetry: Radial/Biradial
  • Coelom: Acoelomate
  • Body Organization: Tissue level
  • Germ Layers: Diploblastic
  • Eumetazoa
  • Development: Protostome
  • Respiratory: Diffusion & Bulk Transport
  • Circulatory: None
  • Skeletal: None
  • Nervous System: Nerve Net
  • Digestive System: Incomplete
  • Excretory System: None
  • Reproduction: Asexual & Sexual
  • Defense: Nematocysts, Toxins
  • Locomotion: Floating, Swimming