Cnidarians
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Transcript of Cnidarians
Cnidarians
Core Concepts
• The cnidarians (or coelenterates), phylum Cnidaria (or Coelenterata), are soft bodied animals that exhibit radial symmetry and posses true tissues.
• The basic structure of cnidarians includes a sac with a central digestive area (gastrovascular cavity or coelenteron) and a single opening surrounded by tentacles bearing stinging cells.
• Cnidarians have two body forms: the sessile/sedentary polyp and the free-swimming medusa. In many cnidarians, the life cycle alternates between these two body forms.
• Phylum Cnidaria is divided into three classes: Class Hydrozoa (hydras), Class Scyphozoa (jellyfish) and Class Anthozoa (corals and sea anemones
Keywords • Cnidaria/Coelenterata• Eumetazoa• polyp• medusa• gastrovascular cavity/coelenteron• epidermis• mesoglea• gastrodermis• cnidoblast• nematocyst• nerve net• Hydrozoa• Scyphozoa• Anthozoa
Characteristics
• the name: Cnidaria from KNIDE (nettle)• Coelenterata from COEL (cavity) + ENTERON
(gut)• includes hydras, jellyfish, corals and sea
anemones• aquatic (mostly marine, few fresh water species)• basic structure: hollow sac w/ single opening
(mouth) surrounded by tentacles bearing stinging cells (cnidoblasts)
• exhibits radial symmetry• diploblastic, with two cell layers of the body
wall separated by mesoglea EPIDERMIS & GASTRODERMIS (+ MESOGLEA – jelly-like, non-cellular)
• exist as both free-swimming medusae and as sessile/sedentary polyps
Digestive and nervous system
• Incomplete digestive system
• Nerve net
Structure and function
• diffuse nervous system NERVE NET (nerve cells distributed all over body)
• • tentacles possess CNIDOBLASTS (stinging cells)
containing NEMATOCYSTS (w/ poison) for defense and catching food
• gastrovascular cavity (coelenteron) is sac shaped with one opening acting as both mouth and anus INCOMPLETE gut/digestive tract
• • digestion EXTRACELLULAR (within the gut)
and intracellular (in cells lining the gut)• • no respiratory, circulatory, excretory systems;
only through diffusion
Body form
• POLYP – sessile, cylindrical body w/ mouth & tentacles at
upper free end– may be solitary or colonial– e.g. corals, hydras, sea anemones
• MEDUSA– free swimming, like an upside down bowl w/
mouth & tentacles facing downward– e.g. jellyfish
Reproduction
• polyp asexual by budding (polyp or medusa)– regeneration of Hydra
• medusa sexual – larval stage (PLANULA) elongate & ciliated– settles, attaches & develops into a polyp
Life cycle
Class Hydrozoa
• alternation between polyp and medusa forms, both stages equally well-developed where the polyp stage is more conspicuous than the medusa
• e.g. Obelia• Physalia (Portuguese man-of-war)• Hydra (only exists in polyp form)
Class Scyphozoa
• medusa is the conspicuous stage; polyp stage reduced to small larva
• although in some, there is no polyp stage• e.g. true jellyfish Aurelia
Class Anthozoa
• “flower animals”• sedentary or sessile polyp; no trace of medusa
stage• e.g. sea anemones • corals (secrete hard external skeletons of
CaCO 3)
• Cnidarians, which are among the most primitive eumetazoans, were the first animals capable of movement. The presence of true tissues, albeit a simple nerve net and contractile cells, has enabled these organisms to sense and react to stimuli.