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.