Phylum Cnidaria (Coelenterata) The simplest of the complex animals . . .

Types of Cnidarians Sea AnemoneHydrasJellyfishSea Coral

Simple Facts Simple CreaturesOver 10,000 living species

Both marine and freshwaterHydras in FWCorals, jellyfish, and anemones in marine

Radial symmetry

Tissue level of organization

Body PlanTwo general body forms exhibitedPOLYPSessileCylindrical bodyRing of tentacles on oral surfaceMEDUSAFlattened, mouth-down version of polypFree-swimming

Show the same internal structure Polyp FormMedusa Form

Body StructureBasic body plan of ALL cnidarians

Sac with a central digestive compartment (GVC)

Single opening serving as both mouth and anus

Ring of tentacles on oral surface

Passive Predators!All cnidarians are carnivores

Tentacles capture and push food into mouth

Tentacles are armed with stinging cellsCnidoblasts / cnidocytesContain stinging capsules called nematocysts

How does the stinging cell work??The tentacle is stimulatedPressure on trigger

Nematocyst is dischargedThread uncoilsEntangles prey

Some species produce toxinsInjects toxin into prey, paralyzing it!

Cnidocytes cover the length of tentacles

Caught ya!! (now what?!?)Food is forced into the GVC

Extracellular digestion beginsEnzymes secreted into GVC

Intracellular digestion completes processPartially digested food engulfed by endoderm cells

Im Feelin it!(Can Cnidarians feel their prey?)First true nerve cells in K. AnamaliaNerve net sends impulses in all directionsCells of epidermis and gastrodermis arranged into contractile fibersDo not have a brain to receive information!

Reproduction in CnidariansVaries among formsHydras, anemones and coralsexist only in polyp form Asexual reproduction (conditions good)BuddingSexual reproduction (conditions unfavorable)Zygotes remain dormant until conditions improve

Budding in Hydra

Some colonial polyp forms have a medusa phase in their life cycle Specialized reproductive polyps produce tiny medusas by asexual buddingSexual reproduction by medusas produce ciliated larvaplanulaPlanula settles and develops into new polyp

What about jellyfish?!?Medusa and polyp stagesSeparate sexes produce gametesFertilization occurs in GVC of femalePlanula develops and is free-swimmingPlanula settles, develops into polypPolyp produces new medusas by budding

*Fig. 32-8, PG 690

The cnidarians may be a relatively small group comprising the Animal Kingdom, but they play a major role in any marine habitat.