The recent setting of specific features of the Cnidaria into evolulionaryand ecological frameworks suggests the centrality of thisphylum in many fields of the life sciences. From an evolutionarypoint of view, the Cnidaria, with their diploblastic pliinulae. mightrepresent the ancestral state of higher Metazwa in the light of aperamorphic origin of animal complexity from a simple, individualorganism. Medusan development in the Hydroidomcdusae viaa medusary nodule, furthermore, implies the formation of a thirdtissue layer (the muscle layer lining the subumbrellar cavity).Cnidarian polyps are diploblastic, whereas at least some of theirmedusae are triploblastic: the evolutionary enigma of the passagefrom a diplo- to a triploblastic organisation takes place every timehydrozoan polyps bud medusae! Cnidarian polyps have also thepremises of the skeletal architecture of higher animals: their chitinousor carbonatic skeletons are similar to those of arthropodsand vertebrates respectively. From an ecological point of view,the coelenterates probably play roles that are much more importantthan usually perceived. Both Cnidaria and Ctenophora feedon the eggs and larvae of most benthic, planktonic and nektonicorganisms and might be crucial (with a keystone role?) in maintainingbiodiversity high, by feeding on potentially monopolisingspecies. The efficiency of gelatinous predators becomes evidentduring periodic outbreaks of their populations, with serious implicationseven on fisheries yields, demonstrating that their impactcan be higher than ours!
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