Cnidarian milestones in metazoan evolution

Cnidarians display most of the characters considered as milestonesof metazoan evolution. Whereas a tissue-level organization wasprobably already present in the multicellular common ancestorof all animals, the Urmetazoa, the emergence of important animalfeatures such as bilateral symmetry, triploblasty, a polarizednervous system, sense organs (eyes, statocysts), and a (chitinousor calcium-based) continuous skeleton can be traced back beforethe divergence between cnidarians and bilaterians. Modularityand metamery might be also regarded as two faces of the samemedal, likely involving conserved molecular mechanisms rulinganimal body architectures through regional specification ofiterated units. Available evidence indicates that the commonancestor of cnidarians and bilaterians, the UrEumetazoa, wasa surprisingly complex animal with nerve cell differentiation.We suggest that paedomorphic events in descendants of this ancestorled to the array of diversity seen in the main extant animalphyla. The use of molecular analyses and identifying the geneticdeterminants of anatomical organizations can provide an integrativetest of hypotheses of homologies and independent evidence ofthe evolutionary relationships among extant taxa.