Departure from the colinearity rules in echinoderms (crinoids and echinoids). (A) Crinoids. (B) Echinoids. The most parsimonious hypothesis regarding the spatial and temporal expression of the Hox genes in crinoids and echinoids indicates that both groups ‘violate’ the spatial colinearity rule as their body (portrayed by a conceptualized bilaterian) is subdivided according to specific Hox expression vectors. The order of Hox genes along the cluster might follow the time vector in crinoids (dashed line indicates that the gene array remains putative), whereas it does not in echinoids in which the temporal colinearity rule is likewise not entirely followed.