The urochordate larva and archichordate organization: chordate origin and anagenesis revisited†

A critical analysis of relevant characters in Epineuralia (Tentaculata + Deuterostomia) in order to trace the emergence of chordate organization demonstrates that a reorganization of already highly adapted planktotrophic larvae is inconceivable. In connection with the clearly regressive state of urochordate organization, an alternative concept is presented. It proposes the chordate origin via adaptation of simply ciliated, short-ranged lecithotrophic *hemichordate* larvae which retained a dorsoposterior primary anus (see Pterobranchia, Enteropneusta-Harrimaniidae, lower Chordata). The extension of larval life to exploit the food-rich layers of the sea enabled the posterior, pre-anal body (rather than a ‘tail’) to develop into a powerful propulsive complex. This was prepared by a mesoblastema heterochrony and was co-evolutively accompanied by (1) the precocious differentiation of the middorsal nerve cord of *Hemichordata* to become a neural plate extending onto the metasoma (meeting the dorsoposterior anus: neurenteric canal) (2) a cerebral sensory centre, and (3) the precociousness of the filter-feeding branchial gut. Such larval, primitive Archichordata were coupled in their life-cycle most probably with semi-vagile *hemichordate* adults (lophophore already reduced; with new endostyle). This biphasic organization subsequently radiated into the two clades of (1) Tunicata (elaboration of the branchial gut with sedentary life, peribranchial cavity, tunica/cuticle, and regressive larvae; secondarily neotene and or pelagic) and (2) Holochordata (prematurity of larvae with extension of the somatic chordate organization forwards over the branchial gut). The latter level gave rise to (a) the Acrania (Cephalochordata) turning to an epibenthic life (fairly stationary habit with asymmetry, regression of the cerebral sensory equipment) and finally becoming infaunal: (b) the Craniata (Vertebrata) elaborating the pelagic existence and changing to a macrophagous biology (selective particle-swallowing rather than mucociliary filter-feeding), correlated with a highly adapted sensory control system (cephalization) and neural crest organization.