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 organiza tion demonstrates that a reorganization of already highly adapted plan ktotrophic larvae is inconceivable. In connection with the clearly reg ressive state of urochordate organization, an alternative concept is p resented. It proposes the chordate origin via adaptation of simply cil iated, short-ranged lecithotrophic hemichordate* larvae which retaine d a dorsoposterior primary anus (see Pterobranchia, Enteropneusta-Harr imaniidae, lower Chordata). The extension of larval life to exploit th e food-rich layers of the sea enabled the posterior, pre-anal body (ra ther than a 'tail') to develop into a powerful propulsive complex. Thi s was prepared by a mesoblastema heterochrony and was co-evolutively a ccompanied by (1) the precocious differentiation of the middorsal nerv e cord of Hemichordata* to become a neural plate extending onto the m etasoma (meeting the dorsoposterior anus: neurenteric canal) (2) a cer ebral sensory centre, and (3) the precociousness of the filter-feeding branchial gut. Such larval, primitive Archichordata were coupled in t heir life-cycle most probably with semi-vagile hemichordate* adults ( lophophore already reduced; with new endostyle). This biphasic organiz ation subsequently radiated into the two clades of (1) Tunicata (elabo ration of the branchial gut with sedentary life, peribranchial cavity, tunica/cuticle, and regressive larvae; secondarily neotene and/or pel agic) and (2) Holochordata (prematurity of larvae with extension of th e 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, regressio n of the cerebral sensory equipment) and finally becoming infaunal; (b ) the Craniata (Vertebrata) elaborating the pelagic existence and chan ging to a macrophagous biology (selective particle-swallowing rather t han mucociliary filter-feeding), correlated with a highly adapted sens ory control system (cephalization) and neural crest organization.