Tornaria of hemichordates and other dipleurula‐type larvae: a comparison*

Abstract

The evolutionary origin of phylum Chordata is the subject of intensive discussion, with the most conflicting views prevalent. One popular theory advocates the separation of chordates from a dipleurula-like ancestor. Thus the dipleurula-type larvae (tornaria of enteropneusts, auricularia and bipinnaria of echinoderms) are considered to recapitulate the ancestral features and the direct evolutionary path from Echinodermata and Hemichordata to Chordata (i.e. Garstang 1894 Zool. Anzeiger 27, 122–125; Grobben 1908 Verh. Zool.-Bot. Ges. Wien 58, 491–511; Dillon 1965 Evolution 19, 436–446; Jollie 1973 Acta Zool. (Stockholm) 54, 81–100; Ivanova-Kazas and Ivanov 1987 Sov. J. Mar. Biol. 13, 67–80; Crowther and Whittaker 1992 J. Neurophysiol. 23, 280–292; Lacalli 1994 Am. Zool. 34, 533–541; Lacalli et al. 1999 Proc. R. Soc. Biol. Series B 266, 1461–1470; Nielsen 1999 Dev. Genes Evol. 209, 198–205). Comparison of the nervous system in enteropneust tornariae and echinoderm larvae has revealed however, striking differences in distribution of biogenic amines and cholinesterase activity. In tornariae, monoamine-containing cells concentrate in the aboral and oesophageal ganglia. In echinoderms, they are located along the ciliary bands throughout their length. The difference in distribution of cholinesterase activity in each group reasonably suggests that acetylcholine-dependent control of locomotion also differs. Our data do not support the homology of the dipleurula-type larvae. Therefore we believe in the course of adaptive evolution, larvae of certain marine invertebrates acquired a set of common morphological and behavioural characteristics, yet retained different physiological mechanisms of behavioural regulation. Thus, similarities in the dipleurula-type larvae (tornaria, auricularia or bipinnaria, and actinotrocha) may have originated from convergence rather then from a common dipleurula-type predecessor. In consequence we must call into question any attempt to trace the ancestors of Chordata to the dipleurula-type animal.