SESSILE LIFE-STYLE AND ORIGIN OF CHORDATES

Contemporary sessile animal groups evidently have evolved gradually fr om sluggish benthic ancestors, resembling holothuroids such as Sphaero thuria and the Psolidae in habit. A dorsal flexure of the digestive tr act with dorsally oriented mouth and anus is characteristic. The moder n near-sessile holothuroids are probably approaching a second permanen tly attached stage in phylogeny. The U-bend of the digestive tubes of echinoderm larvae might represent a dorsal flexure reminiscent of a se ssile ancestry; the gut curvature of the entoproct larva also is presu med to be dorsal. The dorsal position of the chordate nerve cord may b e an inheritance from a near-sessile ancestor with dorsal particle-fee ding arms, and its tubular structure evidently reflects the protective inrolling of a primitive intra-epidermal nerve concentration. Early v ertebrates probably originated from a feeding neotenic tadpole form of an ascidian-like predecessor; the neurenteric canal of vertebrate emb ryology may be a reminiscence of the dorsally located anus of a sessil e ancestor.