The Roles of Motile Larvae and Fixed Adults in the Origin of the Vertebrates

Abstract

The eclectic theory herein developed sets forth the following observations: that notwithstanding wide diversification of adult forms among echinoderms into sessile, stalked, and slow-moving animals, all developed free-floating larvae, which scatter widely enough to insure favorable sites for a sufficient percentage of the adult reproductive phase. That among tunicates the larvae may have utilized the ancient stalk as a vibratile tail, but with a minimum of directive organs in the central nervous system. That Amphioxus, although possibly degenerate, represents a much higher stage than the echinoderms. In slow-moving ostracoderms such as Drepanaspis and Cephalaspis, larval locomotor stages are still unknown, but in the modern lampreys, which appear to be degenerate derivatives of the ostracoderms, the locomotive larval stages are truly jawless and retain resemblances to the orobranchial system of Cephalaspis, while the nasopituitary sac shows how, by great increase in size of the "upper lip" behind the pituitary sac, the latter became displaced to the top of the skull. Thus both fixed adults and motile larvae may have played important roles in the origin of the vertebrates from such attached enteroblastic forms as the earliest carpoid echinoderms. The metameric locomotor system was developed in the motile larvae before the fixed stage was eliminated by neotony.