Existence and Functions of a Gel Filled Primary Body Cavity in Development of Echinoderms and Hemichordates

Abstract

An extensive gelatinous material occupies the primary body cavity of larval echinoderms (auricu laria, bipinnaria, ophiopluteus, and echinopluteus) and hemichordates (tornaria). Its presence and its recovery of shape following application and release of force were demonstrated by dissection of larvae in a suspension of sumi ink. A gel in the primary body cavity explains struc tures that occur in all ofthese five larval forms: (1) con cave body surfaces bounded by thin epithelia and (2) muscles unopposed by other muscles. A gel filled pri mary body cavity invalidates deductions of morphoge netic mechanisms that assume a fluid filled cavity, an as sumption implicit in many models of blastulation, gas trulation, and movement of mesenchyme cells. A gelatinous primary body cavity permits body plans and morphogenetic processes not possible with a fluid filled cavity and permits development oflarge larvae with little cellular material. The taxonomic distribution ofgebfilled body cavities is not known, but gel filled cavities are pos sible wherever fluid motion has not been demonstrated or is not a functional necessity.