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

¹ Friday Harbor Laboratories and Department of Zoology, University of Washington, 620 University Road, Friday Harbor, Washington 98250

An extensive gelatinous material occupies the primary body cavity of larval echinoderms (auricularia, 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 structures that occur in all of these five larval forms: (1) concave body surfaces bounded by thin epithelia and (2) muscles unopposed by other muscles. A gel filled primary body cavity invalidates deductions of morphogenetic mechanisms that assume a fluid filled cavity, an assumption implicit in many models of blastulation, gastrulation, 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 of large larvae with little cellular material. The taxonomic distribution of geb filled body cavities is not known, but gel filled cavities are possible wherever fluid motion has not been demonstrated or is not a functional necessity.

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