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156 NATURE AND AFFINITIES OF THE SPONGES.paraded with so much tfclat, when put to the test was, to use a familiarexpression, found incapable of " holding water " it now remains to be;seen whether Professor Haeckel's new arguments were based upon amore firm foundation. The one essential point brought forward on thisoccasion relates to the composition and significance of those ciliated andmotile reproductive bodies common to all sponges, first discovered by Dr.Grant, and noticed by various subsequent observers, but whose true structureand import have not yet been exhaustively investigated. In accordancewith Professor Haeckel's interpretations, these bodies, or " ciliatedlarvae," as they have since been more commonly designated, abundantlydeveloped throughout the representatives of the Calcispongiae, were allreferable to one common plan, with regard both to external configurationand internal histologic composition. This common plan, as now enunciated,manifested itself externally in the possession of an evenly ovateor subpyriform contour, the broader end representing the anterior pole,as exhibited by the body in its condition of natation. Except at onepoint, the entire peripheral surface of these bodies was clothed with longvibratile cilia, each cilium originating from the centre of a minutely circumscribedpolygonal area, to each of which was assigned the morphologic valueof a single cell. The exceptional region referred to, over which the cilia didnot extend, was limited to the anterior pole, from which point an axialcanal was described as leading from the external surface to a central bodycavity.Round the outer edge of this apical opening were stationed a circularborder of larger subspheroidal non-ciliated cells, which represented theexternally protruding units of a layer of similar cells that lined in a singleand continuous series the entire surface of the hollow internal cavity withwhich the apical aperture was continuous. Taken thus in optical longitudinalsection, these bodies, as interpreted by Professor Haeckel, orborrowing from his own illustrations, as represented at Fig. 2 in theadjoining woodcut, presented the aspect of an ovate sac composed of twoseparate, and histologically distinct, external and internal cellular layers,the outer one being composed of more minute subcylindrical and radiallydisposed monociliate cells, and the inner one of a correspondingly simplelayer of much larger subspheroidal and non-ciliated cells. This sac-shapedbilaminated structure Professor Haeckel denominated a "gastrula," andrepresented it to be the ground or stock-form from which all spongeswere primarily developed. It was further insisted that the outer or socalled" dermal lamina " of this bilaminated structure represented a trueexternal dermal layer or ectoderm, and the inner or so-called "gastrallamina" a true entoderm, as obtains in all ordinary Metazoic organisms.Launching out into the regions of hypothesis, Professor Haeckel claimedfor his so-called " gastrula " a far-reaching and most important significance." I regard the gastrula," Haeckel says, " as the most important and significantembryonic form in the whole animal kingdom. It occurs among the sponges, theAcalephae, the Annelida, Echinodermata, Arthropoda, Mollusca, and the Vertebrata