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NATURE AND AFFINITIES OF THE SPONGES. 193extent, the developmental phenomena of the Spongida are productive ofcomposite structures, the swarm-gemmules or so-called ciliated larvae,which bear the closest superficial resemblance to the segmented ovum andprimary form and disposition of the component blastomeres of the Metazoicembryo. Penetrating beneath this superficial likeness, however, the pointsof analogy are found to diverge and vanish altogether. No Metazoic embryo,and no Metazoic structure, whatever, is distinguished by the possessionof collared flagellate cells, with their attendant properties and functions,as found to exist respectively in these ciliated reproductive bodies, in theessential monadiform constituents of the adult sponge, and amongthe independentDiscostomatous Flagellata. It is this one important histologicelement, the collared cell with its attendant physiologic functions, thatso closely unites together the two sections of the Spongida and independentDiscostomata or Choano-Flagellata, while it isolates them at the sametime from the members of every other organic group. Were the interstitialcanals and chambers of the sponge-stock lined with cells possessing nocontractile vesicles, bearing simply flagella, or corresponding with ordinaryciliated epithelium, and yet capable of ingesting solid food-matter, thegrounds for removing this organic section into the Metazoic series wouldapparently be based on a surer foundation and some analogy would bepresented with the simpler Hydrozoa and many Turbellaria, where, asshown by Kolliker and Kleinenberg, the endoderm cells lining the alimentarycanal develop long flagella or pseudopodic processes, and itwould appear, engulf food-substances after the manner of Amoeba.Even here, however, the hypothetical analogy would be entirely delusive,the matured sponge-stock being the sum total, not of the concourseof sexual elements, but of the essentially Protozoic process of sporedevelopment.In the fashioning of the motile ciliated gemmules, or pseud-embryos,upon the plan of the Metazoic morula and amphiblastula, and in thepeculiar arrangement and separation of the constituent flagellate andamoeboid factors of the adult sponge, nature would certainly seem to havemarshalled her forces preparatory to crossing the border from the Protozoato the Metazoa, and so far as a transitional group between the two seriescan be predicated,it isprobably realized in the section of the Spongida.The step, as a complete one, however, is by no means accomplished in thisgroup. As is at once made manifest by a closer insight, the spongesremain in every detail of structure, function, and development, typical andthoroughgoing Protozoa. Their position in, and affinitiesamong, theseveral groups of this sub-kingdom are evidently close to, and inseparablefrom, that of the naked and independent Discostomata or Choano-Flagellata,and which, having due regard to the clearly defined laws of organicevolution, must be recognized as the ancestral progenitors or archetypes ofall sponge-forms. The phylogeny or backward passage, again, from theseindependent collared types to the simpler monadiform Flagellata,is madeO