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178 NATURE AND AFFINITIES OF THE SPONGES.sac-shaped structural type attributed by Haeckel to the so-called ciliatedembryos, the Haeckelian interpretation of the Mctazoic affinities of thesponges has exerted so widespread an influence, and obtained such favour,that every point has been strained on all sides to reduce these reproductivestructures, one way or another, to the Metazoic formula.Unfortunately for these authorities, however, the one dissentient partyto this seemingly plausible and, so far as the sponges are concerned, mosthonourable correlation is encountered in the very object of their solicitousattention. The ciliated sponge-embryo or, as shown later on, it may bemore appropriately denominated the " ciliated sponge-gemmule " stubbornlyresists interpretation as the exact analogue of any one of the variousembryonic types prevalent among the Metazoa, and seems indeed to derivea pleasurable satisfaction from the exhibition of a varying type of structure,possessing by turns some shadowy semblance of all, but actually conformingin no single instance to any one of them. Sometimes, in fact, not only inthe same order, or in the same family, but in the same genus, in the samespecies, and even in the same individual sponge-colony, an entire series ofdiversely constructed reproductive bodies may be met with.Abundant illustration is afforded of the more important variations ofform and structure found to obtain among the free-swimming spongegemmules,by the figures numbered 22 to 36 of PI. IX., derived partlyfrom the author's personal investigation, and partly from the publishedcontributions of the various authorities previously quoted. Examined attentively,it will become apparent that this entire series exhibits, with variousintermediate gradations, what may be denominated three fundamentalplans of structural differentiation. Thus, in the first of these, as shownin Figs. 22, 23, and 36, such plan presents the simplest possible expression,the so-called body-wall of the more or less ovate body consisting of asimple and even layer of columnar cells, each giving origin peripherallyto a single elongate cilium or flagellum. In the second series, Figs. 27and 29, an entirely distinct and more complex plan is exhibited. Here, thecellular components of the anterior and posterior regions of the gemmulediffer in both size and structure ;those of the former being columnar, andbearing flagella as in those of the first series, while in the latter theyare very much larger, usually more or less spheroidal, and entirely devoidof flagellate appendages. The third and highest state of complexity isarrived at in Fig. 30, where a new element issuperadded in the form ofa central zone of smaller rounded cells, interposed between the anteriorcolumnar and posterior spheroidal series. It isscarcely to be wonderedthat the energies of talented biologists have been taxed to their uttermostto reconcile such entirely diverse structures with the typical developmentalformulae of the Metazoic embryo. By none of these, as yet, can suchidentification be claimed to have been successfully established ; nor, onexamining more closely the very discordant interpretations that have beensuggested by different authorities with relation even to gemmules found