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( 143 )CHAPTER V.NATURE AND AFFINITIES OF THE SPONGES.IT isproposed to devote the present chapter to an extensive discussion ofthat near relationship of the sponges to certain of the Infusoria Flagellata;briefly referred to on several occasions in the course of the precedingpages. This affinity is found, indeed, upon an impartial examination ofthe data here collected, to be so comprehensive and thoroughgoing as torender absolutely unavoidable the correlation of this group with the typicalrepresentatives of the flagellate Protozoa. Those differences which doexist between the two groups are, in point of fact, far less essential thanthose which obtain between many of the subordinate sections of the ordinaryCiliate and Flagellate Infusoria ;such being the case, the present workcould not be considered completeif it did not embrace a more or lessextensive account of the fundamental plan of organization, at least, of theSpongida. Strictly speaking, the sponges, throughoutall their wealth ofform and organization, are here accepted as Mastigophorous Protozoa, andit is on account only of the limited space at disposal, that their full specificenumeration and description, on a scale corresponding with that allotted tothe more typical representatives of the Flagellate Infusoria, is here omitted.Under existing circumstances, it is found possible to submit a brief sketchonly of those broad fundamental characters which either unite with, ordistinguish the members of the sponge-tribe from their nearest allies,supplementing them with the author's personal interpretation of thosesomewhat obscure structural and developmental points which have beenheld by other authorities to indicate an affinityin a different direction.For, although it is confidently anticipated that the evidence now broughtforward must materially assist in securing to the sponges, eventually, ageneral recognition of their intimate relationship to the Choanophoroussection of the Flagellate Infusoria, it can by no means be said that such anaffinity is at the present date universally recognized. On the contrary,the balance of contemporaneous scientific opinion favours the relegationof this organic group to the division of the Metazoa, though upon groundswhich, plausible as they seem to be upon the surface, are fundamentallypurely artificial and untrustworthy.In order to arrive at a position permitting a thoroughgoing andimpartial appreciation of the very voluminous and conflicting evidencethat has been amassed with reference to the much debated affinities of