Studies on Hexactinellid Sponges. III. The Taxonomic Status of Hexactinellida Within the Porifera

Recent findings on the structure and physiology of the enigmatic hexactinellid sponges challenge the conventional definition and classification scheme of the Porifera. The gradual acceptance of the choanocyte element as a diagnostic morphological character of the phylum must be revised in the light of the syncytial (acellular) and enucleate condition of the hexactinellid flagellated chambers. The contrasts between the hexactinellids and all other sponges are reviewed and considered sufficient to require recognition of a primary subdivision at the subphylum level. The major differences include, respectively, syncytial (or symplastic) as against cellular organization, acellular as against cellular (pinacocytic) epithelia, thin as against abundant mesohyle matrix, absence as against presence of contractility, and presence as against absence of a relatively rapid conduction system. The new findings support Reid's 1958 allocation of classes in his subdivision of the phylum, but the names that he employed, Nuda and Gelatinosa, are considered inappropriate for present use because of the hypothetical basis of their origin and of the lack of descriptive value of the names in the present context (R. E. H. Reid, Palaeontogr. Soc. [Monogr.] 111, 1-46 (1958)). Two new subphyla are proposed and defined, the Symplasma to include the single class Hexactinellida, and the Cellularia to include all other extant sponges of the classes Calcarea, Demospongiae and Sclerospongiae.