Phylum Porifera Grant, 1836

Abstract

Phylum Porifera (sponges) are metazoans united by the unique possession of choanocyte chambers, a system of afferent and efferent canals with external pores, lacking a tissue grade of construction but having a^. highly mobile population of cells capable of totipotency, and possessing siliceous or calcitic spicules in many (but not all) species. Four sponge classes are currently recognised, although the monophyly (and indeed the existence of the ‘Phylum Porifera’) has been challenged by recent molecular data and chemical evidence. Three classes have Recent species: Hexactinellida (syncytial choanoderm, discrete cells and pinacoderm, siliceous triaxone spicules, ubiquitous fibrillar collagen, mostly deeper water), Demospongiae (discrete cells, siliceous monaxone or tetraxone spicules, ubiquitous fibrillar collagen), and Calcarea (discrete cells, calcareous spicules, ubiquitous fibrillar collagen). Archaeocyatha was an important group of sessile marine organisms during the Cambrian but is now presumed extinct. It was characterized in having double-walled inverted conular growth forms with spaces between outer and inner walls filled by various skeletal structures, overall architecture of interlocking polyhedral microgranular calcite, and lacking free spicules. Within this systematics there is scope for recognition of subphyla (e.g., Demospongiae + Calcarea vs. Hexactinellida, based on cytological evidence; or Demospongiae + Hexactinellida vs. Calcarea, based on some molecular data), but this level is not applied here as the debate is still embryonic and there are still conflicting molecular data. Fossil sponges are not always easy to classify with respect to Recent taxa, with some fossilised groups such as the ‘lithistids’ and ‘sphinctozoans’ distributed within a Recent classification, but with others treated independently. Keys to the classes of Recent and fossil sponges are constructed independently to avoid presumptions of alleged phylogenetic affinities. This work describes 682 valid genera of the extant fauna (of about 1600 nominal genera) in three classes, 25 orders and 127 families; and mentions over 1000 ‘valid’ genera amongst the fossil fauna in six ‘classes’, 30 orders and 245 families (although the fossil demosponge fauna is not substantially delineated in this work). Fossil ‘Classes’ Sphinctozoa and Stromatoporoidea are clearly polyphyletic and represent grades of construction and not phylogenetic clades.