Abstract
Each animal phylum has its own bauplan. The phylotypic stage is the ontogenetic stage during which the phylum level characteristics appear. This stage refers to different stages of development in different animals. Sponges are one of the simplest, and probably the oldest multicellular lineage of extant animals. On the basis of the analysis of sponge development during (i) sexual and asexual reproduction, (ii) regeneration from small body fragments, and (iii) cell reaggregation, we suggest a hypothetical variant of their phylotypic stage (spongotype): the mono-oscular juvenile—the rhagon. The major feature, which permits to consider the rhagon as the phylotypic stage of the Porifera is the final, definitive position of all the cellular and anatomical elements of the future adult sponge. It seems that at the rhagon stage the pattern of the axial complex of anlagen is already formed, and only growth processes occur at the later stages.
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