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An epithelium is important for integrity, homeostasis, communication and co-ordination, and its development must have been a fundamental step in the evolution of modern metazoan body plans. Sponges are metazoans that are often said to lack a true epithelium. We assess the properties of epithelia, and review the history of studies on sponge epithelia, focusing on their homology to bilaterian epithelia, their ultrastructure, and on their ability to seal. Electron micrographs show that adherens-type junctions are present in sponges but they can appear much slighter than equivalent junctions in other metazoans. Fine septae are seen in junctions of all sponge groups, but distinct septate junctions are only known from Calcarea. Similarly, all sponges can have collagenous sheets underlying their epithelia, but only homoscleromorphs are established to have a distinct basal lamina. The presence of most, but not all, gene families known to be involved in epithelial development and function also suggests that sponge epithelia function like, and are homologous to, bilaterian epithelia. However, physiological evidence that sponge epithelia regulate their internal environment is so far lacking. Given that up to six differentiated epithelia can be recognized in sponges, distinct physiological roles are expected. Recognition that sponges have epithelia challenges the perception that sponges are only loose associations of cells, and helps to relate the biology and physiology of the body plan of the adult sponge to the biology of other metazoans. [ABSTRACT FROM AUTHOR]

Cnidaria have traditionally been viewed as the most basal animals with complex, organ-like multicellular structures dedicated to sensory perception. However, sponges also have a surprising range of the genes required for sensory and neural functions in Bilateria. Here, we: (1) discuss "sense organ" regulatory genes, including; sine oculis, Brain 3, and eyes absent, that are expressed in cnidarian sense organs; (2) assess the sensory features of the planula, polyp, and medusa life-history stages of Cnidaria; and (3) discuss physiological and molecular data that suggest sensory and "neural" processes in sponges. We then develop arguments explaining the shared aspects of developmental regulation across sense organs and between sense organs and other structures. We focus on explanations involving divergent evolution from a common ancestral condition. In Bilateria, distinct sense-organ types share components of developmental-gene regulation. These regulators are also present in basal metazoans, suggesting evolution of multiple bilaterian organs from fewer antecedent sensory structures in a metazoan ancestor. More broadly, we hypothesize that developmental genetic similarities between sense organs and appendages may reflect descent from closely associated structures, or a composite organ, in the common ancestor of Cnidaria and Bilateria, and we argue that such similarities between bilaterian sense organs and kidneys may derive from a multifunctional aggregations of choanocyte-like cells in a metazoan ancestor. We hope these speculative arguments presented here will stimulate further discussion of these and related questions. [ABSTRACT FROM AUTHOR]

Presents information on the symposium "Sponges: New Views of Old Animals," which highlighted diverse aspects of poriferan biology. Description of sponges; Relevance of sponges to studies of animal evolution; Importance of the study of sponge embryogenesis to a person's understanding of early animals; Overview of the ecology of sponges that border Antarctica.