Meeting Abstract
Ciliary urns are cornucopia-shaped coelomic organs known in no other sea cucumbers (holothuroids) except those of the suborder Synaptina (order Apoda). The excretory ability of urns implicates their having a role in holothuroid immune response. Although ciliary urns are widely reported in the literature, the process and histology of urn development has not yet been described. Various forms of microscopy were used to examine the development and structure of urns of the viviparous holothuroid Synaptula hydriformis. Mature urns occurred rarely in 10-tentacled young and commonly in later growth stages, and developing urns were found in post-pentactulae, 10-tentacled young, and released juveniles. Urns developed as rounded protrusions of ciliated collar cells on the mesenteries. The cells increased in number to become pseudostratified with a shallow lumen, forming the cushion stage (5–15 μm diameter). The subsequent spoon shaped stage (15–25 μm width) had a short stalk, deepened lumen, and extensive ciliary field. Mature urns (24.5 μm body width, 9.5 μm stalk height, 29.7 μm body height) had a thin stalk and funnel shaped body with an abluminal epithelium of squamous cells and an adluminal epithelium of densely packed pseudostratified, columnar, ciliated collar cells lining the deep lumen. Urns resembling the cushion stage in S. hydriformis have been described in the sea star Archaster typicus. Considering the basal phylogenetic position of synaptines among holothuroids and the presence of cushion urns in A. typicus, ultrastructural and histological examination of other echinoderm groups should reveal if cushion urns have been overlooked and are a plesiomorphy of the phylum.
