The hyaline layer (HL) around the embryos and larvae ofPatiriella calcaris examined by transmission electron microscopy.P. calcarhatches at the gastrula stage and develops through a lecithotrophic planktonic brachiolaria. The hyaline layer of unhatchedP. calcaris poorly developed and is comprised of wispy fibrils scattered among the epithelial microvilli. Fibrils are also occasionally seen associated with the inner surface of the fertilization envelope. By the hatched gastrula stage, the hyaline layer is organized into three strata: the intervillous layer, the supporting layer and the coarse outer meshwork layer. Seven-day-old brachiolaria also have a hyaline layer comprised of three strata. In these larvae the supporting layer elevates away from the epithelial surface due to the tuft-like organization of the underlying microvilli. This results in the formation of local outpockets giving the surface of the HL a lobed appearance. Bacteria are occassionally seen in the intervillous layer, particularly in association with the outpockets. These bacteria are phagocytosed by the epithelial cells and, in larvae that have bacteria, may play an augmentive role in larval nutrition. The structure of the hyaline layer ofP. calcaris compared with that of the hyaline layer of otherPatiriellaspecies to determine if it is more similar to the external coats around its planktonic (P. regularis) or benthic (P. exigua) developing congeners. The comparison shows that the hyaline layer ofP. calcaris virtually identical to that ofP. regularis, a similarity that may reflect the pelagic life histories of these species.