Development of the protonephridia of Austramphilina elongata

Abstract

In embryos of Austramphilina elongata, ciliary tufts (flames) of three terminal cells are enclosed by cytoplasmic cylinders of a single, proximal canal cell. The cylinders are joined to form a single capillary, connected to the capillary of the distal canal cell by a desmosome. Internal outgrowths of the terminal cells grow into the space between the cylinders and the outermost cilia. During development, more cilia are formed, the parts of the cylinders closest to the terminal cells bulge out to accommodate the additional cilia, and at points of contact between the inner outgrowths and the cylinders external ribs are formed, connected to the internal ribs formed from the inner outgrowths by means of thin, cytoplasmic sheaths (“membranes”). During development and in fully developed protonephridia, the cytoplasmic cylinders are connected to the terminal cells by desmosomes but do not fuse with them.