FAHRENBACH, WOLF H; Oregon National Primate Research Center, Beaverton: Excretory organ of a pycnogonid.

Literature of the past century states that sea spiders (Pycnogonida), a group with unresolved arthropod affinities, lack an excretory system. I found a conventional excretory organ in Nymphopsis spinosissima (Ammotheidae). It consists of an end sac, a straight proximal tubule, a short distal tubule, and a raised nephropore, all in the scape of the chelifore. The end sac is a polygonal chamber, about 150 �m in cross section. Its wall is a basement membrane, 1-4 �m thick, facing the surrounding hemocoel, covered with a carpet of podocytes on the urinary surface. An ultrafiltration slit membrane crosses the gap between adjacent pedicels. The end sac attaches to the proximal tubule over a diameter of about 100 �m. In the center of this zone, the two constituent epithelia form a valve between them. The simple proximal tubule has a deep and irregular brush border on a thick, basally infolded epithelium. The proximal tubule opens through a constriction into the cuticle-lined distal tubule, a small chamber. The nephropore projects above the surrounding cuticle. This organ has astounding similarities to those of primitive crustaceans, specifically to the maxillary gland of Hutchinsoniella macracantha. One can compare the pycnogonid gland to the primitive antennary gland of crustaceans in view the location in the first appendage. Conversely, coxal glands of many chelicerates have substantial similarities with this organ. Thus, this gland might incite further morphological exploration of the pycnogonids and contribute to a resolution of their phylogenetic status. Supported in part by NIH Grant RR00163.