ULTRASTRUCTURE OF THE PSEUDO-PROTONEPHRIDIUM OF THE ENIGMATIC OPISTHOBRANCH, RHODOPE TRANSTROSA (GASTROPODA, NUDIBRANCHIA)

The fine structure of the excretory organ of adult Rhodope transtrosa Salvini-Plawen, 1991 has been investigated by means of transmission el ectron microscopy in order to specify its protonephridial-like appeara nce in previous light microscopic descriptions. The studies revealed a unique fine-structure with a specialized filtering basement membrane and a ciliary flame, but lacking a filtration weir, which is otherwise typical for protonephridial terminal cells. This distinct ultramorpho logy of the unpaired excretory organ is considered as one oi the many autapomorphies of Rhodope, which has secondarily lost its original sit e of ultrafiltration in the pericard. The pseudo-protonephridial featu res of Rhodope once more provide evidence for its non-turbellarian nat ure and confirm ideas of the relevance of body cavity conditions with respect to the ultrastructure of the metazoan excretory system.