Anatomy and ultrastructure of the excretory system of a heart-bearing and a heart-less sacoglossan gastropod (Opisthobranchia, Sacoglossa)

Abstract 

The microanatomy and ultrastructure of the excretory system of the Sacoglossa have been investigated from two species by means of semithin serial sections, reconstructions and transmission electron microscopy. Whereas Bosellia mimetica shows a functional metanephridial system consisting of a heart with ventricle and auricle in a pericardium and a single kidney, Alderia modesta lacks heart and pericardium, possessing only several haemocoelic sinuses and a very long kidney. In B. mimetica podocytes as the site of ultrafiltration could be detected in the pericardial epithelium lining the auricular wall. The flat epithelium of the kidney with extensive basal infoldings and a dense microvillous border towards the luminal surface serves to modify the ultrafiltrate. In A. modesta podocytes are absent. Solitary rhogocytes (pore cells), the fine structure of which strongly resembles podocytes (meandering slits with diaphragms covered by extracellular matrix), occur in B. mimetica and A. modesta, representing additional loci of ultrafiltration. The presence of podocytes situated in the epicardial wall of the auricle is regarded as plesiomorphic for the Mollusca and confirmed for the Sacoglossa in this study, contradicting earlier assumptions of the loss of the primary site of ultrafiltration in the ancestors of the Opisthobranchia. In contrast to the likewise heart-less Rhodopidae with a pseudoprotonephridial ultrafiltration system, A. modesta shows no further modifications of the excretory system.