Résumé
In three species of the Enterogona (Clavelina lepadiformis, Ciona intestinalis and Ascidiella aspersa) and three species of the Pleurogona (Dendrodoa grossularia, Styela clava and Molgula manhattensis) the testis was found to be invested by an epithelium separating the germ cells from the surrounding connective tissue or haemal sinuses. Each epithelial cell probably bears a single cilium, which in C. lepadiformis has a rootlet. Cilia are absent in S. clava. Lipid droplets are common and glycogen-rosettes occur in C. lepadiformis and D. grossularia. The basal plasmalemma varies from smooth to very irregular and in A. aspersa is anchored with hemidesmosomes. Except in S. clava, desmosome-like junctions occur between adjacent cells. Elimination of waste sperm following the reproductive season was observed to be undertaken by the epithelial wall cells in all species except C. lepadiformis. In C. intestinalis, D. grossularia, S. clava and M. manhattensis many of these cells detach and migrate to the interior of the testis where they continue and complete the phagocytosis of sperm. In C. lepadiformis, the non-germinal epithelium plays no role in the elimination of superfluous sperm which is probably phagocytosed, together with the rest of the body, by wandering trophocytes. Within the Urochordata the effectiveness of the testis epithelium as a blood-testis barrier varies, but is not correlated to modes of reproduction as postulated for other taxa.