This study deals with appendicularians from z 1 superficial plankton samples collected in the coast of Argentina, Uruguay and Brazil in October-November, 1972 . Seventeen species were identified and their distribution established and compared with the results of previous authors . Megalocercus abyssorum received particular attention because it is a very infrequent species in the Atlantic Ocean. Specimens of the family Oikopleuridae were measured, taking data from trunk and tail . From the state of development of the gonads three different stages were established . The data from each station, grouped by species and state of maturity, were subjected to a conventional statistical treatment, obtaining the mean and standard error for each parameter. In addition, the use of statistical testing techniques validated the three developmental stages established, because from the analysis of variance for all the measurements highly significant results were obtained . An exception must be made for Oikopleura dioica since the only difference is in the height of the trunk . The same pertains to the difference between males and females . Oikopleura longicauda showed significant differences between specimens from coastal and pelagic samples in all measurements of the tail .

Until 1973, when Van Soest revised Thalia Blumenbach, 1798, this genus comprised only two species: Thalia democratica (Forsskål, 1775) and Thalia longicauda (Quoy & Gaimard, 1824). The former showed great variability, which led Van Soest to distinguish five species: T. democratica, T. orientalis, T. rhomboides, T. cicar, and T. sibogae. Some authors have ignored this revision considering the observed differences of infraspecific level. In the present work new material is studied analysing variables not previously considered, such as the development of blastogenic stolons in the solitary zooids and the location of the tunnel opening through which the bud blocks are liberated. These characters previously permitted a separation of closely related species in the genus Ihlea. The results show that stolon features, even though they permit a separation of solitary zooids of Thalia longicauda, do not discriminate all species of the “T. democratica group” since they are only useful in distinguishing T. democratica from T. sibogae. Other variables, mainly qualitative, do permit differentiation. The sympatric distribution of these taxa supports Van Soest’s hypothesis as well.