Três espécies de Corbicula registradas para o lago Guaíba são comparadas quanto à morfologia externa das partes moles e aspectos relativos à incubação branquial. Observam-se diferenças específicas nos sifões e quanto à incubação branquial: espécimes de C. largillierti apresentam incubação somente nas demibrânquias internas; registra-se pela primeira vez espécimes de C. fluminea com incubação nas demibrânquias externas; e espécimes de C. af. fluminalis apresentam incubação predominantemente tetragenous (i. e. em ambas demibrânquias).

Three species of Corbicula identified in Guaíba Lake are compared in respect to the external morphology of their soft parts and aspects of branchial incubation. Morphological differences are most obvious in the siphons. For the first time, individuals of C. fluminea have been found with incubation occurring in the outer as well as the inner demibranchs. Individuals of C. largillierti showed incubation only in the inner demibranchs. In specimens identified as C. aff. fluminalis, tetrabranchial incubation predominated.

Propõe-se a transferência do gênero monitípico Parodizia Medina, 1959 de Bithyniidae (Prosobranchia) para Pyramidellidae (Heterobranchia). A proposta está baseada em dados morfológicos de espécimens de P. uruguayensis Medina, 1959 procedentes da bacia do rio Tramandaí, Rio Grande do Sul, Brasil.

The transference of the monotypic genus Parodizia Medina, 1959 from Bithyniidae (Prosobranchia) to Pyramidellidae (Heterobranchia) is proposed. The proposal is based on some morphological data of P. uruguayensis Medina, 1959 from Tramandaí river basin, Rio Grande do Sul, Brazil.

Heleobia robusta, new species from plain coast of Rio Grande do Sul, Brazil, is described. Conchology, conchometry, soft-part morphology including head-food, pallial structures, radula and some aspects of the female and male reproductive systems are provided.

Potamolithus catharinae Pilsbry, 1911 is characterized on specimens from Hercílio river, State of Santa Catarina, Brazil, type-locality. Conchology, conchometry, soft-part morphology including head and food complex, pallial structures, radula and some aspects of the female and male reproductive systems are provided.

The reproductive system of Gundlachia concentrica (Orbigny, 1835) and Gundlachia moricandi (Orbigny, 1837) are described. The following structures distinguished the specimens of those species: ovotestis, seminal vesicle, tubular prolongation of dorsoposterior region of uterus and duet of the copulation pouch.

This paper registers 16 species of aquatic molluscs, especially the limnic and estuarine species Heleobia davisi Silva & Thomé, 1985, Heleobia australis australis (Orbigny, 1835) and Erodona mactroides Bosc, 1802, from D'Una river and Mirim, Imaruí and Santo Antônio lagoons and Laguna Beach (Santa Catarina, Brazil). This parameter was very important for the paleoecological analysis conduced for that area, where oscillation of the sea level occurred during the Holocen Period. Heleobia davisi is registered for the first time to the Santa Catarina.

São descritas e ilustrradas rédia, cercária imatura e cercária madura e metacercária de um paranfistomídeo, provavelmente do gênero Paramphistomum (Fischoeder, 1901). Estas formas evolutivas foram obtidas de um exemplar de Drepanotrema kermatoides (Orbigny, 1835) coletado, em maio de 1983, no arroio Chico Lumã (29º56'03"S e 50º35'59"W), município de Santo Antônio da Patrulha, Rio Grande do Sul, Brasil.

Morphological descriptions and illustrations of the redia, the imature cercaria and mature cercaria, and encysted metacercaria, found in a infected specimen of Drepanotrema kermatoides (Orbigny, 1835) are presented. The infected mollusk was found in Chico Lumã creek (29º56'03"S and 50º35'59"W), Santo Antônio County, Rio Grande do Sul State, in May of 1983. These larval stages probably are part of a cycle of a rumen parasite of the genus Paramphistomum (Fischoeder, 1901).