The Preputial Organ of Snails in the Genus Helisoma (Gastropoda: Pulmonata)

The structure of the penis of members of the family Planorbidae has be:n considered by several workers. In this family, as well as in other families of the Basommatophora and the Stylommatophora, it has been used as one of the important genital structures in taxonomic studies largely because it reveals many generic and specific relations. In the classical work on the morphology of planorbids by Buchner (1891), the penis of members of the genera Planorbarius, Segmentina, Planorbis and Anisus, Bathyomphalus and Gyrautus were compared. Rao (1923) and also Baker (1933 and 1945) described the penis of Indoplanorbis in considerable detail. Soos (1935) dealt with the genitalia of Hungarian planorbids, particularly Anisus (Tropidiscus), Gyvaulus and Armiger and he, too, emphasized the i:rnportance of the penis in taxonomy. The presence of a "penial gland" in the North American genus Helisoma was first reported by Baker (1926) and he also found it later in several other genera of American, European, and Asiatic planorbids. This same investigator (1931) thought that the gland was in some way involved in the formation of the spermatophore and, consequently, called it the spermatophore gland. Later, Baker realized that this assertion was not in agreement with the structure of this organ. In 1945 he stated that the function of the "penial gland" "is insufficiently known but it seemed to him that the secretion of the gland might serve to dilate an opening through which the verge protrudes or that the gland might serve as an excitatory organ or sarcobelum. In this present study, special attention is given to the morphological and histological structure of the organ, as well as to the mating habits of the snails, in order to clarify how this gland functions. Helisoma trivolvis was used mainly for this work, but a comparison was also made with certain other members of this genus. Further studies on other genera will be published in a forthcoming paper.