New insights into the functional morphology of the male copulatory apparatus of bullid gastropods

In this study, semi-thin sections stained with histochemical techniques and transmission electron microscopy were used to obtain new data about the morphology and function of the male copulatory apparatus of the cephalaspidean gastropod Bulla striata. The apparatus comprises a vestibule, a penial papilla and a prostate consisting of a coiled unbranched tube ending in a blind caecum. The penial papilla and the coiled tubular prostate are enclosed by a muscular sheath, which is continuous with the muscular tissue of the vestibule. The epithelium lining the lumen of the vestibule is formed by ciliated and mucus-secreting cells. Two new types of subepithelial secretory cells were discovered in this region. The penial papilla is a muscular structure without secretory cells in the epithelium lining the narrow lumen. The tubule that constitutes the prostate possesses a muscular wall and can be divided in three distinct regions: a non-secretory duct connected to the penial papilla, a glandular region rich in large secretory cells and the terminal caecum containing just a few small secretory cells. In the terminal blind caecum, the muscular sheath is fused with the muscular wall of the tubular prostate. Large numbers of spermatozoa were found in the glandular region and in the terminal caecum of the prostate. A new functional mechanism is proposed to explain penial eversion during copulation. This differs from a previous hypothesis in two main aspects: (1) existence of a permanent penial papilla in mature animals acting as a functional penis and (2) functional role of vestibule during copulation, which everts and surrounds the penial papilla, while the latter protrudes outwards.