Gastropod gill morphology à 
Planaxis sulcatus (Caenogastropoda, Planaxidae) 
The caenogastropod P. sulcatus has a single monopectinate gill, positioned on the left side of the 
mantle cavity, attached to the mantle wall. The gill filaments extend from the anterior margin of the 
mantle deep into the mantle cavity. They are positioned parallel to each other and appear 
morphologically identical, although the size of the filaments gradually decreases posteriorly. Each 
filament has a narrow dorsal ridge, which extends ventrally as a thin sheet of tissue (Plate 3A). Each 
ridge is corrugated and covered at the margins with long cilia (Plate 3B). The central area of the 
ridge, consisting of non-ciliated cells, has a very smooth appearance and its surface is covered with 
microvilli (Plate 3C). The thin sheet of tissue extending from the dorsal ridge consists of non-ciliated 
cells and has a rougher appearance than the non-ciliated surface of the ridge (Plate 3A). Light 
microscopic sections confirm the parallel orientation of the gill filaments and show that the filaments 
are joined together at their base (Plate 3D). Haemocoelic spaces can be Observed inside individual 
filaments (Plate 3E). Connective tissue, present inside the filaments and along the base where 
filaments fuse, presumably provides structural support. No secretory cells could be observed (Plate 
3E). 
Littoraria articulata (Caenogastropoda, Littorinidae, Littorininae) 
In the caenogastropod L. articulata, a single monopectinate gill is present, positioned on the left 
side of the mantle cavity, and all gill filaments appear morphologically identical (Plate 4A). Each 
filament possesses a latero-dorsal ridge, which extends as a thin sheet of tissue across the mantle 
cavity. This gives each filament a somewhat triangular and elongated appearance (Plate 4B). The 
margins of the latero-dorsal ridge are covered with long cilia, while the cells of the elongated sheet 
аге non-ciliated (Plate 4B + C). Light microscopic sections show that all filaments are linked to a 
common base (Plate 4D). In each filament a haemocoelic space is visible, forming a wider, round 
canal anteriorly inside the latero-dorsal ridge. Goblet type mucous cells are present in the anterior 
part of the filaments (Plate 4E). 
Bembicium auratum (Caenogastropoda, Littorinidae, Lacuninae) 
The single monopectinate gill of the caenogastropod B. auratum is attached to the mantle wall on 
the left side of the mantle cavity and consists of triangular shaped filaments (Plate 5A). АП filaments 
appear uniform in shape, but from the anterior side onwards they gradually increase in size, and 
posteriorly a small reduction occurs. The dorsal surface of the filament is formed by a ridge showing 
numerous irregular folds (Plate 5C). The margins of the ridge consist of ciliated cells, surrounding a 
central area of non-ciliated cells, which has a rougher appearance than the corresponding area in P. 
sulcatus, The remaining part of the filament is formed by a thin sheet of tissue, composed of non- 
ciliated cells (Plate 5A + B). Light microscopic sections show that all filaments are joined together at 
the base, which is supported by connective tissue (Plate 5D). Each filament shows a haemocoelic 
Space and anteriorly a well developed group of mucous cells (Plate 5E). It can also be clearly seen 
that although the filaments appear uniform in shape, they differ substantially in size. 
Morula marginalba (Caenogastropoda, Muricidae) 
The caenogastropod M. marginalba has a single monopectinate gill, positioned on the left side of 
the mantle cavity. The anterior part of the gill is not aligned with the mantle edge, but was observed 
More posteriorly in the mantle cavity. The gill filaments lie parallel to each other and are slightly 
curved (Plate 6A + B). The rounded dorsal ridge of each filament, extending from the base to the 
apex, is covered with long cilia. Adjacent to the ridge, a second area of ciliated cells is present, 
although these cilia are shorter than the previous ones (Plate 6C). The remaining part of the filament 
IS formed by a sheet of tissue consisting of non-ciliated cells and has a rough appearance. Light 
microscopic sections reveal that the double layer of connective tissue present inside the gill filaments, 
not only encloses the haemocoelic space, but also extends into the base where filaments merge (Plate 
