6 R.H.M. Eertman 
Results 
Austrocochlea constricta (Vetigastropoda, Trochidae) 
External gill structure: The trochid A. constricta has a single bipectinate gill, positioned on the left 
side of the mantle cavity. The anterior third of the gill is supported by a collagenous rod, which 
attaches the gill to the mantle wall. The filaments of the outer row are shorter (1.1 + 0.1 mm) than 
those of the inner row (3.2 + 0.1 mm). Towards the anterior and posterior ends of the gill, the 
filaments gradually decrease in length (Plate 1A). Each gill filament is blade-shaped, and its dorsal 
margin covered with long cilia (Plate 1B + ІС). A row of nodules occurs on both sides of individual 
gill filaments, each nodule being covered with long cilia, with the exception of the basal margin 
(Plate 1B, ІС + 1E). Alternating rows of ciliated and non-ciliated cells give each filament a striped 
appearance. The rows of ciliated cells, which are usually 3 to 4 cell-layers wide, extend from the base 
of a nodule across the filament towards the outer margin (Plate 1C). These cilia, which are shorter 
than those covering the nodules, appear in small tufts (Plate 1D). 
Internal gill structure: Transverse sections through gill filaments reveal a single layered epithelium, with 
columnar epithelial cells that аге 13.8 + 0.7 um long (20 cells measured). Each filament is supported 
axially by a double layer of collagenous connective tissue, enclosing the haemocoelic space (Plate 2A). 
This double layer of connective tissue thickens towards the anterior end of the filament where both layers 
join, forming the main haemolymph canal. The connective tissue layer surrounding the main haemolymph 
canal consists of an inner layer of collagen and an outer layer of proteinaceous prekeratin (Figure 1). 
Ciliated cells are noticeably more electron-dense than non-ciliated cells (Plate 2B). Each cilium is 
composed of a standard 9+2 axoneme, a basal body and associated ciliary rootlets (Plate 2B). All ciliated 
and non-ciliated cells exhibit microvilli on their apical surface. Although individual microvilli were not 
discernible under SEM, their presence is clearly demonstrated in TEM sections (Plates 1F and 2B). 
Microvilli measure 1.1 + 0.1 um in length (20 cells measured), and are surrounded by an electron-dense 
glycocalyx (Plate 1F). All epithelial cells, whether ciliated or non-ciliated, have a round to oval nucleus 
in the centre or basal half of the cell. Mitochondria seem to be concentrated in the apical half of each cell. 
Gap junctions between cells, containing septate desmosomes, are associated with membranes of 
adjoining epithelial cells and presumably serve as attachment zones (Plate 1F). 
Gill filament nodules are triangularly shaped in longitudinal sections and broadly attached to the 
filament (Plate 2C). Each nodule has a maximum width of about 40 uim and is approximately 33 um 
high. Each nodule has a coating of columnar epithelial cells of which the apical ones are ciliated. 
The function of these nodules remains unclear. Although some sensory function could perhaps be 
expected, no obvious sensory cells were observed inside the nodule. 
Two types of secretory cells could be distinguished in the gills of A. constricta: mucous secreting 
cells and secretory cells containing large electron-dense vesicles. The mucous secreting cells have a 
typical goblet cell appearance and are located amidst the epithelial cells (Plate 2D). The cells are 
oval with a narrow apical neck facing towards the surface of the gill filament. The cells, which react 
PAS-positively, are filled with mucus in one large vacuole. The mucus appears as finely granulated 
secretions of low electron density. The nucleus and cell organelles are confined to a narrow area near 
the lateral and basal areas of the plasma membrane. The second type of secretory cell is not restricted 
to a specific location in the gill. It can be found: (1) bordering the surface of the filament, (2) totally 
enclosed by epithelial cells (Plate 2E), or (3) inside the nodules that cover the gill filaments. This 
type of secretory cell shows many characteristics associated with cells that have high metabolic 
activity. Mitochondria with well developed cristae are present throughout the cell. A rough 
endoplasmic reticulum (RER) occupies most of the cytoplasm (Plate 2F), indicating a high level of 
protein synthesis. Secretory vesicles, budded from the Golgi cisternae, presumably fuse to form the 
larger vesicles. All secretory vesicles, irrespective of size, are clearly surrounded by a membrane. 
The function and structure of these secretions remain unknown, although the cells react negatively to 
PAS tests, indicating that mucopolysaccharides are not present. In cells that border the gill surface, 
release of secretory products by exocytosis could be observed. 
