16 R.H.M. Eertman 
6D). The widened anterior parts of the filaments, representing the rounded edges (Plate 6C), form 
the major haemolymph canals. Mucous cells, closely resembling goblet cells, can be observed in the 
anterior part of the filaments (Plate 6E). 
Patelloida mimula (Patellogastropoda, Acmaeidae) 
The gill of the acmaeid limpet P. mimula is triangularly shaped, pointing anteriorly (Plate 7A). It 
is positioned in the centre of the mantle cavity, extending forward to the mantle margin and attached 
to the mantle by a thin membrane. The individual filaments lie parallel to each other, are uniform in 
shape and increase in size posteriorly. Large areas of the filaments are covered by cilia, which differ 
from the ordinary type by the presence of terminal paddles (Plate 7B + D). The paddles appear to 
consist of a solid outer margin covered on one side by a thin membrane. Ciliated and non-ciliated 
cells are not organized in any recognizable pattern (Plate 7C). Large uninterrupted areas of ciliated 
cells are alternated with areas that consist largely of non-ciliated cells, but that include numerous 
small tufts of ciliated cells. Light microscopic sections show that the gill of P. mimula is not 
monopectinate, as it appears to be in SEM preparations, but bipectinate. All filaments join in the 
centre, although in an alternating rather than strictly paired fashion (Plate 7E + F). In the centre of 
each filament the haemocoelic space is visible. No secretory cells were observed. 
Nerita chameleon (Neritimorpha, Neritidae) 
The neritamorph М. chameleon has an elongated triangular shaped gill, positioned slightly left of 
centre in the mantle cavity and pointing anteriorly (Plate 8A). The gill is attached to the mantle by a 
thin membrane. All filaments lie parallel to each other and are corrugated (Plate 8B). The dorsal 
margin and lateral surfaces of each filament are covered with cilia. However, cilia associated with 
the lateral surfaces are longer, and orientated differently from those of the dorsal margin (Plate 8С). 
Light microscopic sections show that N. chameleon has a bipectinate gill, whose filaments are linked 
to а central base. The linkage is unpaired and the left and right filaments alternate. A haemocoelic 
space can be seen in the centre of each filament and towards the anterior end goblet type mucous 
cells were observed. 
Discussion 
External ultrastructure: The present study demonstrates pronounced differences in ctenidial 
morphology among the investigated gastropod species from Moreton Bay. The results show that 
species belonging to the clades Patellogastropoda (P. mimula), Neritimorpha (N. chameleon) and 
Vetigastropoda (A. constricta) possess bipectinate gills, whereas species belonging to the more 
advanced Caenogastropoda (L. articulata, B. auratum, P. sulcatus and M. marginalba) have 
monopectinate gills. The gills of Vetigastropoda, a group to which the trochid A. constricta belongs, 
are characterized by the presence of a skeletal rod, which has a supportive function. In A. constricta 
the gill filaments of the outer row are much shorter (1.1 + 0.1 mm) than those of the inner row (3.2 + 
0.1 mm). This modification, which was observed also in other Trochoidea species and in the gills of 
the related vetigastropod family Lepetodriloidea, is considered to be a transitional condition from a 
bipectinate to a monopectinate gill structure (Haszprunar, 1988). Gill filaments covered with rows of 
nodules and regular patterns of ciliated and non-ciliated cells, as observed in A. constricta, have not 
been reported in ctenidia of other species. Randomly distributed clusters of cilia were observed on the 
lamellae of the secondary gills of the limpet P. vulgata (Nuwayhid et al., 1978) and the pulmonate 
limpet Siphonaria capensis (De Villiers and Hodgson, 1987). However, from a taxonomic — 
phylogenetic perspective ctenidial structures cannot be compared to features of secondary gills, as 
these two types of gills are not homologous. Although there are three ciliated areas on the gill 
filaments of A. constricta (margin — nodules — striped pattern of ciliated cells), it is unclear as to 
what extent they function to generate water currents, disperse mucous and other gill secretions, or 
aid in cleansing of the gill filaments. 
