Gastropod gill morphology i 
The lack of a skeletal rod in the bipectinate gills of the acmaeid limpet P. mimula and the neritid 
N. chameleon may be considered a primitive condition (Haszprunar, 1988). The gill structure of 
these representatives of the clades Patellogastropoda and Neritimorpha is remarkably similar. The 
gill of P. mimula is more distinctly triangular in shape in comparison to the gill of N. chameleon, but 
in both species the gill is positioned in the centre of the mantle cavity, the gill filaments lie parallel 
to each other and the linkage of the left and right filaments to the base is alternating. 
The gills of the caenogastropod species P. sulcatus, L. articulata, B. auratum and M. marginalba 
have a basically uniform composition, although species specific variation occurs. Each gill filament 
consists of a clearly defined (latero-)dorsal ridge, which extends as a thin sheet of tissue across the 
mantle cavity. The shape of the filaments shows species specific differences. The gill filaments of the 
closely related littorinids L. articulata and B. auratum both appear triangular in shape, although the 
gill filaments of L. articulata are very much more elongated than those of B. auratum. The gill 
filaments of the muricid M. marginalba are slightly curved, giving the gills a quite distinct 
appearance. The gill filaments of the planaxid P. sulcatus are recognizable by the corrugated nature 
of the dorsal tidge, which may be a modification to enlarge the total gill surface area and improve 
respiratory gas exchange. 
The gill filaments of all investigated species possessed areas of ciliated alternating with areas of 
non-ciliated cells. In general it can be stated that the main gill margin of all investigated species 
consisted of ciliated cells. In cases where the gill filaments possess a clearly defined ridge, as in the 
caenogastropods P. sulcatus, L. articulata and B. auratum and M. marginalba, both the dorsal and 
ventral margins of the ridge consist of ciliated cells. If present, the enclosed area of the ridge has a 
wavy appearance and is non-ciliated. The dorsal ridge of M. marginalba differs from this general 
pattern by not having a central. area of non-ciliated cells. In this species the outer margin of the ridge 
18 Covered with long cilia, bordered by an area of cells that bear shorter cilia. The cilia were of the 
ordinary straight type with sharp terminals in all species except the acmaeid limpet P. mimula, 
where each cilium had a paddle shaped terminal. Similar cilia have been observed only in the 
secondary gills of the siphonariid limpet S. capensis (De Villiers and Hodgson, 1987), but not in the 
common patellid limpet P. vulgata (Nuwayhid et al., 1978). Paddle shaped cilia have been reported 
to occur in molluscan osphradia, where they perform a chemoreceptive function (Haszprunar, 1985). 
However, De Villiers and Hodgson (1987) were unable to detect any nerve connections to these cilia 
in the pallial gills of S. capensis suggesting that their only function is as water paddles, as originally 
Proposed by Yonge (1952). It has been demonstrated in P. vulgata, that movement of cilia creates a 
water current in the opposite direction to the haemolymph flow through the gill filaments, thus 
establishing a counter current mechanism for respiratory gas exchange (Nuwayhid et al., 1978). The 
flattened tips of the cilia of P. miniula would be expected to magnify the strength of the water 
current, increasing the efficiency of the cilia as water paddles. Ciliary movement also takes part in 
distributing mucous secretions, that serve to capture foreign particles and remove them from the 
gills. 
Internal structure: In terms of internal structure, gastropod gills are less disparate than their external 
ultrastructure suggests. Light microscopical sections showed that gill filaments of all species studied have 
à standard internal architecture. Whether gills were mono- or bipectinate, individual filaments showed 
basic similarities. All filaments were positioned parallel to each other and were linked by acommon base, 
through which the haemolymph is directed and distributed to the individual filaments for respiratory gas 
exchange. Each filament consists of a unicellular epithelium, presumably to enhance a rapid gas 
exchange, and is supported by connective tissue which partly encloses the haemocoelic space. When 
comparing the ultrastructure of the gill epithelium of A. constricta with data from literature one should 
realise that most studies dealt with species that possess secondary gills. A single layered epithelium, 
Consisting of either columnar or cuboidal cells, has been found in most species studied so far: S. capensis 
(De Villiers and Hodgson, 1987), P. vulgata (Nuwayhid et al., 1978), the nudibranchs Archidoris 
pseudoargus апа Peltodoris atromaculata (Jonas, 1986), the freshwater bivalve Anodonta woodiana 
lauta (Nakao, 1975). However, there appears to be a difference in the thickness of the epithelial cells, 
