18 R.H.M. Eertman 
which ranges from 4 um for the cuboidal cells in the gill of S. capensis (De Villiers and Hodgson, 1987) 
to 10—50 um for the columnar cells in the gills of the nudibranchs A. pseudoargus and P. atromaculata 
(Jonas, 1986). The columnar cells in the gill epithelium ofA. constricta have an average thickness of 13.8 
um, which is similar to the thickness of the columnar cells in the bivalve Lucina costata (Giere, 1985). 
De Villiers and Hodgson (1987) suggested that there is a direct relationship between the thickness of 
epithelial cells and the degree of protection. Unprotected gills would be prone to desiccation and therefore 
would need to have thicker epithelial cells. This may be the case for some species, but the present results 
do not support this theory. Although the columnar cells in the gills of A. constricta could be classed as 
being moderately thick, the gill itself is located inside the mantle cavity and therefore is well protected 
from any desiccating influence. 
Mucous secreting cells were observed in all species except P. sulcatus and P. mimula. They were 
usually of the ordinary goblet type except in the littorinid B. auratum, where the mucous cells were 
larger and were grouped in the anterior region of the filament. The presence of mucous cells and 
other secretory cells in gill filaments is not uniform throughout molluscan species. In the gills of the 
common limpet P. vulgata (Nuwayhid et al., 1978) and the freshwater bivalve A. woodiana lauta 
(Nakao, 1975), mucous cells or other secretory cells were not observed. Jonas (1986), however, 
located three types of granular gland cells and an additional mucous cell in nudibranchs. The TEM 
study into the internal gill ultrastructure of A. constricta showed that the goblet type mucous cells 
contain one mucus-filled vacuole and, like the surrounding epithelial cells, possess microvilli. The 
structure of the mucous cells is comparable to those described previously. The second type of 
secretory cell observed in the gill of A. constricta has not been reported in any other molluscan gill. 
Large membrane bound secretions are present throughout the cell. Since this cell is found totally 
enclosed by other cells or bordering the filament surface, it is unclear whether the secretory products 
are secreted externally, or into the haemolymph or both. Further cytochemical study is certainly 
needed to clarify the functioning of this cell type. De Villiers and Hodgson (1987) suggested that the 
possession of mucous cells may be related to gill location. Exposed gills, as observed in the 
pulmonate S. capensis, in contrast to internal gills would not need mucous cells, as ciliary currents 
would be adequate to keep the lamellae clear of debris. The gills of all investigated species in this 
study are well protected inside the mantle cavity and the absence of mucous cells in P. sulcatus and 
P. mimula does not agree with Hodgson’s suggestion. The presence or absence of mucous cells does 
not seem to be related directly to the habitat of the investigated species, as mucous cells were 
observed in species inhabiting the shore at low to mid tidal level (A. constricta, M. marginalba) as 
well as in species inhabiting the shore at high tidal level (L. articulata) (Eertman & Hailstone, 
1988). 
Concluding, it can be said that apparent species specific differences in external gill ultrastructure 
do not imply functional differences between gills. Each species has developed its own gill 
morphology presumably to optimize respiratory gas exchange. Features that characterize gill 
Table 2. A summary of the features that characterize the ctenidia of the investigated gastropods. 
Species Clade Type of Uniform Clearly Skeletal го4/ ^ Shapeof Paddle Secretory 
gill rows of defined nodules filaments shaped cells 
filaments dorsal ridge cilia 
A. constricta —— Vetigastropda bipectinate - - * blade - * 
P. mimula Patellogastropoda ` bipectinate + - - X = 
М. chameleon Neritimorpha bipectinate + - - 9 - + 
P sulcatus Caenogastropoda ` monopectinate + - corrugated - - 
L. articulata Caenogastropoda топоресіпаіе + - triangular - + 
В. auratum Caenogastropoda ` monopectinate + - triangular - + 
M. marginalba Caenogastropoda ` monopectinate + - curved - * 
* Individual filaments do not have a characteristic shape. 
