ANATOMY AND LIFE HISTORY OF MOLLUSCA. 167 



a remarkable illustration of this. If a calcareous operculum is to 

 be regarded as a modified second valve, it must be remembered 

 that such an organ is possessed by a comparatively few species. 

 A very large number of shells have horny opercula. The family 

 of Trochidse is a large group of nacreous shells with horny opercula. 

 Some of these differ in important characters from the calcareous 

 opercula, not only because they are chitinous, but also they have 

 a central nucleus from which the organ increases in size by a many 

 whorled circular spiral. In this there does not appear to be any 

 centre of nervous matter, though there are clouds and bands of 

 pigment and nerve-sheaths, and rarely, sense-organs in the shape 

 of dark pigmented bodies which may be eyes. Such an operculum 

 as this is found in many widely removed genera and families, as 

 for instance, Cerithium ebeninum, Brug., Trochocochlea tceniata, 

 Lam., Risella melanostoma, Gmel., &c. 



There is also a horny operculum with a lateral nucleus in some 

 species, as for instance, Littorina mauritiana, Reeve, Tectarius 

 pyramidalis, Quoy, and all the Purpuridse, Tritonidse and many 

 other families. In all these there is the same peculiarity as to 

 the clouding and banding with dark crimson-brown pigment. In 

 the case of Littorina mauritiana, Reeve, there is somewhat 

 questionable evidence of a nerve-ganglion. That is to say at the 

 side of the nucleus corresponding with the columella there is an 

 aggregation of ganglionic cells, but it is different in character from 

 the nerve-cells in the case of the calcareous opercula and it is not 

 connected with the sheathed nerve-fibres already described. I 

 think that possibly the organ has the same office. 



I am inclined to regard the pigment-cells and bands of colour 

 in the chitinous opercula as connected with sense-organs, but as 

 any investigations made on the subject are incomplete, I must defer 

 giving any definite opinion. The whole question of the nature 

 and office of the operculum requires a careful revision. Amongst 

 the land-shells there are calcareo-chitinous opercula, and some with 

 a lateral nucleus on one surface and a multispiral central nucleus on 

 the other (Hybocystis); and there are curious modifications of the 

 same organ amongst common littoral species in Australia ; but 

 for the present I wish to confine my observations to such as are 

 connected with the possession of nervous centres and sense-organs. 



Trigonia margaritacea, Lam. This is the large Tasmanian 

 species to which reference has already been made. It is not usually 

 so compact or so regular in appearance as T. lamarckii, Gray. 

 The eyes are about the same size, though the species is much larger. 

 But the character of the nerve-ganglia is different. A section 

 shows a mass of whitish iridescent tissue under the upper plate 

 containing the eye-capsule. It is composed of a very dense mass 

 of nerve-fibres anastomosing with each other in all directions, and 



