166 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



But when this is said, it must be admitted also that the relative 

 number of shells examined is but small, and I have seen quite 

 enough to convince me that characters as wonderful as those of 

 Trigonia lamarckii, Gray, will be easily found, and that in respect 

 to the sense organs the species is not so very exceptional after all. 



I have already stated that there is a nervous ganglion in the 

 columella of certain univalves, with accessory sense-organs. Hence 

 I regard the shell of these univalves as being a cephalic ganglion 

 case. But if this be so, we naturally enquire what about the 

 operculum, for this is justly regarded as the homologue of the 

 second valve amongst bivalves. In answer to this I may say that 

 I have found, in the calcareous opercula of a few species, well- 

 marked nervous ganglia as well as accessory sense-organs. In 

 nearly all the species of Nerita known to me, the operculum is 

 covered with small oval or hemispherical projections of a 

 transparent or glassy nature. These when examined are found 

 to be abundantly provided with sense-organs and attached nerves. 

 Moreover amongst many species of Turbo, the calcareous operculum 

 is thickly studded with opaque or glassy tubercles and projections, 

 some of considerable size. All of these are abundantly supplied 

 with nerves, and some of them have well-formed eyes upon them, 

 while others have abundant sheaths of nerves for sense-organs. 

 There can be no doubt that when the animal is comfortably housed 

 in its shell with the door of the operculum shut against intruders, 

 it is able to take the fullest cognizance of what is going on outside 

 by means of the asperities and tubercles with which its door is 

 beset. 



Moreover the position of the ganglion in the operculum is 

 precisely similar to that which it would occupy in the spiral of a 

 univalve shell. In all the opercula now referred to, the nucleus 

 is lateral, and it represents the spiral of a univalve shell, the outer 

 curve corresponding with the outer lip, and the nucleus with the 

 columella. Just inside this nucleus the ganglion is situated, 

 and from it proceed an abundance of fine silky nerve-strands going 

 in parallel lines to the periphery. In fact the whole operculum 

 is, when seen in section, one mass of nervous structure. The silky 

 lustre of the sheaths as well as the white shining character of the 

 ganglion cells, give a peculiar brilliancy to the structure, which I 

 think is ultimately due to the small highly refractive nerve-cell& 

 which are always found in neurospongium. At any rate this white 

 iridescent character is a very good distinguishing feature for 

 the tissue. 



But it must be admitted that the structural differences between 

 one species and another and one genus and another are often so 

 great and perplexing that it is very difficult to pronounce any 

 particular organ as exceptional or not. The operculum furnishes: 



