132 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



shell-bearing Mollusca. It is not here, however, that the nerves 

 for the organs of sense communicate between tne shell and the 

 softer tissues. If we examine the cup-shaped interior of the simple 

 conical shell of Patella we shall find a horse-shoe shaped depression 

 in the interior lining. In this there are special perforations for 

 the passage of nerve-fibres and vessels of circulation. In all the 

 univalve Mollusca there are muscular attachments which perform 

 the same office of fixing the shell in its habitation, and where a 

 communication is established between the hard and soft structures. 

 It is obvious that the passage for the nerves, etc., could not be 

 looked for at the periphery or peristome of the shell, where 

 the tegmentum overlaps ; because, though the mantle usually 

 stretches down to it and continues adding to its structure, it can be 

 withdrawn far within the shell when occasion requires it. But 

 the overlapping is important as showing in every case the 

 existence of the tegmentum and the inner plate, as much in every 

 family as amongst the Chitonidae. In Patella the overlapping 

 plate forms a conspicuous, though very narrow margin. In some 

 species of Acmcea it is wide, and of a different colour from the 

 tegmentum ; that is to say, sometimes when the latter is variegated 

 the overlapping margin has a uniform band of colour or vice versa. 

 In Trochocochlea there are three coats : the tegmentum, the nacre, 

 and a white shelly coat forming an inner lip round the mouth, of 

 broad, dark brown bands and narrow green ones. In Siplwnaria 

 denticulata, Quoy, it is an extremely narrow margin, not at all 

 easy to see, and sometimes itself overlapped by the broken and 

 split edges of the periostraca. Other conical shells need not be 

 particularized. 



In Risella melanostoma, Gmel., this margin is not very difficult 

 to see as a light-coloured, narrow band round the external lip and 

 so on to the bp.se of the columella. But the line of junction between 

 the different plates requires a hand-lens to make out. In all of 

 our Australian species of Littorina it forms a conspicuous addition 

 to the ornamentation of the mouth of the shell. Of all the 

 Trochidse the same may be said. 



This digression has such an important bearing on the 

 subject matter of these eyes, that I must be pardoned inserting 

 it here for the sake of non-scientific readers, and with a view to 

 certain inferences which I shall have to lay stress upon hereafter. 

 But the important point which has to be here insisted upon is 

 that there are in all shells two plates, between which are 

 nerve-fibres, and as I shall show afterwards, in which are nerve 

 centres. 



As far as my observations have gone, I have come to the 

 conclusion that the shell-eyes are by no means confined to the 

 Chitonidse, and that in fact a multiplicity of eyes of this kind is 



