174 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



animal. It is probable also, now that we know more of the 

 economy of Mollusca, that we can assert that the tissue subserves 

 the purposes of seeing as well, an instance of which I believe we 

 have in Australia. 



In Sydney Harbour Triton spengleri, Lam., is a common species, 

 and it is conspicuous, like a good many of the Tritonidoe, for the 

 possession of a very transparent yellow periostraca which invests 

 the whole shell. It clothes this so closely as to follow every 

 rib and grooving, and is in itself in fact a perfect reproduction of 

 the whole surface. But it has organs which the shell has not. 

 The membrane is thickly studded with long, sharp, curved 

 points like teeth. These are doubtless organs of feeling, as they 

 are seen to be connected with nerve-fibres of the utmost delicacy. 

 Portions of this periostraca, submitted to microscopic examination, 

 reveal certain organs so like the shell-eyes that one can scarcely 

 refrain from classing them together. First of all there are 

 transparent clusters of minute round vesicles, some with dots 

 like pupils and some without them ; all being refracted to some 

 extent as if they were made of glass. Next there are clusters of 

 what may be eyes about the same size, for they 'have a distinct 

 outside ring of pigment and a pupil as well. Next there are 

 small, black, rounded bodies sometimes gathered in clusters, and 

 sometimes like little strings of beads. The centre of these little 

 bodies is so highly refracted as to be quite brilliant. There is a 

 pupil and an intensely black outside coat. These clusters are 

 rather larger than the clusters of vesicles first spoken of. 

 Finally there are large oval organs, five or six times larger than 

 those already described, with only a faint pigment coat around 

 the clear horny substance which encloses them. In one of these 

 I was able to observe a structure something like the dorsal eyss 

 of Onchidium, though the section was not quite satisfactory. 

 There was apparently an optic nerve on the outside, and on the 

 surface of this, the position being the reverse of that which is 

 found in Onchidium, but corresponding with what Prof. Mosely 

 found amongst the Chitonidse. This, as already observed, is 

 what is usual amongst the Mollusca. Fine nerve-fibres can be 

 traced in connection with all these organs, showing the periostraca 

 to be truly a portion of the living tissue as much as the skin of 

 any other animal. Its points of attachment during life are with 

 the mantle, where additions are made to its growth, and also 

 most probably nerve-fibres reach it from the shell. 



It cannot be absolutely asserted without further examination 

 that these organs are eyes, though I do not see what else they 

 •can be. It shows us, however, what an important field for 

 observation the periostraca may become. Amongst the highly 

 ■coloured and ornamental shells such as the cones, this membrane 



