ANATOMY AND LIFE HISTORY OF MOLLUSCA. 147 



that this is their true character. What is meant, will be better 

 seen when I am describing the shell-eyes of Trigonia; but it may- 

 be just mentioned here, that the tubes are cylindrical when they 

 are on a plane surface, but when they radiate from a tubercle, 

 they become conical at the base. If I do not use the word cone, 

 it is because I do not wish to adopt a terminology which would 

 seem to take for granted the supposition that the structures are 

 identical in function. There are, however, so many points of 

 resemblance between the facetted eyes of insects and the shell-eyes 

 in some species that no one need be suprised at finding them 

 classed under the same terminology. On the tubercles of the large 

 Tasmanian Trigonia, the cornea of the eyes or sense-organs 

 whichever they may be, becomes detached, and then it is seen so 

 closely to resemble the facetted cornea of an insect's eye, 

 such as the common house-fly, that no one could help identifying 

 •one with the other. Moreover there is a very considerable 

 resemblance between the arrangement of the large cerebral ganglia 

 of many insects and the ganglion of the shell. Those who have 

 examined these objects will remember that there is a cerebral 

 ganglion and a ganglion in connection, called the peri-opticon. 

 These various ganglia being separated by comparatively clearer 

 spaces where the nervous fibres decussate, it would be quite easy 

 to point out parts and structures in some shells, Trigonia in 

 particular, where a closely similar arrangement exists. 



But in addition to the deep-seated eyes just referred to, even in 

 the substance of the shell, we find, as already stated, deep-seated 

 eyes cut off, so to speak, from all communication with the outer 

 world. In the nacre we see fine lines, which represent the former 

 lines of growth in the structure. These have been formerly the 

 edge of the shell, which frequently, as it seems to me have borne 

 eyes. By the growth of the shell this has been covered over with 

 nacreous plates, and the eyes entombed. Thus in more than one 

 place, in the substance of the shell, I have come across eyes which 

 are conspicuous from their large size. A figure of one of them is 

 given at pi. vi., fig. 8, in which the nerve-channels formerly 

 connected with it have entirely disappeared. The eye it will be 

 seen has the structure already described of the Invertebrate type, 

 with the arrangement of the rods reversed. The rods themselves 

 are similar to those observed in the last section of the eye of a 

 limpet just described. 



If the nerves have an exit from the plate which forms the inner 

 lining, we might expect to find a channel for their exit similar to 

 that for their entry. And this is what we do find as a matter of 

 fact. There is a small plate of fibrous shelly matter, lining the 

 nacre, and this is densely perforated with membranous tubes which 

 continue through all the fibrous tissue, which here is of the thinnest 



