136 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



the surface. Their position is commonly arranged so as to give 

 vision at every angle to the animal. Of course the majority of 

 the eyes upon the shell-surface could only look upwards if they 

 have no power of movement, about which nothing can be asserted 

 as yet. But I find that every little point or elevation is chosen 

 as a location for an eye or a cluster of eyes, and thus there is a 

 close connection between the surface form and these organs of 

 vision. On the ribs of shells it will often be observed that there 

 are raised nodules, points, or bosses at regular intervals, gradually 

 increasing in size in a radiate manner from the summit to the 

 periphery. Such ornaments, which often form the special beauty 

 of a shell, reminding one of the crockets on spires in Architecture, 

 are lit up by these crystalline optical arrangements, making a 

 shell in which they are well preserved look like a city adorned with 

 rows of street-lamps, especially when the surface is wetted or 

 oiled. The eyes are nearly always so minute as to be only visible 

 with the microscope. When the number and variety of them is 

 considered, some will wonder that they have not been noticed 

 before, but they are seldom preserved on the surface of shells in 

 museums and collections. The scrubbing, washing, decalcifying, 

 and polishing to which they have been subjected has long ago 

 swept away these fragile little crystals. My opinion also is 

 that the animal tissues connected with these visual organs are of 

 a very perishable nature, and that their places are only indicated 

 by pits after a little dessication. The calcareous matter contained 

 in the cornea soon ceases to reflect light, and becomes white and 

 opaque. In very old shells the former presence of these eyes is 

 indicated by innumerable pits, as close as small-pox markings on 

 the human face. In this manner it is not impossible that they 

 may be detected in fossil species. 



I have found these visual organs as common amongst the 

 bivalves as amongst the univalves. In some genera they are 

 present in extraordinary numbers. This I regard as specially 

 the case with Trigonia lamarckii, already referred to. I have, 

 moreover, good reason for the opinion that such eyes are still to 

 be found amongst shells whose upper plates or tegmenta are 

 formed of tine lamella?, such as the common oyster. 



Isolated Eyes. — Besides these tegmentary eyes, solitary eyes 

 of larger size are found on the edge of the shell, on the operculum 

 and on the periostraca when it is horny. There are peculiarities 

 about these organs which show them to be quite different from 

 the tegmentary visual organs. (1) They are of much larger 

 size ; (2) they occur solitary, in pairs, triplets, or even little 

 clusters ; (3) they are irregularly spherical or oval, of dark 

 colour, and highly refractive in the centre ; (4) they are probably 

 of the vertebrate type, that is to say the nerve penetrates through 



