ANATOMY AND LIFE HISTORY OF MOLLUSCA. 145 



It must not be supposed that this analysis of the oblique 

 fibrous layer is applicable as a description of every shell. Let it 

 be understood that I am simply describing now what is observed 

 in one particular species, that is Patella tramoserica. I should 

 further state also in reference to this species that the 

 membranaceous nerve-channels to which I have referred are silky 

 and opalescent, of bluish-white or golden colour, and to this the 

 tint of the nacre is owing. A similar structure in other shells 

 has always been identified with the prismatic shell-structures, and 

 it may have been in this case, but I claim for it the interpretation 

 which I here describe.. 



Besides the eyes on the tegmentum, I have been able to obtain 

 occasional sections of visual organs of another kind on the same 

 species. As these were of larger size they gave greater facilities 

 for examination. 



At pi. vi., fig. 8, there is an illustration showing an eye which 

 was revealed in making a section through the substance of the 

 shell of Patella tramoserica. It had formerly been on the edge of 

 the periostraca, but had been covered up by the growth of the 

 shell. It was a larger eye than any seen on the shell-surface. It 

 will be seen that the structure is of the type common to the eyes 

 of Invertebrata generally, and different from those which have 

 been described by Dr. Semper as occurring in the dorsal mantle 

 of Onchidium. The eyes receive the optic nerve at the base of 

 the rods, and it does not pass beyond them. In the Chitonidse, 

 as described by Prof. Mosely, the optic nerve which supplies the 

 eye gives off from its strands nerve-fibres which pierce through 

 the capsule and come to the surface issuing from the larger or 

 smaller pores. These nerve-fibres are named by the professor, 

 Macraesthetes and Micraesthetes. Here we have a different 

 arrangement : a small branch of the optic nerve becomes detached 

 before it enters the eye-capsule and makes its way to the outside 

 where it joins on to the pigment-coat near the cornea. 



Possibly we have in Patella micreesthetes and macreesthetes 

 also, and they may emerge to the surface through some of the 

 sheaths described, which probably are not all connected with 

 visual organs. Sometimes it is very difficult to see the passage 

 of the nerve from the membranaceous tube or sheath here 



minute fibrillse, similar to the minute network of primitive fibrillse 

 described by Gerlach in the mammalian brain and spinal cord. This 

 description of the minute structure of the opticon applies equally to the 

 epi-opticon and principal ganglia of the body. As this tissue is very 

 commonly met with in the animal kingdom, and has not, as far as I 

 am aware, yet received any separate name, I propose to call it a. 

 neurospongium. In many insects the neurospongium of the opticon is 

 traversed by fibrils, and in some cases it contains a few scattered nerve 

 or even ganglion-cells." 



J— July 4, 1888. 



