ANATOMY AND LIFE HISTORY OF MOLLUSCA. 163 



like the knob or handle of a door. This is a crystalline lens, 

 and the basal structure is very similar to the facetted structure 

 of an insect's eye. The facets are certainly no bigger and of 

 uniform size. The valves are provided with about 20 ribs beset 

 at intervals with flattened raised tubercles, and these tubercles 

 are covered with round bull's-eye-like lenses instead of the hexagonal 

 facets on the body of the shell. Towards the outer edge of both 

 species (T. lamarckii, Gray, and T. inargaritacea, Lamarck, ) the 

 facetted outer coating becomes gradually merged into a corrugated 

 membrane or periostraca, from which the facetted structure 

 gradually disappears. At the most moderate computation I can 

 make, there are at least 12,000 eyes on each valve but probably 

 this is far within the estimate. (See figs. 25 and 26.) 



On making a section through the shell-substance of the valve 

 and using high magnifying powers, we find that every facet covers 

 a small eye-capsule similar in office to the pseudocone of the 

 insect's eye, except in this that there is a line of division in the 

 middle of the capsule which contains the rods and the optic nerve 

 beneath. The optic nerve also sends forth small fibres towards 

 the outer surface. The manner in which the optic nerve is 

 conveyed from the large ganglion in the shell substance to the 

 base of the eye-capsule, is better conveyed by a diagram than by 

 any description. At pi. x., fig. 16, I have given a small portion 

 of a section made transversely to the ribs, that is to say, cutting 

 through three ribs in succession. Here it will be seen that there 

 is a crowd of eye-capsules inclined at every angle according to 

 their position on the slope of the ribs ; some of these are covered 

 with highly refractive round bull's-eye-like lenses and others with 

 a facetted arrangement and the knob-like lenses already described. 

 From these the nerves radiate out in a fan-like fashion when they 

 come from the valley between the ribs, or collect into one column 

 from a reversed fan-shape when they come from the rib or its slopes. 

 A reference to the plate will make the meaning of this quite clear. 

 A section made at right angles to this, gives slightly different results, 

 but only arising from the different position in which the same 

 structures are seen. It will be noticed also in the plate that when 

 the optic nerves have spread out in a radiate fashion for a certain 

 distance they seem to terminate, though in reality there is no 

 termination : there is simply an absence of pigment cells, and the 

 nerve-sheaths become more transparent until theyjoin a large optical 

 . ganglion. Between the base of the capsules also, or rather beneath 

 the base of the capsules, there is a dark line or stratum of " tissue 

 where the nerves are more difficult to trace. It seems as if the 

 darkness is due to pigment cells, and the line where they cease is 

 -as clearly marked as the line on the upper surface separating 

 the clear glassy-looking eye-capsules from this dark layer. 



