554 William Patten 



be confident that itmust be the normal couditiou. This globular structure 

 is found in the colorless cells of HaUotis and Patella , and is probably 

 characteristic of these Clements. When macerated either in osmio and 

 acetic acid (according to Hertwig's formula), or in dilute snlphuric 

 acid, the globular contents of the celi shrink into a small and refractive 

 body situated in the middle of the celi ; the remainder of the celi then 

 appears to be filled with a clear fluid, while, just back of the nucleus, a 

 partition is formed, dividing the celi into an external part containing the 

 nucleus, and alarge internai portion once filled with refractive globules. 

 In sections , and in partly macerated specimens, it is seen that the 

 retinophorae are capped by an oblong, transparent and refractive body, 

 which at once suggests that it is a corneal facet, or cuticular lens. This, 

 however, I am convinced is not its true nature, but, on the coutrary, 

 its structure, and abundant supply of nerve fibres, shows that it is 

 the light perceiving element of the eye , or the rod. This rod is 

 brilliantly refractive and perfectly colorless. The inner surfaee has 

 the same curvature as the outer wall of the underlying celi. The 

 somewhat broader, distai end is well curved in Arca Noae . but is 

 somewhat flatter in Arca barbata-, in cross sections it is per- 

 fectly spherical. The same fact may be observed in the living eye by 

 looking directly into the ommatidium, when one sees through the 

 perfectly transparent rod the black pigment beneath, producing the 

 effect of a deep and round, black spot surrounded by light chocolate- 

 colored pigment cells. In longitudinal sections, one sees an extremely 

 thin, cuticular-like outer layer, covering the outer surfaee of the rods, 

 and continuous with the cuticular of the pigmeuted cover cells. 

 Toward the edge of the rods, it is more distinct, and there passes over 

 to form the very much thicker, cuticular layer of the cover cells. It is 

 in specimens macerated in sulphuric acid, that one is best able to de- 

 monstrate this membrane, for by this method, large pieces with severa! 

 rods attached may be easily separated. I bave spoken of these struc- 

 tures as the rods, while, in reality, they are formed by the fusion of 

 two pieces, but so closely and intimately applied that it is not always 

 easy to demons träte their dual origin. To do this with fresh and but 

 little macerated specimens is well nigh impossible, except in abnormal 

 cases. I bave found two such cases, in which the freshly macerated rod 

 was formed of imperfectly joined halves, the configuration of each half 

 being as distinct , as though they belonged to two separate cells 

 ^fig. 52). Such cases are, however, rare. On the other band, if the cells 

 were thoroughly macerated , in every rod could be seen a median divi- 



