680 JUSTUS OAERIERE. 



pigment spots at a distance of about 0*5 mm. from one another. 

 In Area Noae they are black-brown_, close crowded^ at the 

 back end of the mantle larger and more distant from one 

 another, about thirteen in a length of 2 centimetres. These 

 organs consist (PI. XLV, figs. 3 and 4) of a small number of 

 large cells of the form of an elongated cone, with the point 

 directed inwards. From their lying close side by side results 

 the outwardly convex form of the organ. 



The pigment is found in the periphery of the cells, and 

 surrounds as a sheath the cell body ; the nucleus lies in the 

 exterior half of each cell. 



In Pectunculus the cuticular border is tolerably thick, 

 and the exterior surface of every cell is convexly arched ; the 

 organ distinctly contrasts by its colour with the mantle fold, 

 which is here without pigment. 



The optic cells of Area are larger, and the whole organ 

 higher developed, for every cell possesses a kind of lens formed 

 by its cuticular border, which is convexly arched, not only 

 outwardly, but also inwardly, Since the pigment sheath 

 reaches the exterior end of the cell this lens is only visible in 

 cells, the pigment of which has been removed in the cutting of 

 a section. Here the substance of this cuticular lens is dis- 

 tinguished plainly from the cell body by its power of refraction 

 and its behaviour when treated with staining reagents. 



The cells which form the organ of sense, the optic cells, are 

 not limited sharply against the epithelial cells of the mantel, but 

 pass quickly through long and narrow intermediate forms into 

 the epithelial cells. Consequently the eyes of Area and Pec- 

 tunculus, as much as those of Patella and the Ocelli of the 

 medusse Oceania and Aurelia aurita, may be classed among 

 the most beautiful examples for the study of the origin of 

 organs of sight by the modification of epithelial cells. 



Moreover, I think that these simple fan-shaped eyes of Area 

 and Pectunculus offer us a new proof of the truth of the view 

 that similar organs of sight in different classes and orders of 

 tiie animal kingdom may have originated independently, and 

 in fact may still originate without in any way implying that 



