702 William Patten 



sections or maceration, the segmeutally arranged pigment spots in A^ite- 

 rope candida and Toìnopteris (sp. ?), and found that they resembled so 

 strongly the ommatidial areas in the Lamellibranchiata , that there can 

 he little doubt that they represent aggregations of simple ommatidia. 

 Although the cutieular covering is thin, the ommatidia are very sharply 

 defined and regulär, consisting of rather large, colorless retinophorae 

 surrounded by a complete ring of pigment cells. To settle the question 

 beyond doubt, it would be necessary to show that the colorless cells 

 end inwardly in points, bave axial nerve fibres, and contain two nuclei, 

 at least one of which is nucleolated. When irritated , Asterope gives off, 

 apparently from the colorless cells, a beautiful, emerald green secretion. 

 This fact militates against supposing the organs in question to be eyes. 

 But ìt is not improbable that the secretion may be a i)athological result 

 of the violent contractions , similar to what Chun has described for 

 the Ctenophorae, where a strong Irritation causes the Chromatophores to 

 burst; the contents in this case, however, being forced inwards. 



In theVertebrates,there seems to be little evidence for supposing 

 that isolated ommatidia are preseut. The structure of the so-called 

 luminous organs of the Scopelidae is too imperfectly known to allow 

 any conclusion as to whether they are composed of ommatidia, or not. 

 If they should prove to be eyes instead of luminous organs , we would 

 bave good reason for supposing that ommatidia are present in the 

 general epithelial covering of the body. Sense hair cells may be found 

 in ali parts of the body , and it is only necessary to bring any part of 

 the sensitive epithelium into Constant relation with certain agents to 

 develop at that point a special organ of touch, taste, or hearing, as the 

 case may be. Gkaber has shown by experiment upon blinded Eeptiles 

 that the skin has, to a certain extent, the power of distinguishing light 

 from darkness. This would lead us to suppose that scattered omma- 

 tidia were present in the skin. Now if animals provided with scattered 

 ommatidia lived in dark places, as at great depth in the sea, then, in 

 order to see in the dim light, their visual organs would undergo changes 

 by which a diminution in the amount of light would be accompanied 

 by a corresponding ìncrease in the area and complexity of the sensitive 

 surface, and would probably result in the appearance of numerous eyes 

 at various parts uf the body. But we bave seen, in those cases when 

 it was necessary for a great amount of light to be used, or to make an 

 economie use of very little light, that refleeting surfaces are developed, 

 either in each retinophora, behind the rod [Haliotis, Arca etc.), or as 

 a continuous membrane behind the inner layer of rods (the argentea in 



