EYES OF MOLLUSCS AND ARTHROPODS. 



WILLIAM PATTEN, PH.D. 



During the year 1885 it was my good fortune to enjoy a 

 prolonged stay at the Zoological Station in Naples, that Mecca 

 to which all good disciples of zoology hope to make at least 

 one pilgrimage. 



My observations there made on the eyes of Molluscs and 

 Arthropods were published in full in the sixth volume of the 

 Mittheilungen aus den Zool. Stat, zu Neapel. . The more impor- 

 tant of those observations are described in the following sum- 

 mary, which has been prepared for the Journal of Morphology, 

 at the suggestion of Dr. Whitman. 



I found the retina of Molluscs, as well as of Arthropods, 

 to be composed of circles of pigmented cells surrounding central, 

 colorless ones, characterized by constant and remarkable struc- 

 tural features. Believing that these groups of cells constitute 

 the structural elements of most, if not all eyes, I have called 

 them ommatidia ; but it must be borne in mind that, accord- 

 ing to my observations, their structure is quite different from that 

 which Carriere, who first suggested the term, supposed them to 

 have in the compound eyes of Arthropods. 



The simplest ommatidia that I have seen are to be found in 

 the pigmented areas of epithelial cells distributed over the 

 exposed parts of the body of Lamellibranchiata, especially 

 upon the mantle and sipho. They consist of a single circle of 

 four to six pigmented cells surrounding a colorless, central one 

 (Fig. 6) ; the latter is the most important part of the ommatid- 

 ium, for it is mainly upon this element that those structural 

 improvements are consummated that lead to the forma- 

 tion of the most perfect eyes. This central body of the omma- 

 tidium is a double cell whose broad outer end contains two nuclei, 

 one of which (Fig. 6, «, rf^) is often difficult to see, stains faintly 

 and, at first sight, has little resemblance to a nucleus ; an axial 

 nerve-fibre passes through the centre of the cell, and issues from 



