COMPOrXD EYES OF AliTIlROI'ODS. 289 



Until tliese (iiu'stioiis be iiiiswercd satisfactorily, tlio most 

 laborious technique of histology is often powerless to discrim- 

 inate the essential from the secondary features in the component 

 cells of a given ommatidium ; and in the absence of an accurate 

 conception of its fundamental structure, the various modifications 

 of the cells and fibres of an ommatidium may be made to assume 

 any significance to suit our speculative ideas. 



With these considerations in mind, I have sought for an 

 ommatidium in which all the features of its structural elements 

 may be made out plainly, and from which we may derive the 

 ommatidial elements of a more complex type. 



Although the number of forms studied in this connection is 

 not quite so extensive as I wish, I feel justified in believing that 

 any view which explains the structural peculiarities of the 

 ommatidia of each and all of the forms in the list above given 

 and reduces them to the same fundamental structure, may fairly 

 • be considered as in all probability generally applicable to the 

 eyes of Arthropods. 



In what follows, the attempt will be made to approach the 

 subject from this standpoint, viz. the consideration of the 

 ommatidium as the morphological unit of the compound eye in 

 Arthropods, just as each little circle of rods with a cone in its 

 centre may be considered as the morphological unit of the 

 " mosaic layer " (Henle) of the human retina. 



II. — The Moepkology of the Ommatidium. 



Following the plan given in the Introduction, I will consider 

 the morphology of the ommatidium, taking it as the structural 

 unit of the compound eye. After the nature of this unit has been 

 considered, we may proceed to examine the state of aggregation 

 of the units in the different forms of tlio Arthropods. For this 

 purpose it is convenient to select one particular ommatidium 

 which may be made the basis of comparison. Accordingly, the 

 ommatidium of SeroUs will be considered at the outset, and the 

 attempt will be made to homologize the ommatidial elements of 

 Serolis with those of other forms. I am indebted to Prof. M. 

 McDonald for the opportunity to study the eyes of the four 

 South American forms of Serolis recently brought back by the 

 U. S. Fish Commission steamer Albatross. 



