MUSEUM OF COMPARATIVE ZOOLOGY. 31 



natural to expect that this structure would be accessible to the light. 

 As I have already shown, the cone-cells form a transparent axis, which 

 leads directly to the rhabdome, and through which light could readily 

 reach that structure. Once having penetrated to the rhabdome, it is 

 probable that, either in the substance of the rhabdome itself, or in the 

 superficial layer of pigment which immediately surrounds the rhabdome 

 and in which the fibrilke are, the light is transformed into that kind of 

 energy which is transmitted by nerve-fibres. 



The Development. 



In discussing the development of the eyes in Arthropods, the more 

 recent investigators have given much attention to the general plan of 

 these organs. One of the objects of their researches has been the 

 reduction of the eyes of these animals to a single structural type. If 

 such a type were found to exist, it would very probably reproduce the 

 essential structural feature which the eye of the ancestral Arthropod 

 possessed. The desirability of ascertaing if there is a common type for 

 the eyes of all Arthropods is evident, for upon the nature of the answer 

 to this question must depend to some extent the conclusions concern- 

 ing the phylogenetic relationship of the group, and its different classes,, 

 Possibly in the phylogeny of two classes of Arthropods the eyes may 

 have originated independently. Of course organs independently devel- 

 oped could not be homologous, and they might be so differently con- 

 structed that it would be impossible to reduce them to a common type. 

 Notwithstanding the difficulty in homologizing the eyes of one class of 

 Arthropods with those of another, the homologies among the members 

 of a single class are much more readily determined, and many impor- 

 tant comparisons can be safely made. It would, therefore, seem more 

 prudent to limit investigations to the eyes of a single class until they 

 were well understood, rather than institute comparisons between the 

 eyes of different classes, where, from the limitations of our knowledge, 

 such comparisons must be more or less hazardous. 



The Plan of the Eye. 



The work of different investigators has already suggested several 

 structural types for the compound eyes of Arthropods, but the differ- 

 ences which some of these types present are of such a fundamental 

 character, that to accept one is to reject another. It was with the hope 

 of gaining confirmation for some one type that I was led to study the 



