MORPHOLOGY OF COMPOUND EYES OF ARTHROPODS. 155 



and " eucone " types refer to the condition of the cells and 

 their products in the middle stratum — the vitrellse. 



Morphologically; then, the compound eye of an Arthropod 

 is strictly single-layered^ although, as is evident, the pre- 

 sent conception is entirely different from the monostichous 

 theory maintained by some recent writers. From Limulus 

 to Squilla we have a series of forms showing all degrees of 

 modification in the general structure of the eye as well as the 

 structure of its individual elements, and there is not here a 

 single form which invalidates the view maintained in the pre- 

 sent paper. Moreover, this view has the advantage of greatly 

 simplifying our conception of these structures, reducing, as it 

 does, all of them to one primitive structure, a depression in 

 skin, in which several organs of ectodermal nature, often of a 

 very complicated type, find their common morphological origin. 

 And when thus the nature of the unit is reduced into a simple 

 invagination of the skin, the formation of the compound eye 

 appears to be but another instance of the well-known method 

 in the formation of a morphological organ, namely, the vege- 

 tative repetition of a similar structure. 



Summary. 



In studying the structure of the ommatidium of the com- 

 pound eye of Serolis it has been found that it may be 

 reduced to a simple ectodermic invagination of the skin. 

 Extending my researches over several other Arthropods, of 

 which Talorchestia, Cambarus, Homarus, and Calli- 

 nectes were mentioned in the preceding pages, the same inter- 

 pretation of the ommatidium may be applied without excep- 

 tion. This view of the ommatidium finds its strongest support 

 in the fact that in Limulus the ommatidium is an open pit 

 of the skin. 



By supposing that the ommatidial pit of Limulus became 

 deeper, and that this was accompanied by modifications in the 

 structure and arrangement of the component cells, we can 

 show the probability of our first supposition that the omma- 



