COMPOUND J'JVL'S OF ARTHliOroDS. 323 



Lave a 8el•ic^; of fbnns showing all degrees of modification in the 

 general stnieture of tlie eye as well as the structure of its indi- 

 vidual elements, and there is not here a single form which invali- 

 dates the view maintained in the present paper. Moreover, this 

 view has the advantage of greatly simplifying our conception of 

 these structures, reducing, as it does, all of tliem to one primitive 

 structure, a depression in the 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 mor- 

 phological organ, namely, the vegetative repetition of a similar 

 structure. 



V. — Summary. 



In studying the structure of the ommatidium of the compound 

 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, 

 Camharus, Homarus, and CaUinectes were mentioned in the 

 preceding pages, the same interpretation of the ommatidium 

 may be applied without exception. This view of the omma- 

 tidium finds its strongest support in the fact that iu Limutus, 

 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 ommatidium of 

 the compound eye of an Arthropod is an independent invagina- 

 tion of the skin. If this view is correct, the unit of the compound 

 eye of an Arthropod is not, after all, so complex a structure as 

 has been supposed by some; and the enormous increase in the 

 number of ommafidia in a given area of the skin which results 

 in the formation of the compound eye finds its parallel in the well 

 known method of the formation of morphological organs, viz. the 

 duplication of a simple unit. 



Baltimobe, April 30, 1889. 



