AND EMBRYOLOGY OF. LIMULUS. 21 



According to Grenacher, the conical lenses are not homologues of the crystalline 

 lenses of other Arthropods, and the eye of Limulus cannot, he holds, be compared with the 

 eyes of any other Arthropoda. There can be, he claims, no genetic connection between 

 the eye of Limulus and those of any other Arthropods, and the two types of eye, i. e., 

 those of Limulus and all other Arthropods, agree only in the fact that they are compound. 

 Among the Arachnida, he states, one may seek in vain for such an isolated type 

 of eye. He adds : " But it is not only possible but also probable, that the Poecilopoda 

 are related by their eyes to Myriopoda. In Cermatia, the eyes are wholly unlike those 

 of the spiders or insects, and they seem to have something in common with those 

 of Limulus." We shall see further on, however, that the type of eye of Cermatia is not 

 fundamentally unlike that of Bothropolys, and other Myriopoda, as figured by Graber. 



We have seen, then, that there is in the eye of Limulus an entire absence of rods and 

 cones, a common feature of the Arthropod eye. The "corneal lens of Limulus corresponds 

 to the cornea or facet of each individual Arthropod eye, but there are no rods and cones, no 

 optic ganglion, no scattered ganglionic cells, but the end of the long, solid, conical, corneal 

 lens is simply enveloped by the pigment mass, and the end of the cone is succeeded by 

 a rhabdom, partly enveloped by the retinula, the terminus of the optic nerve 

 passing into the axial part of a retinula cell. 



Comparison of the compound eye of Limulus with that of Trilohites. Beyond the fact 

 that the entire eye of certain Trilohites, and enlarged views of the outer surface of the 

 cornea of the eye, have been described and figured in Burmeister's work on the organ- 

 ization of Trilohites and in various palaeontological treatises in Europe and North 

 America, especially by Barrande in his great work on Trilohites, I am not aware that 

 any one has given a description of the internal structure of the hard parts of the 

 eye of Trilohites. 



The full bibhography of treatises relating to these animals in Bronn's Die Classen 

 und Ordnungen des Thierreichs, carried up to 1879 by Gerstacker, contains references 

 to no special paper on this subject, and the resum^ by Gerstacker of what is known of 

 the structure of the eye, only refers to the external anatomy of the cornea, the form 

 of the facets and their number in different forms of Trilohites. He shows that observers 

 divide them into simple and compound; the former (ocelli) are found in the genus 

 Harpes. These "ocelli" are said to be situated near one another, and are so large 

 that the group formed by them can be seen with the unaided eye; the surface 

 of the single "ocellus" appears, under the glass, smooth and shining. From the 

 description and the figure of the eye enlarged, from Barrande, it would seem as if 

 each eye was composed of three large simple ones ; so that these eyes are really 

 aggregate, and not comparable with the simple eye or ocellus of Limulus and the 

 fossil Merostomata.^ Moreover, the situation of these so-called oceUi is the same as 

 that of the compound eyes of other Trilohites. 



The Trilohites with compound eyes are divided into two numerically very dissimilar 

 groups ; the first comprising Phacops and Dalmanites alone, and the second embracing 



1 The eyes of the fossil Merostomata (Eurypterus and judging by Mr. Woodward's figure, exactly homologous with 

 Pterygotus) are evidently in external form and position, the ocelli and compound eyes of Limulus. 



