208 PROFESSOR E. RAY LANKESTER AND A. G. BOURNE. 



Although there is this strange exaggeration in parts, the essen- 

 tial agreement of the central eyes of Limulus with those of 

 Scorpions is obvious. No other known eye approaches it in 

 constitution. This agreement is more marked than that of the 

 lateral grouped eyelets of Scorpions with the lateral polymenis- 

 cous eye of Limulus, though there is no parallel to the mono- 

 stichous eyelet of the Scorpion with its "epistatic" ommateum 

 to be found so close as that presented by a single element of the 

 King Crab's lateral eye, also monostichous and epistatie. 



When the two kinds of eyes are compared in the two 

 animals, each to each, we are abundantly justified in saying 

 not only that there is no other animal which presents so close 

 an approach to either of these animals in respect of its eyes as 

 they do to one another, but even more emphatically that the 

 agreement is one comprising such a large number of important 

 details that we are compelled to conclude from it that the 

 Scorpions and the King Crabs are closely allied representatives 

 of one class, the Arachnida. 



Summary and Tabular Statement. 



The facts set forth in the preceding memoir are to a large 

 extent summarised by the plates which accompany it, and the 

 explanatory description of those plates. 



The more general conclusions to which our observations 

 tend may be gathered from the following tabular arrangement 

 of some of the chief varieties of Arthropod eyes which are at 

 present known. The technical terms which have been intro- 

 duced in the present memoir and recur in the tabular state- 

 ment are explained in a list appended. 



Without assuming to assign their due significance to all 

 varieties of the Arthroj)od eye, some of which may very pos- 

 sibly (e. g. those of certain Crustacea) find no proper place in 

 the scheme here submitted, we yet think that it is useful to 

 tabulate the principal facts of structure known as to a large 

 number of Arthropod eyes in the following way. 



We take as primary divisions those eyes with monostichous 



