173 PROFESSOR E. RAY LANKESTER AND A. 0. BOURNE. 



that most important points of agreement would be revealed in 

 the comparison of the minute structure of the soft parts of the 

 eyes in question. And we consider that the results we have 

 obtained confirm the opinion expressed by Prof. Lankester, to 

 the effect that the minute structure of the eye, when thoroughly 

 studied, will furnish even more valuable evidence than that 

 given by any other structural features, in the attempt to trace 

 out the genetic relationships inter se of the great groups of 

 Arthropoda. 



Previous observations. — The classical researches of 

 Grenacher^ have laid the foundations of a new and rational 

 study of the minute structure of the Arthropod eye. Our 

 observations have led us to accept, as thoroughly justified, the 

 main conclusions of that anatomist in reference to the nature 

 and structure of the morphological factors of the simple and 

 compound eyes of Arthropoda. To these we shall have to 

 allude in the course of our descriptions. Here, we have to 

 point out, that in his large work Grenacher has not given any 

 account of the eyes of the Scorpions, and only a fragmentary 

 account of the lateral eye of Limulus. 



Von Graber,^ writing subsequently to the publication of 

 Grenacher's large work, has endeavoured to " correct '^ the 

 conclusions arrived at by Grenacher, and has offered some 

 original observations on the structure of the lateral and central 

 eyes of Scorpions. So far as Graber's " corrections " relate to 

 the fundamental points, such as the ultimate structure of a 

 retinal or optic cell, and the relation of optic cells to the cells 

 of the vitreous body, we have no hesitation in stating that he 

 is totally wrong and that Grenacher is right. With especial 

 reference to the Scorpion's eye, Graber's observations and 

 drawings are very defective and, indeed, altogether misleading 

 in regard to simple and fundamental features of structure. 

 Apparently the method of manipulation, the great thickness 

 of the sections, and similar circumstances, are the cause of 

 Graber's errors. 



^ ' Sehorganen der Artbropoden,' Gottingen, 1879. 

 - 'Arcbiv. f. Mikro&k. Auatomie,' vol. xvii, ISSO. 



