MUSEUM OF COMPARATIVE ZOOLOGY. 177 



wise much alterea, lose most of their pigment. It is from ihese last 

 that the hest results were obtained. 



X no eyes in scorpions are situated on the prosomatic shield. According 

 to their position they may be classed into two natural groups, the median 

 and the lateral eyes. As their name implies, the median eyes are situated 

 close to the sagittal plane. They are a little in advance of the centre of 

 the shield, two in number, and always symmetrically placed. The lateral 

 eyes form two isolated groups, one on either side, at the edge of the shield 

 where its anterior border meets its lateral margin. In different genera, 

 the number of eyes in each group varies from two to seven. Two kinds 

 of lateral eyes have been distinguished ; the larger or " principal," and 

 the smaller or " accessory " eyes. As will be shown later, no essential 

 difference exists between these two groups ; the smaller and larger eyes 

 are constructed on the same plan. 



On account of the marked dissimilarity in the structure of the median 

 and lateral eyes, they will be described separately. 



The Median Byes. 



xrenacher ('79, p. 40) first pointed out tnat the vitreous and retinal 

 layers in the eyes of spiders were separate. Graber in the same year con- 

 firmed this discovery, and showed that the median eyes of scorpions had 

 a similar structure. These two-layered eyes were designated by Lan- 

 kester and Bourne ('83, p. 195) as diplostichous, and among them were 

 included the median eyes in scorpions. Up to this time all authors 

 agreed that the median eyes of scorpions were two-layered. 



One of the results of Locy's work ('86, p. 85), as Mark has indicatea 

 ('87, p. 71), is that in spiders the so-called diplostichous eyes are in 

 reality three-layered, or triplostichous. The embryological facts on which 

 this statement -is based will be referred to later. For the present it is 

 sufiicient to note that an interesting question presents itself, namely, if 

 the so-called diplostichous eyes in spiders have been shown to be triplo- 

 stichous, may we not look for a similar condition in the median eyes 

 of scorpions 1 Some of the reasons for believing this have already been 

 stated by Mark ('87, pp. 55-58), but the final settlement of the question 

 can only be reached through embryological means. It was my principal 

 object in beginning these studies to reach a satisfactory conclusion in 

 this matter. 



Patten ('86, p. DV2j had alreaay ciaimea taai; the median eyes in 



VOL. XIII. — NO. 6. 12 



