MUSEUM OF COMPARATIVE ZOOLOGY. ERG 
wise much altered, lose most of their pigment. It is from these last 
that the best results were obtained. 
The 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 Eyes. 
Grenacher (’79, p. 40) first pointed out that 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 indicated 
(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 
sufficient 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? 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. 672) had already claimed that the median eyes in 
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