Visual Organs of Insects and Crustacea. 129 



or vitreous humour, whose anterior surface corresponds to 

 the convexity of the cornea. The broad basis of the conical- 

 shaped optic nerve is in contact with a membrane investing 

 the vitreous humour : the axis of the nerve being also that of 

 the eye. In the vicinity of the eye the fibrous structure of the 

 nerve disappears ; but there the nerve seems to be traversed 

 by dark longitudinal lines. A very thick and black pigment 

 surrounds the vitreous humour on all sides, except at the 

 parts where it is in contact with the base of the optic nerve 

 posteriorly, and with the lens in front ; the pigment extending 

 as far as the periphery of the lens and the margin of the 

 cornea. 



iStorpio tunensis, S'corpio aegyptiacus. {Jigs. 18, 19.) — In this 

 animal there are two large eyes placed near the middle of the 

 cephalothorax. They consist of a cornea, a spherical lens, a 

 vitreous body, and a membranous retina. These eyes are very 

 near to each other, being separated only by a projecting ridge, 

 and th^y are so disposed that their axes diverge from each 

 other at a very considerable angle. The cornea [Jig» 18. «) 

 seems formed of the common integu- 

 ments, which, of course, for that pur- 

 pose become transparent, and elevated 

 into a convex prominence. The centre 

 is thinner than the circumference, 

 owing to the internal concavity being 

 greater than the external convexity; 

 and the circumference seems to be 

 bordered by a dark band, which ap- 

 pearance is produced by the subja- 

 cent dark pigment (c). If the cornea be carefully separated 

 from the internal parts, the crystalline lens (6) will always be 

 found remaining attached to the under concave surface. The 

 lens of the African scorpion is perfectly spherical, very hard, 

 and of an amber colour ; it is in contact with the cornea only 

 at the centre, for at the circumference of the latter there 

 exists between it and the lens a triangular zone, of which a 

 part only, toward the margin of the cornea, is filled with the 

 bluish black pigment. Immediately behind the lens is the 

 vitreous body (2), convex anteriorly and posteriorly, the lens 

 not being, in this animal, embedded in the vitreous body. 

 The breadth of the vitreous body is three times greater than 

 that of the lens ; its longitudinal axis is to its tranverse dia- 

 meter nearly as 1 to 2; and it has a greater convexity posteriorly 

 than anteriorly. Its apparent concavity in front is simply 

 owing to the pigment already noticed as in part filling up the 

 zone surrounding the lens. The choroid pigment [c] invests 

 Vol. i V. ~~ No. 18. k 



