Visual Organs of Insects and Crustacea. 131 



for the most part round, rarely elliptical ; and they are 

 generally situated on the upper part of the head, sometimes 

 in a single line, at others in the form of a triangle. They 

 have an organisation similar to that of the eyes of spiders and 

 scorpions, possessing a cornea, a crystalline lens, a vitreous 

 body, and a choroid membrane and pigment. If the exterior 

 form of the eye is elliptical, that of the internal parts is gene- 

 rally ellipsoid* The diameter of the lens is always less than 

 that of the cornea. The optic nerves of the stemmata in the 

 perfect insects do not always unite together into a single 

 trunk; they arise, however, from the same portion of nervous 

 matter. 



Aggregates of Simple Eyes, 



The Myriapodes, the genera Oniscus, Zulus, Lepisma, LithcV 

 bius, Cymothoa, &c., have aggregated simple eyes, which 

 must be distinguished from the true compound eyes to be 

 hereafter described. From these latter the aggregated simple 

 eyes may be known by their smaller number, which varies 

 from twenty to forty, by their not being in contact with each 

 other, and by their having no hexagonal facets. Often, how- 

 ever, the differences can be strikingly seen only when the 

 parts are examined with the microscope. 



In a large Cymothoa, examined by Professor Mliller, there 

 were found about forty of these simple eyes aggregated toge- 

 ther : and the general cornea exhibited a similar number of con- 

 vex elevations, and in its under surface a corresponding number 

 of concavities. Each of these portions was therefore a dis- 

 tinct cornea ; and immediately in contact with its under con- 

 cave surface was a perfectly distinct lens or crystalline globe. 

 The lenses were hard, transparent, amber-coloured, and 

 nearly round. Beneath the lenses lay a dark- coloured mass, 

 having in its anterior surface as many depressions as there 

 were lenses, to the posterior convexity of which the depres- 

 sions corresponded. They are not, however, similar to the 

 cup-shaped receptacles in the iScolopendra, but are situated in 

 the anterior surface of other bodies larger than the lenses, 

 transparent, tolerably hard, nearly globular, and amber- 

 coloured. The sides and posterior part of these globular or 

 vitreous bodies are covered with pigment, and posteriorly 

 there is attached to each a filament of the optic nerve, which 

 probably is expanded into a cup-shaped retina, placed between 

 the vitreous body and its surrounding stratum of pigment. 

 The optic nerve passes under the aggregated mass, distribut- 

 ing its filaments to the individual eyes. 



K 2 



