Flsual Orga?is of Insects and Crustacea. 225 



structures forming the simple eye of the vertebrated animals. 

 In fact, we find in these compound eyes a nervous filament 

 attached to the extremity of a transparent body representing 

 the vitreous humour and crystalline lens ; a transparent cornea 

 covering externally this apparatus ; and a choroid membrane, 

 represented here by a coloured pigment, which surrounds, as 

 in the vertebrated animals, these minute organs of refraction 

 and sensation. We may still further remark that the pig- 

 ment, continuous in all parts, although varying in thickness, 

 forms between the cornea and the transparent or crystalline 

 cylinder an iris {Jig. 37. I), or at least a uvea, which allowsj 

 the light to pass only through the centre of the apparatus. 

 There is also a perforation, a true pupil> which appears black, 

 as in man, when examined with a powerful magnifier. The 

 whole of these pupils, whose axes correspond to that of the 

 eye of the observer, form the black and mobile spot which 

 has often been a source of embarrassment to those examining 

 these parts. 



M. A. Dug^s states that he has frequently thought that 

 this black spot underwent changes of magnitude, like the 

 pupils of the mammiferous animals. Is there, as in these 

 latter, a contractile diaphragm ? If such be the fact, we must 

 suppose that the superficial pigment, which forms these in- 

 numerable irides, adheres to the cornea only at the circum- 

 ference of each facet; that it has elsewhere throughout a 

 perfect freedom ; and consequently that there exists a minute 

 space between these two organs. This, indeed, M. Dug^s 

 believes he has seen in a very clear section of a compound 

 eye. It appeared that, behind each facet, there existed a 

 little cavity {^fig. 3 8. m), a kind of anterior chamber, filled with 

 an aqueous fluid. But it is very certain that the minute size 

 and extreme softness of these parts, the violence to which 

 they must necessarily be exposed in their preparation for 

 examination, however carefully we may proceed, and also the 

 high magnifying power required for such observations, must 

 always render the result of these investigations somewhat 

 doubtful and confused. 



In the Dytiscus marginalis, the vitreous or crystalline bodies 

 beneath the cornea are conical ; which is their form also in 

 nearly all the articulated animals provided with compound 

 eyes. In the Libellula?, as already noticed, they are cylin- 

 drical. In the Dytiscus marginalis, the lateral surfaces of 

 these cones are coated with dark and opaque pigment, which, 

 however, does not appear to extend quite to the cornea : it 

 ceases also at the apices of the cones ; so that the nervous 

 filaments, for the greater part of their course, are without 



Vol. IV. — No. 19. q 



