246 SUPPLEMENT 



distinguish objects at a sufficiently great distance, while others 

 seem to possess the power of vision only for those which are 

 very close. Finally, some of them are absolutely destitute of 

 eyes altogether. 



We have already seen that the eyes of these animals are of 

 two kinds, simple and compound. The former, which are 

 sometimes called stemmata, are too small to be dissected in a 

 satisfactory manner. 



x\,s to the compound, or complex eyes^ they are better 

 known. We need hardly repeat that they are divided, like 

 those of the insects, into a number of hexagonal facettes, 

 slightly convex, and which form so many small particular 

 cornese, whose substance is very transparent, and thicker at 

 the middle than at the edges. M. de Blainville informs us 

 that their internal surface in the Palinuri, which we may 

 take as an example, is invested with a sort of black vascular 

 membrane, which must be considered as a true choroid. In 

 fact, it is evidently pierced in the middle of each little cornea, 

 by a small orifice, which should be analogous to the pupil. 

 From this orifice proceeds a small membranaceous produc- 

 tion, in the form of an extremely short tube, which is applied 

 on a corresponding nipple of a considerable, subgelatinous 

 translucid mass, and which is indubitably the analogue of 

 the crystalline, or vitreous humour. M. de Blainville has been 

 unable to ascertain whether this mass is divided into as many 

 parts as there are small tubes, by the prolongation of their 

 very transparent envelope. But he has clearly recognized that 

 this mass of vitreous humour, convex on one side, and concave 

 on the other, is applied on a thick ganglion, or enlargement 

 of the extremity of the optic nerve, which ganglion appeared 

 to him also to present at its surface, as many small alveoli, as 

 there are small ocular tubes. 



M. Cuvier has not found in the eyes of Astacus all the 

 details of organization which M. de Blainville states that he 



