Visual Organi of Insects and Crustacea, ' 125 



accuracy of Miiller's researches, was lately given in the An^ 

 nales des Sciences Naturelles, The most important facts thus 

 made known on the structure of the simple and compound 

 eyes of insects and the Crustacea I will endeavour to compress 

 into the following pages. 



For the visual perception of distinct images, it is evidently 

 necessary that the light from the different parts of the object 

 to be seen must also be insulated in a corresponding manner 

 upon the different parts of the sentient organ, A retina, 

 without an exterior apparatus for the fulfilment of this con- 

 dition, can only communicate to its possessor the simple sens- 

 ation of light. Whatever may be the objects presented to 

 such a retina, since the light, shade, and colours proper to 

 their several parts, are all transmitted indifferently to every 

 part of the sentient organ, it will inevitably result that these 

 qualities (light, shade, and colours) will produce a common 

 and simultaneous impression upon the whole of the organ ; 

 the light being no longer distinct from the shadow, nor the 

 colours from either of the two former, as they are in the ex- 

 ternal object. If there are in nature beings possessed of this 

 simple sensation of light and colours, without an optical organ 

 of vision, such sensation cannot extend so far as to insulate 

 the colours, so as to correspond with those of the exterior 

 object ; and this may be the case in some Annelides, which 

 have what are regarded as ocular points or eyes, in which, 

 however, no distinct and separate structures can be dis- 

 covered. 



That parts of many, or of most plants, are affected by the 

 sun's rays, is sufficiently proved by the opening and closing 

 of flowers, by the directions which the leaves take in order 

 to expose their surface to the light, &c. ; and the opinion of 

 M. Dutrochet, that vegetables possess, dispersed through their 

 several organs, a substance analogous to nervous matter, and 

 to whose agency this susceptibility to the action of light may 

 be attributed, is certainly far from being improbable. We 

 may fairly assume that, in the organised bodies placed lowest 

 in the scale of the animal kingdom, this nervous matter is less 

 disintegrated, less mingled up with the other textures, than in 

 plants ; and that, as it becomes more and more concentrated, 

 and separated from the rest of the animal organisation, the 

 adumbrations, as it were, of organs of sense will simultaneously 

 make their appearance ; and thus at length distinct sensations 

 will be produced by the action of exterior agents. It can 

 scarcely be denied that the Annelides, and other allied tribes 

 of the lower animals, possess the sensation of light; but it 

 must also be inferred, from our present knowledge of their 



