of Insects and Crustaceans Animals. 153 



insuperable difficulties in the way of all attempts of explaining 

 the nature of the function, and naturally enough has been 

 quoted in support of the extravagant doctrine which refers the 

 seat of vision in the eyes of animals to the choroid. 



The observations of Dr. Muller refer to the four different 

 forms of eyes as they occur in Insects and Crustacea, viz. : 

 1. Simple Eyes. 2. Aggregates of Simple Eyes. 3. Com- 

 pound Eyes with facets on the external surface. 4. Com- 

 pound Eyes without facets. 



1. Simple Eyes. The eye of Scorpions and Solpugae have 

 all the parts of the eyes of higher animals, viz., a retina sur- 

 rounded by a layer of black pigment, a lens and vitreous 

 humour, and lastly a cornea, convex externally. The black 

 pigment, surrounding the cup-shaped retina, forms at the 

 anterior edge of the vitreous humour a projecting belt, closely 

 embracing the greatest posterior convexity of the lens. In 

 Scolopendra morsitans there are four such simple eyes on each 

 side of the head, of which three are circular, and the fourth 

 and largest, elliptical. In all there is a hard, amber-coloured, 

 and almost circular lens, in immediate contact with the 

 posterior surface of the cornea. Each lens is lodged in a cup- 

 shaped retina, coated externally by black pigment. In these, 

 as in most other simple eyes, there is either not any vitreous 

 humour, or it is so small as to escape notice. In other cases, 

 on the contrary, as Mantis religiosa, Gryllus hierogliphicus, and 

 the larva of Dytiscus marginalis, there is reason to suppose it 

 exists. 



2. Aggregates of Simple Eyes. Of this kind are the eyes of 

 Oniscus, Julus, Lepisma, Cymothoa, &c. In a large species 

 of Cymothoa, where the number of eyes thus aggregated was 

 about forty, Dr. Muller found as many crystalline globes or 

 lenses, one in contact with the posterior surface of each cornea ; 

 they were hard, transparent, and amber-coloured. Behind 

 each lens was a larger globular mass, also transparent and 

 amber-coloured, with a pit on its anterior surface, in which was 

 lodged the posterior convexity of the lens. This larger mass 

 was coated externally and posteriorly by a layer of black pig- 

 ment, and in contact at its back part with a fibre from the 

 common optic nerve, which probably is expanded into a cup- 

 shaped retina, situated between it and the stratum of pigment. 



3. Compound Eyes with polygonal facets. In many Crus- 

 tacea, the existence of crystalline cones or prisms between the 

 facets of the cornea and the fibrils of the optic nerve has long 

 been known. Such were described in Astacus fluviatilis, by 



