196 THE ENTOMOLOGIST. 



section it exhibits a stellate appearance. In the Crustacea 

 the structure of the facetted eyes is in the main the same as 

 that of the euconic eyes of insects ; but the number of the 

 cells from which the crystal-cone is developed is variable. 

 In the great majority of the Crustacea there are four, as in 

 insects, but there are only two in Araphipods, Gammarus 

 and Hyper in a ; in Isopods, Aseliina and Oniscoidea ; and in 

 Schizopods, Mysis ; there are five in the Cladocera, Daphnia, 

 &c. ; and in the Phylopodous genus, Estheria ; but there are 

 only four in Apus and Branchipus. The number of elements 

 in the retinula is also variable ; there are usually seven, as 

 in the majority of insects ; there are only five in Hyperinis, 

 Apus, and Branchipus; and ibur in Gammarus. Each 

 retinula appears externally to have the same form throughout 

 its entire length ; but in many genera the rhabdoma shows 

 enlargements, which are stellate in section, as in insects. 



III. On the Morphological Relations of the Simple 



AND Compound Eyes. 



Aconic eyes are comparable in structure to the simple 

 ocelli ; each consists of a transparent lenticular swelling of 

 the cuticle, of certain modified cells of the hypoderuiis, 

 the vitreous in the ocellus and the crystal cells of the aconic 

 eye, and of the retina in the ocellus and the retinula in the 

 compound eye. Dr. Grenacher concludes that the two 

 forms of eye are the result of the modification, in two 

 opposite directions, of a primitive but rudimentary type; 

 in the one the tendency has been towards the multiplication 

 of retinal elements and the perfecting of a dioptric appa- 

 ratus ; in the other towards the reduction of the retinal 

 elements to a single receptive structure, which attains its 

 highest form in the rhabdoma of the euconic eye — the lens 

 remaining as a rudimentary structure, or being altogether lost 

 as in the Hyperidce, the perfection of the compound eye being 

 attained by the multiplication of the component ocellulae. 



IV. On the Function of the Compound Eye. 



As has been already stated, Dr. Grenacher returns to the 

 theory of Johanes Midler; and it will be seen that the 

 anatomy of the couipound eye is entirely in favour of the 



