220 patten: " [Vol. I. 



and remained separated. This supposition is rendered all the 

 more probable since there are indications that the primitive 

 ocellus and its dorsal extension, from which, in my opinion, the 

 compound eye arose, are also in some cases (Hydrachna) com- 

 pletely separated. In fact we do find a separation of the com- 

 pound eye into its primitive parts in the adult Phronima, in 

 which the dorsal and ventral eyes are apparently distinct and 

 are supplied with separate retinal ganglia and optic nerves, 

 although there is but a single optic ganglion. In Insects there 

 are numerous instances of this kind. In Gyrinnus natator there 

 are two pairs of compound eyes which are supplied, according 

 to Carriere (20) with separate nerves and retinal ganglia, the 

 middle ganglion being paired, but united, while the inner is 

 unpaired. Carriere considers that the two pairs of eyes arose 

 from the division of a primitive pair. This statement, however, if 

 I am correct, may need modification, since the facts I have pre- 

 sented render it probable that double compotmd eyes have not 

 arisen by the division of a primitive homogeneous eye, but by 

 the separation of two originally different parts. In most cases 

 these parts are alike, and so closely united as to form one eye, 

 the difference being discernible only in the youngest stages. 



Another instance of double eyes, a description of which we 

 owe to Carriere, (19) is found in Bibio and Cloe diptera. In 

 these cases, however, only the males have double eyes, while in 

 Gyrinnus both sexes are thus provided. In Bibio and Cloe, the 

 accessory eye, as Carriere calls it, arises at the beginning of the 

 pupal stage as a thickening of the epithehal cells, very similar 

 to the primitive thickening for the compound eye-, on the dorsal 

 edge of the " Gattungsauge," from the upper, inner edge of 

 which it appears to take its origin. 



Although Carriere distinctly states that there is no transition 

 of the ommatidia of the lower eye into those of the developing 

 upper one, I feel inclined to doubt the complete accuracy of 

 this statement when the condition described in Acilius and Vespa 

 is borne in mind ; for, in the former case, there is, at first sight, 

 no connection between the dorsal extension and the ocellus in 

 the older larvae, although this connection is apparent in the 

 embryos. Perhaps Carriere did not see the very first stages of 

 the accessory eye, since he states that it appeared at the begin- 

 ning of the pupal stage, while the dorsal half of the compound 



