2i6 PATTEN. [Vol. I. 



Now that the ocelli of Dytiscus and Hydrophilus, which for so 

 long have been regarded as open cups, are seen to be in reaHty 

 composed of three layers of cells, it is all the more necessary 

 to supply a satisfactory confirmation of Lankester's observa- 

 tions on the lateral eyes of Limulus and Scorpions, 



In the larvae of Hydrophilus, Dytiscus, and Aciliiis, there is 

 a remarkable organ on the dorsal side of the posterior dorsal 

 ocellus (Fig. 8), formed of two layers of cells; the thin outer 

 layer, judging from embryological data, is probably continuous 

 with the corneagen, and the thick inner one with the retina of 

 the adjacejit ocellus. The inner layer of the organ in question 

 is composed of long, spindle-shaped cells drawn out to single 

 nerve-fibres at the inner end, and terminating at the opposite 

 extremity in double rods {I'd^ similar to those in the ocelli of 

 Vespa. In the oldest larvae there is an irregular layer of pig- 

 ment around the base of the rods. Although there is no lens- 

 like thickening of the cuticula over this organ the disposition 

 of the pigment and the structure of the cells point to the con- 

 clusion that it is some form of ocellus. 



In Dytiscus this organ is pigmented only in the oldest lar- 

 vae; in Acilius the pigment appears much earlier. In just- 

 hatched larvae of Hydrophilus the organ is a thickened patch 

 of cells (Fig. lO ab), apparently continuous with the retinal 

 layer of the eye on its dorsal side, which at that point is 

 practically open. The outer cell layer of this organ is probably 

 formed later than the inner by the dorsal growth of the cor- 

 neagen. In the older larvae the two structures are apparently 

 perfectly distinct (Fig. 8), although touching each other. It 

 is thus evident that the organ we have been describing is 

 nothing more than a dorsal extension of the ocellus to which 

 it is attached. 



This double ocellus, in its youngest stages, resembles the 

 compound eye of Vespa in its early stages. 



In other drawings, not published here for lack of space, the 

 resemblance is greater than in those in the plate, which were 

 selected to illustrate other points. At present it seems to me 

 probable that the compound eye of Arthropods has developed 

 from a single ocellus, which increased in area by a process sim- 

 ilar to that observed in the ocelli of Dytiscus and Acilius. The 

 ventral pit of the compound eye in Vespa, with its bundle of 



