2l8 PATTEN. [Vol. I. 



ventral side with a dorsally directed fold (Figs. 4 and 5), while 

 on the opposite side the inner wall of the pouch gradually passes 

 into the thickened ectoderm of that side without the formation 

 of a fold. The dorsal fold, which develops much later than the 

 ventral one, is quite different from it in structure and general 

 appearance, and may be regarded as a secondary formation. 

 The compound eye grows, like the double ocellus, toward the 

 dorsal side by the formation of ommatidia on its dorsal edge. 



If the compound eye of Arthropods arose from such an ocel- 

 lus with its dorsal extension as I have described in Acilius, 

 Dytiscus and Hydrophilus, we ought to find traces of a similar 

 structure in the larvae of other Insects. In the larvae of Chironi- 

 m7is, I have seen on the dorsal side, and a little in front of the 

 ocelli, an oblong patch of hypodermic cells which has a strong 

 superficial resemblance to the dorsal outgrowth of the ocellus 

 in Dytiscus, the only important difference being that, in the 

 former case, the colorless patch of cells is apparently completely 

 separated from the ocelli. 



In young Corcthra larvae there is a similarly thickened patch 

 of cells on the dorsal side of the compound eye. In this case 

 it is probable that the larval compound eye is a modification of 

 an ocellus, while that of the adult represents the larval com- 

 pound eye, plus the dorsal patch of colorless cells, which, in the 

 adult, have developed ommatidia like those in the rest of the 

 eye. 



In NeopJialax there is a thickening on the posterior dorsal 

 side of the larval compound eye, similar to that in Corcthra. 



In the Hydrachnida the apparently single ocellus is in reality 

 double, or at least has two distinct lenses imbedded in a common 

 mass of pigment; on the dorsal side of each double ocellus is 

 a rosette-shaped organ which I believe to be homologous with 

 the dorsal extension of the ocellus in Aciliits. In Hydrachna 

 the organ in question is rosette-shaped and, in surface views, 

 appears to be composed of a circle of wedge-shaped cells with 

 peripheral nuclei ; in the centre is a distinct granular core. The 

 inner ends of the large cells are prolonged into nerve-fibres, 

 which unite to form a bundle of nerves. The outer ends are 

 capped with a hyaline layer of rods which resemble those of 

 the dorsal extension in Acilius. Neither the rosette-shaped 

 organs of Hydrachna, nor the similar structures in Ckironimus, 



