No. I.] EYES OF ARTHROPODS. 21 3 



pod eyes, are in reality closed vesicles, and composed primarily 

 of three layers of cells. 



With this object in view I have examined the ocelli in the 

 larvae oi HydrophUtis, Dytiscus, and Acilius, and find the facts 

 such as my supposition would lead one to expect. From Gren- 

 acher's description, there would be little room to doubt his 

 assertion that the ocelli of these Coleopterous larvae are com- 

 posed of a single layer of cells. Graber (Arch f. mik. Anat. 

 xvii., Bd. Hft. I, p. 6y), however, hinted that Grenacher was 

 in error on that point, and then asserted, in a very short foot-note, 

 that he had subsequently found a " membrana limitans" in the 

 ocelli of Dytiscus. Sograff (Zool. Anz. No. 18, 1879) re- 

 marks that the eyes of Lithobida: and Scohpennda are 

 exactly like those of Coleopterous larvs (Acilius), and Spiders, 

 although, in this preHminary notice, we can find no evidence in 

 confirmation of his statement. There are a number of very im- 

 portant differences in the structure of the ocelli of these three 

 groups. It is not clear in what points he regarded them as 

 similar. 



In the larvae of Act/ius there are six pairs of ocelli. Longi- 

 tudinal vertical sections of the head show that the two dorsal 

 pairs are very deep, and resemble the two- layered ocelli of cer- 

 tain spiders. The space between the lens and retina is com- 

 pletely filled by a layer of very long cells, corneage^i, whose 

 deep, nucleated ends are somewhat swollen and bent away from 

 the centre of the eye. The axial cells of this layer are the longest, 

 and their nucleated ends terminate about half way between the 

 centre of the retina and its periphery (Fig. 8). The abaxial 

 cells are shorter and thinner on the periphery of the eye. It 

 thus happens that there are no nuclei of the corneagen just 

 above the centre of the retina, while there is a distinct layer 

 of them over its periphery, as well as on the walls of the inner 

 half of the eye. 



The periphery of the corneagen contains a thin layer of very 

 large, dark globules, many of which contain a still darker cor- 

 puscle. This layer of pigment-like bodies extends from the 

 edge of the lens to the retina. 



The floor of the eye is formed by a layer of upright retinal 

 cells, each provided with a double rod. In the middle of 

 the retina is a deep furrow, filled with two rows of very large 



