THE ARTHROPODA. 419 



We have no indication of the manner in which the facet-eye has 

 arisen in the series of the Crustacea. Xone the less must we 

 consider that this eye, which closely resemhles the facet -eyes of 

 the Insecta, arose in the same way as the latter. Any deviations 

 that may occur, such as the presence of another cell-layer in the 

 ommatidium (Fig. 197 D, l.g), are to be explained simply by the 

 inclusion of another row of cells in the hypodermal depression, as 

 already shown. 



The structure, development, and relations of the unpaired median 

 eyes in the Crustacea are still little understood. It has recently 

 been asserted that they arise by inversion (Claus, Xo. 3), and since 

 this method of formation is characteristic of some of the eyes 

 found in Limulus and the Arachnida, relations between the median 

 Crustacean eye and the median eye of Linndus and the Scorpiones, 

 as well as the so-called principal eyes of the Araneae, are suggested. 



The eyes of the Arachnida belong to a third ontogenetic series. 

 They have only one lens, and are thus devoid of the characteristic 

 feature of facet-eyes, but in the eyes of Scorpio we find a grouping 

 of the cells into retinulae and the formation of rhabdoms within 

 these latter, and in this respect they may claim to be compound 

 eyes. We considered ourselves justified in explaining the common 

 lens as having arisen by the flowing together of distinct corneal 

 lenses (p. 71, etc.), and find in the lateral eyes of Limulus, which 

 also show rhabdom -formation, an indication of such a fusing of 

 the lenses. We tried further to show it to be probable that the 

 ■eyes of the Araneae, which in their present form appear to be simple 

 eyes, are to be derived from compound eyes, this origin being still 

 indicated in their development and their structure. It is highly 

 probable also that the compound eyes of the Arachnida, like those 

 of the Insecta, arose through the accumulation of simple hypodermal 

 depressions resembling ocelli. 



When we turn to the ontogenetic formation of the Arthropod 

 -eyes, we find that the simple forms arise as pit-like depressions of 

 the ectoderm. In the higher forms, i.e., in the compound eyes, 

 this primitive method of formation is obliterated. The single eyes 

 here arise merely through the differentiation of a cell-layer without 

 special depressions. Where such a depression is found in the 

 development of a compound eye, it leads to the formation of the 

 eye as one whole. In this last process, as in the differentiation of 

 the single eyes out of a multilaminar cell-layer, we have secondary 

 phenomena representing a simplified method of formation of the 



