SURVEY OF THE ARACHNIDAN EYES. 



73 



single optic cells. According to Grenacher, the rods in the eyes 

 of all Araneids consist of two parts, and thus appear split longi- 

 tudinally ; in Phalamjium, each rod is composed of three parts, and, 

 in transverse section, has the shape of a trefoil. Although it is 

 expressly asserted that the rods lie in the cells, we cannot avoid 

 conjecturing that this hi- or tripartite character of the rods is 

 perhaps a vestige of the rhabdom- and retinula-formation.* 



The relations of the various pairs of Araneid eyes to one another are rendered 

 difficult to understand by the differences in structure and development found in 

 them. We are at first disposed to connect the anterior median eye with the 

 median eye of Scorpio, and the other eyes with the lateral eyes of Scorpio. We 



a 



36. 



Fig. 3S. — A, anterior, B, posterior median eyes of an AraneM (diagrammatic, after Grenacher 

 and Bertkau). ch, chitinous cuticle passing into the cuticular lens (?); gl, vitreous body ; 

 h, hypodermis ; 1, lens ; n, optic nerve ; r, retina ; st, rod ; t, tapetum. 



cannot, however, reconcile with this view the fact that the posterior median eyes 

 and the lateral eyes develop in almost the same manner as the anterior median 

 eyes, while the lateral eyes of Scorpio form in a very simple way (Fig. 11, p. 17). 

 We might therefore better trace back all the Araneid eyes to a breaking-up of 

 the median eyes into separate complexes, such as has been assumed for the 



* In the eyes of some Araneae {e.g., Atypus), the vitreous body layer appears 

 to be exceedingly thin. Bertkau (No. 50) compares these eyes to the ocelli 

 of the Insecta, and states that they obliterate the distinction between uni- 

 laminar and multilaminar eyes. Those ocelli with which these Araneid eyes 

 are compared already show a vitreous body separate from the retina (in 

 Phryganea and Vcspa, Grenacher), i.e., the vitreous body and the retina 

 no longer form one continuous layer. In this respect, these eyes have a 

 structure similar to that of the Araneid eyes, and are removed from that of 

 the unilaminar ocellus. According to the view adopted above, the median eyes 

 of the Araneae, which so strikingly agree with the Inscctan ocelli, must not be 

 considered as homologous with the latter, but we must assume that this appar- 

 ently similar development was brought about in different ways. 



