68 ARACHNIDA. 



they would show great agreement with the eyes of the Scorpiones. 

 The anterior median eyes of the Araneae, which develop through 

 inversion, would then correspond to the median eyes of Scorpio, which 

 arise in a similar manner ; the posterior median and the lateral eyes 

 of the Araneae would be comparable to the lateral eyes of the 

 Scorpiones, which also originate as simple ectodermal depressions 

 {without inversion). If so, Bbrtkau's division of Araneid eyes into 

 "principal" and "accessory" eyes would have some significance. 



The fact that these very important results obtained by Kishinouye were not 

 accorded the first place in our account is partly due to their becoming known to 

 us only during the publication of this book. Again, compared with the state- 

 ments of former investigators, they did not appear sufficiently supported. It 

 should be noticed, however, that researches made by F. Purcell, independently 

 of the work of Kishinouye, on the structure and development of Araneid eyes, 

 led to similar results. Purcell also thought that the lateral eyes arose in the 

 form of slight depressions of the ectoderm, over the bases of which (retinae) 

 the lateral parts grow in as vitreous bodies. This seems to agree with the 

 structure of the adult eyes, which, together with Purcell's ontogenetic 

 observations, will be further examined later on. 



C. Survey of the Arachnidan Eyes. 



In order to understand the development and structure of the 

 Araneid eyes, it is necessary to compare them with the eyes of 

 the Scorpiones. Whereas the eyes of the Araneae are usually 

 regarded as simple eyes, i.e., as so-called ocelli, the retinae of which 

 show no regular grouping of cells (retinula formation, Fig. 34 A and 

 B) ; the eyes of the Scorpiones have retinulae, i.e., groups of cells 

 with a central rhabdom* (Fig. 11 B, p. 17, and Fig. 10 C, p. 14). The 

 eyes of Scorpio, especially the median eyes, thus show an essential 

 feature of the compound eye, although, like the Araneid eye, they 

 possess a single corneal lens not divided into facets, and therefore 

 are without another important feature of the compound eye. We 

 must, nevertheless, regard them as compound eyes, if only as 

 modified eyes of this sort, as will be proved by the following 

 considerations. 



* In the simple Arthropodan ej*e {ocellus, stemma, etc.) the retina consists of 

 a layer of similar optic cells (Fig. 36, r) ; in the compound eye, however, it 

 breaks up into groups of cells, the retinulae (Fig. 10, p. 14). These cells, which 

 vary in individual cases, carry, at the sides which are turned towards the centre 

 of the group, optic rods (rhabdomcres), which may fuse to form a single structure, 

 the rhabdom (Figs. 37, rh, and 10, p. 14). Each group of the retinulae, 

 together with the superjacent cells of the vitreous body and the corneal facets 

 belonging to them, forms one of the ommatidia (Fig. 198 D), which in larger 

 or smaller numbers compose the eye. This at least applies to the typical form 

 of compound eye ; the Arachnidan eye now under consideration is somewhat 

 different. 



