72 ARACHNIDS. 



agreement. Both the lateral and the median eyes of the Scorpion 

 appear to be compound eyes, the one kind, however, showing more 

 than the other a tendency towards the suppression of the distinction 

 of the primary single eyes and towards the formation of one homo- 

 geneous eye. The fusion of eyes formerly distinct would, if we 

 may follow this idea further, become continually closer, and finally 

 lead to the formation of an eye in which the single eyes as such 

 would be hardly recognisable or altogether indistinguishable. It 

 does not appear to us possible that the eyes of the Araneae have 

 reached this stage, although it will perhaps be considered para- 

 doxical first to trace the evolution of the simple eye into the 

 compound eye, and then to derive from this latter an eye which 

 we are accustomed to claim as a simple eye. 



The eyes of the Araneae show, especially in their manner of 

 development, the greatest agreement with the eyes (median eyes) 

 of the Scorpiones, apart from the fact that by their position 

 they prove themselves to be homologous structures. The process 

 of infolding so strikingly resembles the similar process in the 

 Scorpiones, that Ave are obliged to consider the two as equivalent, 

 and thus also to consider the Araneid eyes as more highly developed 

 than their structure leads us to expect. The eyes of the Araneae 

 are usually regarded as ocelli, and are compared with the ocelli of 

 the Insecta. Their structure appears to justify this view, for the 

 retina is composed of a continuous layer of similar cells (Fig. 38 A 

 and B). The development of this layer is, however, more com- 

 plicated than in the simple eye ; in manner of formation it resembles 

 the eye of Scorpio (Fig. 35, p. 65, and Fig. 10, p. 14). The multi- 

 laminar character is not, as in the ocellus, caused simply by the 

 hypodermal layer pressing forward over the retina, but is the result 

 of a process of infolding (Figs. 35 and 10). This striking resem- 

 blance to the eye of Scorpio alone inclines us to regard the Araneid 

 eye as a compound eye* By the breaking-up of the retinulae, the 

 uniform character of the retina would again be attained. There 

 are besides, in the structure of the Araneid eye, certain indications 

 which seem to support this view, and which would lead us to 

 conclude that the retina is not composed of a uniform series of 



* [Purcell (App. to Lit. on Opiliones, No. VI.) finds that the eyes of the 

 Opiliones are three-layered inverse eyes, arising in a precisely similar way to 

 the principal eyes of the Araneae and the median eyes of Scorpio. They contain 

 retinulae, each composed of four cells surrounding a single rhabdom. Purcell 

 claims to have definitely proved that a retina composed of retinulae or of a 

 modification of these occurs in the Opiliones and in the Araneae. — Ed.] 



