74 



ARACIIXIDA. 



lateral eyes of Scorpio* In this case the lateral eyes would be altogether 

 wanting in the Araneae. The differences in structure in the different pairs of eyes 

 are, indeed, very remarkable ; but, in the present state of our knowledge, it is 

 not possible to explain them in a satisfactory manner. 



In connection with the infolding of the Arachnid eyes and the 

 secretion of the lens over that part of the fold which lies near the 

 hypodermis (Figs. 35 and 10, p. 14), the elements of the retina 

 undergo rearrangement. The part of the cells formerly turned out- 

 ward is now directed inward. It carries the rods, as it also does even 

 in the so-called accessory eyes (lateral and posterior eyes) of the 

 Araneae (Fig. 38 B). The nerve-fibres become connected with the 

 ends of the cells which were formerly turned inwards, but are now 

 turned outwards. At these ends lie the nuclei of the retinal cells. 

 This seems to be the definitive condition in the accessory eyes, and 

 corresponds at the same time to the condition before infolding, apart 

 from a few r modifications that may still take place in the innervation 

 (p. 67). It is otherwise with the so-called principal (anterior median) 

 eyes of the Araneae and the median eyes of the Scorpiones. In the 

 former, the rods lie at the outer (distal) ends of the retinal cells, 

 while the nuclei lie internally (Fig. 38 A). The nerve-fibres join 

 the nucleated proximal ends of the cells. This is also the case in 

 the median eyes of the Scorpiones (Fig. IOC). A displacement has 

 therefore taken place here as a consequence of the infolding. The 

 absence of this displacement in other Araneid eyes may perhaps be 

 explained by the development of the tapetum by which the light is 

 reflected back on to the ends of the rods that turn inwards. 



The observations of Kishinouye and Purcell quoted above, according to 

 which inversion does not take place in the developing accessory eyes, again 

 throw doubt on the manner of innervation of these eyes which has till now 

 been considered likely, and lend probability to the repeated assertions that the 

 nerve enters from behind. 



From what is as yet known of the structure and development of the Arachnid 

 eyes, we are not justified in doubting that displacement occurs. We have not, 

 indeed, sufficient details of this process, which has been followed chiefly by 

 Mark and Parker, to be able to judge fully as to its occurrence. Mark has 

 specially devoted his researches to the morphology of the process. In order to 

 understand his view we must, however, briefly touch upon another theory 

 concerning the origin of the Arachnid eye. 



We derived the compound eyes of the Scorpiones and that of Limulas from 

 more highly-organised (facet) eyes ; another view, adopted specially by Ray 



* The recent statements regarding Kishinouye and Puroell's observations 

 (p. 68), according to which the anterior median eyes arise through inversion, 

 while the other eyes do not, render highly probable the first conjecture that the 

 former are to be traced back to the median eyes of the Scorpiones, and the latter 

 to the lateral eyes of that animal. 



