64 



ARACHX1DA. 



their position is so slight that nothing definite can be asserted about 

 them. It appears, however, to be certain that the four pairs of eyes 

 arise from different infoldhigs (Mark). 



The way in which the different eyes arise varies, the anterior 

 median pair differing in this way from the two posterior median eyes, 

 and from the two lateral pairs. There is a corresponding difference 

 in structure : in the anterior median eyes the rods of the retinal cells 

 lie external to the nuclei (Fig. 34 A, st), while in the other eyes the 

 rods are found internal to the nuclei (Fig. 34 B, st). In the first case, 

 the nuclei lie at the base of, in the second, at the outer ends of, the 

 retinal cells. The anterior median eye, further, has no tapetum 

 (Fig. 34 B, t), i.e., no layer of cells lying behind the retina filled 



a 



36. 



Fig. 34.— Anterior (A) and posterior (77) median eyes of an Araneid (diagrammatic, after 

 Grenacher and Bertkau). eh, chitinous covering of the body ; gl, vitreous body ; 

 h, hypodermis; 1, lens; n, optic nerve; /•, retinal cells ; st, rods; t, tapetum. 



with shining granules, such as occurs in the other eyes, and appar- 

 ently also determines a difference in their methods of development. 

 Following Bertkau, we shall, for the sake of brevity, call the 

 anterior median eyes principal eyes, the posterior and lateral eyes 

 accessory eyes. These two kinds of eyes are distinguished, not only 

 in structure, but possibly also in the method of their development 

 (see below), a fact which justifies us in making this distinction. 

 The last point, however, requires further elucidation. 



Our present knowledge concerning the origin of the eyes in the Araneae is 

 chiefly due to the researches of Locy (No. 64) and Mark (No. 67), carried out 

 on Agalcna naevia. The following account, therefore, refers specially to this 

 form. Kishinouye (No. 62), recently made some investigations of the develop- 



