368 THE PINEAL ORGAN 



in the invertebrate phylum Arthropoda has involved the median paired 

 eyes of certain groups — for instance Apus, Branchipus, Scorpio, Limulus — 

 whereas in other examples the median paired eyes have retained the 

 primary upright character, as in Acilius (Fig. 82, Chap. 11, p. 120), the 

 blowfly (Fig. 9, Chap. 3, p. 14), and the median or parietal eyes of verte- 

 brates (Fig. 134, Chap. 17, p. 188, Fig. 183, Chap. 20, p. 259, and 

 Fig. 252. 



Besides this differentiation in the extent of the superficial area of the 





Fig. 251. — A Transverse Section of the Right Median Eye of an Adult 

 Scorpion. (After G. H. Parker.) 



Compare with Fig. 253, D. The primary cavity has been obliterated by 

 adhesion of the retinal and post-retinal layers, along the line marked fis. 



c, c'., c." : outer, middle, and inner 



layers of cuticle. 

 hyp. : hypodermis. 

 lens : cuticular lens. 

 nib. : basement membrane. 

 m. pr. r. : pre-retinal membrane. 

 mes. : thin mesodermal layer between 



retina and pre-retina. 



n. pr. r.c. : nucleus of pre-retinal cell. 



pig. : pigment. 



po. r. : post-retina. 



pr. r. : preretina (lentigen or vitreous 



layer), 

 r. : retina. 

 rhb. : rhabdites. 

 scl. : sclera. 



derma which is involved in the formation of an optic placode, there is a 

 tendency in the more highly differentiated eyes to increase in depth. 

 The central neuro-sensory cells become more and more depressed and 

 at the same time elongated ; moreover, their distal ends become modified 

 to form vertical rods, whereas the distal ends of the cells lining the sides 

 of the pit form short rods which tend to become horizontal, and near the 

 mouth of the pit secrete a vitreous mass which serves as a lens. In the 

 adult eyes of the higher types of insects and many of the Crustacea the 



