MUSEUM OF COMPARATIVE ZOOLOGY. 203 



the phylogenetic antecedent of the median eyes, a condition has been 

 implied which agrees with the essential features of the lateral eyes. Of 

 all the eyes in spiders and scorpions, the lateral eyes in scorpions are un- 

 doubtedly the least complicated, and they may be looked upon as deviat- 

 ing least from the probable ancestral type. 



Summary of Results. 

 No9. 2-11 refer to the median eyes ; No8. 12-17 refer to the lateral eyes. 



1. The retinas of the median and lateral eyes are strictly hypodermal in 



their origin. 



2. The median eye is triplostichous, and is formed by an involution of 



hypodermis accompanied with an inversion of the middle layer, 

 which forms the retina proper. 



3. The first layer or leiitigen, is modified hypodermis immediately ex- 



ternal to the pocket of involution, and, in addition to secreting 

 the lens, serves a purpose which gave to it its earlier name of 

 ■'vitreous." 



4. The lens is the specialized cuticula produced by the lentigen. it 



differs from ordinary cuticula in containing no pore-canals, a^id, 

 excepting the external hyaline layer, in being stainable through- 

 out. 



5. The lentigen can produce cuticula independently of the general 



hypodermis. 



6. The second layer, or retina, is inverted, and consists of +wo kinds of 



cells, — retinal (nerve-end) cells and pigmftnt cells. It contains 

 phaospheres. 



7. The retinal (nei've-end) cells contain pigment ; their walls are thick- 



ened into prenuclear rhabdomeres, and a nerve fibre emerges from 

 their deep ends. They are so arranged in groups of five, that five 

 rhabdomeres are united to form one rhabdome. 



8. Each of the pigment cells is reduced to two sacs, connected by a stiff 



fibre. The external sac contains pigment ; the internal, the 

 nucleus and pigment. 



9. The third or post-retinal layer is the " sclera matrix " of Graber. 



It becomes intimately fused with the retina. 

 10. The fibres of the optic nerve in the embryo emerge from the external 

 ends of the inverted retinal cells ; in the adult, from the opposite 

 ends. 



