MUSEUM OF COMPARATiyE ZOOLOGY. 89 



I know of no example among Arthropods in which this condition is 

 strictly realized, provided the still problematic development of the lateral 

 eyes of scorpions is not taken into consideration. Even the simplest are 

 considerably modified. 



It is not certain along what line of modifications the eye with inverted 

 retina has been developed. Not all triplostichous eyes are necessarily 

 like the pre-nuclear type in spiders. A triplostichous condition might 

 be produced by a simple depression of the retinal area and a subsequent 

 closing together of the surrounding hypodermis, ultimately giving rise to 

 an inner and an outer corneal layer, as in many of the mollusks. The 

 condition of the eye in Peripatus suggests such a method of formation in 

 this primitive Tracheate. 



It is not unreasonable to suppose, however, that all the triplostichous 

 eyes have passed through a condition of simple sac-like depression, in 

 which originally the retinal cells are not inverted, and that from this 

 simple condition two others have originated, — (1) By a closing together 

 and fusion of the lips of the original depression a more or less volu- 

 minous cavity (filled with a so-called lens) is formed in front of the still 

 uninverted retina and behind a double layer of hypodermis, — a triplos- 

 tichous condition such as is realized in Peripatus (Carriere, '85, p. 124 ; 

 Kennel, '86, p. 32, Taf. Ill, Fig. 34). (2) By an approximation of the 

 walls of the depression its cavity is reduced to an axial fissure ; the cells 

 corresponding to the " outer cornea " in the first case become the " lenti- 

 gen ; " those corresponding to the " inner cornea " become a " vitreous ; " 

 the retina still remains uninverted, — a monostichous (potentially tri- 



(Pecten, Spondylus, etc.). It seems to me there is little doubt but that in both 

 these cases there has been at some time an inversion of the retinal area. The pecuHar 

 course of the optic-nerve fibres and their method of joining the sensory cells (at their 

 anti-bacillar ends), as well as the position of the bacilli, point to this conclusion. 

 They are not, it will be observed, in any sense monostichous eyes. 



The eyes of Planaiians, also, may possibly be interpreted as having bacilli of the 

 "post-nuclear" type; but here, too, the course of the nerve-fibres points to an in- 

 version of the retina, and, in addition, it is doubtful if the eye is monostichous. 



Postscript. — Although Dr. Patten informs me that there is no inversion of the 

 retina in the case of Pecten, I believe that an inversion at some time during the 

 phylogeny of the eyes of Pecten has been the cause of their present condition. But 

 whether there is an inversion during the ontogeny of Pecten or not, the question im- 

 mediately before us is little affected by it ; for eyes like those of Pecten are already 

 too complicated to have served as the primitive condition of the triplostichous ocelli 

 of Arthropods. It may therefore still be safely assumed that the cells of the primitive 

 ocelli had ^re-nuclear bacilli. 



