64 T. H. MORGAN. 



the slight development of lens in some forms), and this could not 

 be accounted for had the group first acquired an eye so different 

 from the Annelids, and afterward, without change of habitat, 

 degenerated. Further, we find a remarkable agreement with the 

 Arachnid eye, and it seems much more probable that this (even 

 alone, but we can consider it now in connection with many other 

 peculiarities, both in structure and development, common to the 

 two groups) has a phylogenetic meaning, and forces one to 

 abandon the theory advanced by Dohrn. 



First 1 must state that the principal papers which I have used 

 in the following comparison are those by Lankaster and Bourne 

 on the Simple Eyes of Limulus, Locy on the Development of the 

 Spider's Eye, and Mark on the same subject, Parker on the Eyes 

 of Scorpions, and Laurie on the Development of the Eye of the 

 Scorpion. Of the more speculative papers of Patten and Watase 

 I shall speak separately. 



The adult eyes of the Sea-Spiders show three distinct layers — 

 the corneal hypodermis, the retinal or middle layer, and the pos- 

 terior or pigment layer. It has been demonstrated beyond doubt 

 that the Arachnid median eyes are composed of three layers, 

 and, further, that these are more apparent in the embryo than in 

 the adult Arachnid. Further, the arrangement of the layers is 

 essentially the same, for we find a corneal hypodermis, a middle 

 sensory layer, and a posterior narrow layer. 



The middle layer in the embryo Spiders (where the eyes have 

 been studied in most detail) contains at the inner ends of the 

 retinal cells clear (stainable) rod-like bodies, or bacilli, which 

 seem to be identical with the similar bacilli of Pycnogonids. 

 The bacilli in the Pycnogonids are at the inner ends of the cells — 

 in other words, the retina is inverted — and in the embryo Spiders 

 we find that similar rods are also at the inner ends of the cells — 

 that is, the retinal elements are here also inverted. A secondary 

 shifting, or change, takes place in the Spider's eyes, so that in 

 the adult the eyes and retina become righted ; but how this later 

 change comes about is at present, I believe, unknown. 



Lastly, the eyes of the Pycnogonids are formed by a process of 

 invagination of the same nature as the invagination in the 

 Arachnids (Locy, Parker, Laurie, Brooks). This invagination, 

 instead of being an open cup (as in Insects), turns to one side 



