(374 William Patten 



the niimber and specializatiou of the ommatidia. The centrai eyes i 

 only diifer frorn the lateral ones in the larger number of ommatidia, and 

 in the greater development of the vìtreous layer; in both, the retino- 

 pliorae of each ommatidium are only increased to five in number. Both 

 Limuìus and Scorpio must be regarded as diverging- in the sanie direc- 

 tion from the ancestral form. Limulus being the more remote. 



The Myriapods are characterised by the small number and uni- 

 formity of their ocelli. They in this respect approach more uearly the 

 ancestral form. It is in no way clear what the structure of the omma- 

 tidia in the Myriapods, is. Graber's observations are not complete 

 enough, while Grenacher seems to bave been more intent upon making 

 pretty drawings, than upon elucidating the actual structure of the retina. 

 The observations of both authors are so widely dififerent that it is diffi- 

 eult to select what seems to be the most probable. It would be of great 

 vaine, in determining the relative position of the Myriapod eye, to show 

 whether the rods are terminal, that is forming a retineum, or axial, 

 forming an ommateum similar to those of Scorpions and Spiders. 

 If the former should prove true , as seems probable , then the eyes of 

 Myriapods would be the most nearly related to the presumed, ancestral 

 form. Whether the lateral, horizontal rods are normal is not certain : it 

 might be supposed that they were really situated on the floor, and as- 

 sume their horizontal position by a lateral collapse of the vesicle, caused 

 in some way by the reagents ; or it is possible that they are not rods 

 at ali, but simple stratifications of the vitreous body. 



The comparatively small number of eyes in Spiders indicate that 

 they bave in that respect not departed very much from the ancestral 

 form. But the most interesting fact connected with them is the differ- 

 entiation of the ommatidia in the anterior and posterior ocelli. In ali 

 the ocelli, bere, as in Myriapods, there is an undoubted vitreous layer 

 present. In many anterior ocelli the rods are apical, that is, retain 

 their primitive, terminal position, forming a retineum. In the larger, 

 posterior eyes, there is a tendency for the rods to become axial, form- 

 ing au ommateum. That this type of retina has not departed far from 

 the originai condition, is shown by the primitive double rods, and con- 

 sequently, the double retinophorae. The manner in which a double, 



i 



1 Of course it must be recollected that both Lankester and Grenacher 

 deny the existence of a vitreous layer in the lateral eyes of Scorpions and Limulus, 

 while Graber states that in the former such a layer is present ; for theoretical 

 reasons I aui obliged to assume that this layer is always present. 



