202 THE APODID^: PART n 



nerves. Their very length, when compared with the 

 usual distance between eyes and brain throughout the 

 animal kingdom, is a clear indication of displacement. 



The anterior pair of Annelidan eyes, which in Apus 

 went to form the unpaired " eye," are represented in 

 Limulus by a pair of ocelli. The wandering of these 

 ocelli on to the dorsal surface can still be traced in 

 the course of the animal's development. According 

 to Packard, the ocelli at their first appearance in the 

 embryo are on the ventral side, and travel on to the 

 dorsal side before the young animal is hatched. The 

 true significance of this fact has already been dwelt 

 upon, and has been compared with a similar, though 

 not so pronounced, wandering of the eyes in the 

 Nauplius as shown in Figs. 36 and 37. The presence 

 of the ocelli on the ventral surface of any ancestor 

 of Limulus would be difficult to explain by any 

 other theory than that of our bent Annelid. The 

 nerves to these ocelli branch from the brain from the 

 same place as do those to the unpaired " eye " in 

 Apus, i.e. from between the optic nerves. 



Judging from the lateral retinae of the sensory body 

 of Apus, and also from the fact that the posterior 

 eyes are compound, we should have expected com- 

 pound eyes and not ocelli as the anterior pair in 

 Limulus. In certain Trilobites (e.g. Harpes), accord- 

 ing to Barrande, these ocelli are not single but com- 

 posed of groups. 1 It seemed to us that these might 

 perhaps form an interesting connecting link between 



1 See Packard's paper on the structure of the eye of Trilobites. 

 American Naturalist^ July 1880. 



