96 THE APODID^: PARTI 



eyes has taken place. The length of the stalks of 

 the optic nerve, and the secondary drawing out of 

 the longitudinal commissures of the anterior ventral 

 ganglia, point to the same conclusion. 



2. We find the eyes close together, ?>., about as 

 near to one another as they probably were on the 

 prostomium. This point is important, because it is 

 often assumed that the eyes of Apus have moved 

 together from the sides towards the middle line, 

 whereas, on the contrary, we hold that the eyes of 

 Apus have kept about the same distance apart as 

 they were on the prostomium of the Annelid 

 ancestor, and that it is the eyes of the higher 

 Crustacea which wander apart and take up positions 

 at the sides. The gradual reduction of the dorsal 

 shield, in the majority of the descendants of Apus, 

 facilitates the wandering to the sides. The case of 

 Limulus is particularly interesting. Here, as will be 

 described later, the eyes wandered from the first, not 

 forwards and upwards, but sideways and upwards, so 

 that the brain could not follow as in Apus, but, being 

 drawn in two opposite directions, remained where it 

 was, the extraordinary length of the optic nerves 

 showing clearly that the eyes must have wandered 

 considerably. 



Almost more interesting, however, are the water- 

 sacs which spread out over the eyes of Apus, 

 between them and the integument. These have never, 

 we think, been described before, at least in detail, 

 and here deserve particular attention as lending 

 support to our theory of the migration of the eyes. 



