THE PYCXOGONIDS. 67 



The middle layer (and this is more to the point) shows a strong 

 bilateral arrangement — first, in respect to the nuclei which lie 

 largely on the two sides, and, secondly, in the arrangement of 

 the retinal elements to the right and left of a median raphe. 

 The third layer has a bilateral arrangement, the cells along the 

 middle being different from those along the sides of the eye, and 

 those at the sides approximating in structure to the retinal 

 elements. 



It does not seem to me that it is possible to interpret this 

 bilaterality on Mark's hypothesis, because it furnishes not the 

 slightest reason why an eye so derived should show a two-sided 

 arrangement, nor is it likely to be explained as a secondary 

 arrangement. 



On the other hand, if we assume the inversion to have arisen 

 by a turning in of two sides of an optic cup — a process already 

 shown in the Insects' simple eyes — we see clearly why the eye 

 should present so perfectly a bilateral plan. To explain this 

 more in detail, we may start with a simple invagination, forming 

 a thick-walled cup with nerve fibers entering the inner ends of 

 the cells. The early stages of development of Insects' simple 

 eyes show such a condition. Next, the lips of the invagination 

 touch and fuse, forming an almost continuous layer over the eye, 

 giving the corneal hypodermis. The bilateral arrangements of 

 this layer in the Pycnogonids may be the expression of this union 

 from the two sides. At the same time that this occurs, the more 

 horizontally-lying retinal elements in the upper part turn into 

 the cavity of the cup, so that their morphologically outer ends 

 now come to point inwards. The nerve fibers which ran up to 

 the back part of the eye continue to do so, and, running from 

 their point of contact around the outer wall of the cup, supply 

 the elements with fibrils. This is practically the condition of 

 the simple eyes of Insects. Such an eye may be said to be 

 (imperfectly) three-layered at this stage, as we may now count 

 the partially inverted elements as a middle layer. The elements 

 lying at the base of the cup are in a disadvantageous position for 

 vision, being covered by a sensory layer (the inverted fibers) 

 lying between them and the light. So these elements of the 

 bottom of the cup and at the sides degenerate and become a pig- 

 ment layer at the back of the eye. JSTow we have reached the 

 type of eye found in the Sea-Spiders. 



