THE PYGNOGONIDS. 69 



I am inclined to push this a step further and attempt to show 

 that the median Arachnid eyes have developed along a similar 

 path, and this must be a necessary corollary to the position to 

 which I have assigned the Pycnogonids, and believe, as 1 have 

 before said, that the inversion of the Spider's eye is its Pycno- 

 gonid stage of evolution and the righting of it a secondary and 

 later change. I also believe there is no evidence that in the 

 evolution of the Spider's eye a change of position in the lens has 

 taken place. 



General Consideration of the Simple Eyes. 



Before leaving the subject there are two points to which I 

 must refer — the meaning of the invagination of Arachnid (and 

 Pycnogonid) eye, and the explanation of the grouping together 

 of the retinal cells into bundles or retinulse. 



It will be at once seen, with respect to the first point, that I 

 have placed more faith in the evidence furnished by the adult 

 eye of the Pycnogonids and Spiders than in that given by the 

 ontogeny of those eyes, while Mark has utilized the ontogeny 

 as furnishing a basis for his considerations. 



It seems to me that the development of the eye, as we find it 

 in the Pycnogonids, is entirely an ontogenetic process, i. e., it 

 represents here an abbreviated condition. Instead of recapitu- 

 lating all the early stages, forming first a cup, then turning in 

 the walls, it performs the whole process at one time and thus 

 complicates the matter. In the Pycnogonids the presence of the 

 eyes in all the larval stages, presumably functional in each, must 

 have changed the original process to a very great extent. 1 Yet 

 through all these stages we have been able to recognize the 

 three-layered condition of the eyes. We must believe, from 

 what has been said of the growth of the eye, that in the Pycno- 

 gonids the invagination (of the Arachnid type) has been retarded, 

 so that, one end of the invagination being formed, the in turned 

 and inverted cells have functioned as larval eyes, and as the 

 animal increased in size the invagination kept pace, adding more 

 and more cells to the layers of the eye, so that all of the stages 

 had presumably functional eyes, and at the same time the larva 

 retained the original type of Arachnid invagination. 



1 We do not know whether the ancestral Arachnids had free-living larva 1 

 or not. 



