THE PYCNOGONIDS. 65 



and comes to lie under another part of the hypodermis, which 

 forms the outer layer of the eye, and the wall of the invaginated 

 tissue lying outmost forms the middle layer, and the inner the 

 inner layer of the adult eye. The invagination of the cells to 

 form the eye does not take place all at once in the eye of the 

 Sea-Spiders, but is a gradual process, so that a few cells at a time 

 are added to the eye, and this is connected with the presence of 

 a small, free larval form having the beginnings of the eyes. In 

 the Arachnids, on the other hand, the process of invagination is 

 more marked, in that all the cells are invaginated at once. 

 These similarities in development and adult structure, it seems 

 to me, point unmistakably to a community of descent between 

 the Pycnogonids and the Arachnids, and further, they point to 

 the same conclusion which we reached in studying the embry- 

 ology of the group, that the Sea-Spiders show in their adult 

 structures close resemblances to the embryos of the Arachnids, 

 inasmuch as the inversion of the eye of the Pycnogonid agrees 

 with the inversion of the embryo Spider, and the well-defined 

 three-layered condition of the latter in the embryo is seen fully 

 developed in the eye of the adult Pycnogonids. 



Inversion of the Eye. 



The early inversion of the eye of the Spiders has been noticed 

 by Locy, and Mark has extended this discovery to the solution 

 of certain theoretical problems. The latter suggested that the 

 process of invagination of the Spider's eye repeats the method by 

 which such an eye arose, that is to say, if we start with a simple 

 cup-shaped invagination and suppose the walls on *One side to 

 have increased greatly, so that they ultimately filled up the cen- 

 tral cavity, rolling over into this cavity, the inversion of the 

 eye was explained. The opposite wall of the cup remaining thin 

 and developing pigment, became the inner laj^er of the eye, and 

 the general hypodermis of the body formed the secreting cells of 

 the lens or the outer layer. Patten has suggested that the 

 Arachnid eye is but a later stage of development of that seen 

 in the Insect eye. The walls of the optic cup turn in from the 

 two sides, the elements at the sides becoming first horizontal, then 

 turning into the cavity of the vesicle, the retinal cells at the 

 bottom of the vesicle forming a posterior (inner) layer to the 



