14 



ARACHNIDA. 



probable that, before the complete separation of the brain from the 

 cephalic pits, the apertures of the two invaginations (Fig. 9, A, e) 

 shift backward and towards the middle line, so as to fuse later to 

 form a common depression. This change is evidently due to a 

 process of growth, by which that portion of the body lying between 

 the pits is gradually drawn into them. If we rightly understand 

 Laurie's description, the common pit seems to be of large extent, 

 but rather shallow (Fig. 9, B, e). It lies immediately in front of the 

 chelicerae which have already developed as pincers. 



The position of the chelicerae in relation to the cephalic pit is 

 somewhat difficult to determine from the illustrations available. In 

 Fig. 12, the pits at several stages are indicated in outline; but 

 unfortunately Patten's description does not enable us fully to 

 understand his figure. 



n .-,_ 



Fig. 10.— Sections through three stages of development of the median eyes in Scorpio (A, 

 after Parker, B and C, diagrammatic), g, brain (?) ; gl, vitreous body; h, hypodennis; 

 I, lens; n, optic nerve ; pr, post-retinal layer; r, retina ; rh, rhabdom. 



The outer edge of the pit grows towards the middle line, thus 

 roofing it in and causing an approximation and ultimate fusion of 

 the two apertures, and the formation of a single bilobed pit in place 

 of the two originally distinct ones (Figs. 9, C, and 12, F). The 

 outer wall of the pit lying under the ectoderm or hypodennis 

 thickens (Fig. 9, C), while the inner wall remains thin and unilam- 

 inar. The whole depression, which is somewhat closely apposed to 

 the hypodennis, becomes flattened dorso-ventrally, so as to appear like 

 a flat pouch '(Fig. 10, A). A right and a left portion can, however, 



