MORPHOLOGY OF BEATN AND SENSE ORGANS OF LTMULUS. 73 



This method of growth forces the concave margins together 

 and produces a choroid fissure. The shape and method of 

 growth of the vertebrate retina are inexplicable, unless we 

 assume it to be inherited from ancestral forms whose eyes 

 must of necessity have had the shape and position that the 

 eyes of Vertebrates would have if carried back to the exterior 

 where they originally belonged. The only Invertebrates having 

 such eyes are the marine Arachnids, Trilobites, Limulus, and 

 Mirostomata, &c. 



If we turn again to fig. 24 we see that the space over the 

 mid-brain uncovered by the marginal folds {m. /. cr,) repre- 

 sents the imperfectly roofed-over cavity of the Sylvian aque- 

 duct, and that it extends forward around the posterior median 

 margins of the hemispheres into the uncovered lateral ven- 

 tricles as a shallow groove (F. M.) which corresponds in 

 position to the foramina of Monroe. Before a complete roof 

 can be formed to the mid-brain {Ne}), and to the third fore- 

 brain segment or the ''tween brain" (iVe.^), by the union 

 of their medullary folds, the cerebral hemispheres grow back- 

 ward, as they do in Vertebrates, and, uniting in the median 

 line, completely cover them (fig. 25). Therefore the cavity in 

 Limulus corresponding to the ''iter" must communicate freely 

 above with the fifth ventricle, or the space between the inner 

 median faces of the hemispheres (fig. 52, v.). But if the 

 marginal folds of the embryo persisted they would occupy 

 about the position indicated by the dotted line in fig. 52, and 

 if they united completely two distinct chambers {v.^ and i.) 

 would be found corresponding exactly to the " iter'^ and to 

 the fifth ventricle of Vertebrates. 



If the marginal folds of the ventral cord united (fig. 32) 

 they would produce a medullary canal, bounded at the bottom 

 by the Mittelstrang, on the side by the ventral cords, and on 

 the roof by the united medullary folds. 



Owing to the divergence of the cords in the hind-brain 

 region (fig. 48) a great rhomboidal cavity with a thin roof 

 would be formed that would correspond to the fourth ven- 

 tricle. 



