MORPHOLOGY OP BRAIN AND SENSE ORGANS OP LIMULUS. G9 



neither these cavities nor the invagination cavities persist 

 till the adult stage is reached. 



Lateral Ventricles. — Referring now to figs. 24 and 25, 

 it is evident the fold (m. c.f.), growing over the cerebral 

 hemispheres, encloses a very broad but flat cavity, correspond- 

 ing to the lateral ventricles of Vertebrates. The cavities are 

 as extensive as the whole upper surface of the cerebral hemi- 

 spheres, and they increase in extent as the latter increase in 

 size. They communicate with each other in front (fig. 25), 

 as in many fishes, and below with the cavity of the semicircular 

 lobes or infundibulum. The roof consists of a thin, non- 

 nervous membrane or pallium, and the floor of the thick pos- 

 terior, anterior lateral, and the median internal, lobes of the 

 cerebral hemispheres. 



The Infundibulum. — On comparing figs. 24, 25, 41, 

 and 43, it is seen that the anterior wall of the cerebral hemi- 

 spheres passes downward and backward into the cavities of the 

 semicircular lobes {s. /.). These two cavities, which are at first 

 separate (fig. 59, s. /.), disappear about the time the lobes in 

 which they lie unite. Had they persisted a little longer they 

 would communicate with each other (as they do in Scorpions), 

 and this broad, backwardly directed cavity thus formed would be 

 similar to that in the infundibulum of Vertebrates. In 

 my first paper on the " Origin of Vertebrates " I advocated the 

 old view that the infundibulum represented a primitive oeso- 

 phagus. I have now abandoned that position for what I con- 

 sider a much stronger one, and we are thus left to account for 

 the ancestral oesophagus in some other way. It is possible that 

 the primitive Arthropod oesophagus broke through the narrow 

 band of nerve-tissue in front of it, and moving forward, was 

 converted into the one we now see in Vertebrates. However 

 that may be, there are excellent reasons for regarding the 

 semicircular lobes of Arachnids as homologous with the 

 infundibulum of Vertebrates : (1) they agree in a general way 

 in shape, and in their position on the hsemal surface of the 

 brain; (2) they develop from invaginations in such a way as 

 to bend the anterior end of the brain-tube downward and back- 



