386 W. H. GASKELL. 



pineal eye. These nervous masses have enclosed the walls of 

 the stomach in this region to such an extent that only a small 

 portion of its surface remains free, and is not utilised for the 

 purpose of lining the cavities of the brain in this region. Its 

 anterior rounded extremity forms the lamina terminalis, 1. 1., 

 the surface of which continued backwards up to the ganglia 

 habenulcC is slightly folded^ and spans the space between the 

 two optic thalami forming the choroid plexus i of Ahlborn. 

 Passing round to the ventral surface,, the stomach wall forms a 

 bulging known as the recessus chiasmaticus. Laterally it 

 forms the lining of the optic thalami, and is compressed by 

 the growth of the nervous matter of the cerebral and olfactory 

 lobes so as to form the diverticula called by Ahlborn the 

 lateral ventricles (fig. 6, PI. XXV ; also Ahlborn's fig. 37, PI. 

 xv). The appearance presented by a longitudinal vertical sec- 

 tion in the middle line through the brain of an Ammocoetes is 

 given by Ahlborn in fig. 41, PI. xvi ; and if the choroid plexuses 

 were dilated we should have an appearance somewhat as in the 

 diagram on Pl.XXyiII,fig.29,where the red line represents the 

 contour of the cephalic stomach of such an animal as Limulus, 

 and the yellow line represents the modifications of contour 

 which have occurred by the growth and compression of nervous 

 material in the case of the Ammocoetes. In fact, it is perfectly 

 clear that in this primitive form of Vertebrate the non- nervous 

 epithelial bag which was originally the cephalic stomach of the 

 Crustacean-like ancestor of the Vertebrate is more conspicuous 

 and stands out more prominently with respect to the nervous 

 system than in the higher Vertebrates ; in other words, the 

 relation between the size of the nervous system and of the 

 cephalic stomach approaches to that which occurs in the Crus- 

 tacean as we descend from the highest to the lowest Verte- 

 brates. 



Sect. 2. — The Mouth and (Esophagus. 



Again, we see in this animal more clearly than in any other 

 Vertebrate the remains of the old Crustacean oesophagus. 

 Prom the central cavity a broad passage (the infundibulum) 



