VERTEBRATES FROM A CRUSTAOBAN-LIKE ANCESTOR. 385 



Sect. 1. — The Cephalic Stomach. 



The brain of the Ammocoetes^ then, differs markedly in ex- 

 ternal appearance from that of higher Vertebrates; and one of 

 its most striking peculiarities consists in the fact that nearly 

 the whole of its dorsal surface is composed of membranous 

 folds, and not of nervous material. The large amount of sur- 

 face which these folds represent is manifest upon inspection of 

 the external appearance of the brain, and is clearly visible in 

 transverse sections such as are represented by figs. 2, 3, 9, PI. 

 XXV. These membranous folds form the choroid plexuses ii 

 and iii of Ahlborn, and may be looked upon as a folded crumpled 

 globular bag which stretches from the commencement of the 

 fourth ventricle to the posterior commissure and ganglion 

 habenulse ; this folded bag has been constricted into two 

 parts, which form the roofs of two cavities, viz. the fourth 

 ventricle and the cavity of the mesencephalon, by the presence 

 of a narrow constricting band formed by the fourth nerve and 

 the incipient cerebellum (see also Ahlborn). This bag, if un- 

 folded, would, in accordance with the view put forward in 

 my previous papers, form the main part of the large globular 

 cephalic stomach of the Crustacean-like ancestor ; it is only 

 possible to imagine its expansion on the dorsal side, for ven- 

 trally the epithelial walls of this bag form the lining mem- 

 brane of the ventral nervous masses of the brain, and in the 

 epichordal portion of the brain at all events the internal 

 surfaces of the most ventral portion are brought close together 

 by the pressure of the two lateral nervous masses, thus forming 

 the raphe (see p. 394). In other words, the infra-oesophageal 

 and upper ganglia of the ventral chain, which, on my theory, 

 form the epichordal portion of the brain, have by their growth 

 nipped and compressed the ventral surface of the cephalic 

 stomach which was lying dorsal to them. 



Lying on the anterior part of the cephalic stomach we see, 

 in the position of the supra-oesophageal ganglia (see fig. 1, 

 PI. XXV), the nervous masses known as the cerebral and olfac- 

 tory lobes, optic thalami, ganglia habenulae, together with the 



