162 COMPARATIVE ANATOMY 



elements, mucli as in Cyclostomes, The olfactory lobes are 

 closely applied to the prosencephalon, which gives rise 

 anteriorly to cerebral hemispheres containing lateral ventricles 

 (Fig. 132). 



The well-developed thalamencephalon has a marked ventral 

 flexure and from its roof arises a strong pineal peduncle, the distal 

 end of which extends into a hollow in the cranial roof, but 

 undergoes atrophy, in Amia becoming completely separated off 

 from the brain.^ Well-developed lobi inferiores are present, 

 and the hypophysis^ and saccus vasculosus are voluminous: 

 the latter consists largely of glandular tubules which open 

 into the infundibulum, as is also the case in Elasmobranchs 

 (p. 160). 



The large cerebellum gives rise to a valvula cerebelli (comp. 

 Fig. 134) extending forwards into the ventricle of the mid-brain ; 

 the optic lobes are also large. 



The brain of Amia on the whole most nearly approaches that 

 of the Teleostei in structure. 



Teleostei. — As is the case in many other Fishes, the brain in 

 most Teleosts by no means fills the cranial cavity, and it is separated 

 from the roof of the skull by a greater or less amount of a fatty 

 and lymph-like fluid. It never attains to so large a relative size 

 as does that of Elasmobranchs. Its form varies greatly, more by 

 far than in any other Vertebrate group, and only the following 

 essential points can be mentioned here. 



The pallium is entirely epithelial in structure (Figs. 133-135), 

 and, moreover, it presents no median involution dividing the 

 anterior part of the prosencephalon into two lateral hemispheres : 

 there is a median prosoccele. The lower part of the prosen- 

 cephalon is made up of large paired basal ganglia (corpora 

 striata) connected together by an anterior commissure. The 

 olfactory lobes are either closely applied to the prosencephalon 

 and contain a small rhinoccele, or they become differentiated into 

 olfactory tract and bulb, as in Elasmobranchs (p. 159). 



The thalamencephalon is very small. The epiphysis (Figs. 

 133, 134) is plainly distinguishable, but it does not pass into 

 the roof of the skull ; an outgrowth arising from the roof of the 

 brain in front of the epiphysis represents the parietal organ, 

 but this becomes constricted off from the brain and disappears 

 during development.-' Marked lobi inferiores, as well as a 



^ In Polypterus the pineal body gives rise to a peculiar and extremely large 

 epithelial vesicle. In Devonian Ganoids there was » parietal foramen (comp. 

 p. 171). 



^ In Polypterus and Calamoichthys the hypophysis communicates with the 

 mouth-cavity by a hollow duct, even in the adult (comp. p. 155). 



^ A parietal foramen is, however, often present in tlie embryo, and persists 

 throughout life in Callicthys. 



