ACTJNOPTER YG1I 



305 



branchiostegal rays and the fulcra, may in the course of divergent 

 evolution undergo great modification and reduction. In the primi- 

 tive forms the cranial bones conform to the ordinary Teleostome 

 plan ; the preoperculum does not cover the cheek, as is so often the 

 case in the Osteolepidoti. 



The brain in the Actinopterygii acquires a very characteristic 

 structure. The preponderance of the hind- and mid-brain over the 

 fore-brain, which has already been noticed in the Teleostomes in 

 general, becomes more pronounced, especially in the Teleostei. 

 There is a large cerebellum, which, instead of bulging almost entirely 

 on the surface, as, for instance, in the Selachian, extends forwards 

 and inwards into the ventricle of the mesencephalon. In the 



E 



Fin. 281. 



Diagrams of transverse sections through the secondary fore-brain of A, Pdypterus (after 

 Burckhardt) ; B, Acipenser (after Goronowitsch) ; C, Amia ; D, Salmo ; B, Protopterus (after 

 Burckhardt). 



Chondrostei the solid cerebellum projects but little into the 

 mesocoele, and forms but a rudiment of this valvula cerebelli (Fig. 

 282), which becomes very large in the Teleosts, filling almost 

 completely the cavity of the mid-brain (Fig. 352). Large paired 

 hollow optic lobes are conspicuous, except in the Chondrostei, their 

 roof (tecturn opticum) covering the valvula. The diencephalon 

 becomes shortened and partially hidden above ; below there is 

 a large infundibular outgrowth, with very well developed lobi 

 inferiores and saccus vasculosus (Figs. 282-3, 353). The fore-brain 

 is remarkably undeveloped ; no cerebral hemispheres are formed, 

 the lamina terminalis becomes almost horizontal, the basal ganglia 

 (corpus striatum and epistriatum) are the only conspicuous paired 

 thickenings, and the whole of the roof remains epithelial (Fig. 281). 

 A large velum transversum is present, but the ventricle remains 

 undivided in the middle line. Tire olfactory lobes are sessile in the 



20 



