BRAIN 



245 



The brain deserves particular notice : 

 few organs in the Dipnoi so clearly show 

 the isolation of this sub-class from other 

 fish. On the whole, the brain is primitive 

 in structure ; it lacks the special character- 

 istics of the Elasmobranch or of the Teleo- 

 stome ; so far as it is distinctly advanced, 

 it approximates to the Amphibian type 

 (Beauregard [35], Burckhardt [69], Bing 

 and Burckhardt [73]). The brain is long 

 and narrow, and has large ventricles. A 

 large and simple medulla is followed by a 

 very small cerebellum (Figs. 214-15). The 

 mid-brain is long and fully exposed ; the 

 optic lobes are fused in Protopterus. There 

 is a velum transversum. Most remarkable 

 are large paired cerebral prolongations 

 projecting far beyond the lamina terminalis. 

 These ' hemispheres ' are separate, and in 

 the Dipneumones have thickened nervous 

 walls above and at the sides as well as 

 below (Fig. 281); in Ceratodus, however, 

 which has a less developed and perhaps 

 somewhat degenerate brain, the roof of the 

 prosencephalon is epithelial, like that of the 

 greater part of the brain. In the Dipneu- 

 mones the relatively small olfactory lobes 

 are attached in front to the cerebral hemi- 

 spheres; but the rhinencephalon of Cera- 

 todus is drawn out into a tract expanding 

 into a large hollow olfactory bulb near the 

 nasal capsule. The ganglion cells are 

 stratified in the fore-brain. 



The dentition is most characteristic. 

 The dental plates, already described, are 

 firmly fused to the underlying bones, and 

 not replaceable. They are composed of 

 thick dentine, round numerous tubular and 

 often branching pulp-cavities. The grind- 

 ing surface, in modern forms, is provided 

 with high radiating ridges. These ridges 

 are really formed in the embryo by the 

 growth and fusion of originally distinct 

 small conical teeth (Fig. 210A, Semon [401], 

 Fig. 21 OB). All external trace of their 



19 



-15 



I 



-<), 



3 



1 



FIG. 214. 



Dorsal view of the brain of 

 rrotopterns annectens, Owen. 

 (After Burckhardt, from Sedg- 

 wick's Zoology.) 1, spinal cord ; 

 2, dorsal root of first spinal 

 nerve ; 8, diverticula of 4, the 

 saccus endolymphaticns ; 5, 

 medulla oblongata ; 6, fourth 

 ventricle ; 7, cerebellum ; 8, 

 mesencephalon (fused optic 

 lobes) ; 9, stalk of pineal body ; 

 10, thalamencephalon ; 11, velum 

 transversum; 12, pineal body; 18, 

 lobus hippocampi ; 14, choroid 

 plexus ; 15, cerebral hemisphere 

 (proscncfphcloii) ; 10, olfactory 

 lobe. 



