376 



FISHES 



CHAP. 



and the absence of corpora restiformia. This type of brain is 

 most strongly marked in the Teleostei. but in other Teleo- 

 stomes some, like Acipenser, 1 are typically Teleostean in this 

 respect (Fig. 216), while others, such as Lepidosteus, have 

 small cerebral hemispheres with lateral ventricles as well as a 

 prosencephalon. 



The most obvious feature in the brain of the Dipnoi is the 

 great development of the cerebral hemispheres. In this respect 

 these Fishes approach the Amphibia, but in other features of brain- 



pn.o. 



p.ch.p. 



opxh 



FIG. 216. Vertical longitudinal section of the brain of a Sturgeon (Acipenser ruthemis.} 

 c.p, Posterior commissure ; c.r, cranial roof ; me, mesocoele ; op. ch, optic chiasma ; 

 p.ch.p, posterior choroid plexus ; v.c, valvula cerehelli ; v.t, velum transversum ; 

 r.iii, v.iv., the third and fourth ventricles ; other lettering as in Fig. 210. (From 

 Goronowitsch.) 



structure they present points of agreement with most other 

 groups of Fishes without being closely related to any one of them. 

 In Protopterus 2 (Fig. 217) the hemispheres are quite distinct 

 except behind, and the walls of their spacious lateral ventricles 

 are entirely nervous. Olfactory lobes are sessile on their anterior 

 extremities, and behind and below they enlarge into ventral lobes 

 which probably represent the hippocampal lobes of the higher 

 Vertebrates. A vesicular pineal body at the end of a slender 

 stalk overlies a singular conical projection from the roof of the 

 thalamencephalon or "pineal pillow." The optic lobes form a 

 single oval body, and, as in Petromyzon and the Amphibia, the 



1 Goronowitsch, Morph. Jahrb. xiii. 1888, p. 427. 



2 Burckhardt, Das Central-Nervensystem v. Protopterus annectens. Berlin, 

 1892. 



