374 



FISHES 



CHAP. 



melct. 



inner ones extend into the hemispheres as lateral ventricles, 

 and the two outer as rhinocoeles into the olfactory lobes (Fig. 

 214). In connexion with the infundibulum there is a pair of 

 sacci vasculosi, consisting mainly of gland-tubules, opening into 

 the infundibular cavity. 1 The cerebellum is exceptionally large, 



but it does not form a " valvula cere- 

 belli." Large ear-like corpora resti- 

 formia are present. The third and 

 fourth ventricles alone retain an epi- 

 thelial roof in relation with choroid 

 plexuses. 



In all essentials the brain of the 

 Holocephali is a repetition of the 

 Elasmobranch type, more especially of 

 the elongated form seen in Notidanus. 

 Indications of a higher grade of struc- 

 ture are, however, to be seen in the, 

 reduction of the prosencephalon which, 

 with its prosocoele, is now scarcely 

 distinguishable from the thalamen- 

 - cephalon and its ventricle ; and in the 



riG. 214. Horizontal longitu- 

 dinal section of brain of chilo- more complete differentiation of the 



a^SiSLSHTS cerebral hemispheres from one another 

 Origin of cerebeiiar ventricle and from the rest of the brain. Large 



or epicoele ; dia. third ven- f -,-, j , -,. 



tricle ; iter, mesocoele ; meta, frilled corpora restllormia are COn- 

 fonrth ventricle ; opt, opto- spicuous structures on each side of the 



coele, or cavity of an optic '-,,, T1 T> i ^ i 



lobe ;.para, lateral ventricles ; medulla oblongata. Besides the usual 

 ^ro S . P ro.socoele;rA,rhinocoeie. intra-cranial pituitary body, there is 



(From Parker and Haswell.) . * . 



also a separate extra-cranial portion 



lodged in a pit on the ventral surface of the basis cranii : in the 

 embryo the two are continuous. 



In the Teleostomi the brain is distinctly of a more primitive 

 type than in any other Fishes (Fig. 215). 2 The most striking 

 feature is the absence of cerebral hemispheres, the evolution of 

 the primary fore-brain proceeding no farther than the formation 

 of an undivided prosencephalon with a non-nervous roof, and 

 a prosocoele which forms a continuous cavity with the third 

 ventricle, or at the most is only separated from it by an infolding 



1 The sacci probably secrete the fluid contents of the ventricles. 

 - Haller, Morph. Jahrb. xxvi. 1898, p. 345. 



