XIII 



PHYLUM CHORDATA 



sisting of a small posterior portion, the cerebral liemispherc'\ci'l). h.), 

 and a larger anterior portion, the olfactory lobe (olf. I.}. The 

 diacoele communicates in front with a small prosocosle or common 

 fore- ventricle, which is roofed over by a choroid plexus (d. pi. 1) 

 and from which a transverse passage goes off on each side and 



tful. 



otf.l 

 pn. 



cK-pl .1 

 L&ns.Hb 



med-.obl 



- m-cd.obl 



Fir;. 7">3. Petromyzon marinus. Dorsal (A) and ventral (B) views of brain, ch.pl. 1, an- 

 terior choroid plexus forming roof of pros- and diencephaloii ; cli. pi. 2, aperture in r roof of 

 mid-brain exposed by removal of middle choroid plexus ; cli. pi. 3, metacoele exposed] by 

 removal of posterior choroid plexus ; crb. cerebellum ; crb. li. cerebral hemispheres ~~cr. crb. 

 crura cerebri ; dicn. diencephaloii ; i/if. infuiidibuluni ; I. <jn. lib. left ganglion habenulse : 

 nii'A. olf. medulla oblougata ; jYV. 1, olfactory; JVY. ~, optic; Nr. 3, oculo-motor ; J\V. :". tri- 

 geminal, and A'r. 8, auditory nerves ; olf. 1. olfactory lobes ; opt. 1. optic lobes ; /. <jn. lib. right 

 ganglion habeiiulse. (After Ahlbom.) 



divides into two branches, a rhinoccele going directly forwards into 

 the olfactory lobe, and a paracoele backwards into the hemisphere. 

 The pineal apparatus consists of three vesicles placed in a vertical 

 series : the dorsal-most of these is the vestigial pineal eye (Fig. 754, 

 pn. e.) : it has a pigmented retina, a flat and imperfectly formed 

 lens, and is connected with the right ganglion habenulae. The 



