XIII 



PHYLUM CHORD ATA 



123 



sisting of a small posterior portion, the cerebral hemisphere 1 ' 

 and a larger anterior portion, the olfactory lobe (olf. L). The 

 diacoele communicates in front with a small prosocoele or common 

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

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



Fir;. 703. Petromyzon marinus. Dorsal (A) and ventral (B) views of brain. -T c/t. pi. 1, an- 

 terior choroid plexus forming roof of pros- and diencephalon ; ch. pi. 2, aperture in roof of 

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

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

 cmra cerebri ; dien. diencephalon; inf. infundibulum ; I. gn. lib. left ganglion habenulae ; 

 in i,'/. oil. medulla oblongata ; Nr. 1, olfactory; Ni\ 2, optic; Nv. 3, oculo-motor ; Nc. 5, tri- 

 geminal, and Nr. 8, auditory nerves ; olj>. I. olfactory lobes ; opt. I. optic lobes ; r. gn. hb. right 

 ganglion habenulaj. (After Ahlborn.) 



divides into two branches, a rhinocoele going directly forwards into 

 the olfactory lobe, and a paracosle 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 habenulse. The 



