xm 



PHYLUM CHORDATA 



127 



the lateral wall of the diencephalon are the two ganglia habenulce, 

 the right (r. gn. hb.) much larger than the left (I. gn. Tib.} : they are 

 connected with the pineal apparatus. Below the diencephalon is 

 a small flattened pituitary body (Fig. 808, pty. &.). In front of the 

 diencephalon are paired bean-like masses, each consisting of a small 

 posterior portion, the olfactory lobe (crb. h.), and a larger anterior 



A'v I 



FIG. 807. Petromyzon marinus. Dorsal (A) and ventral () views of brain, ch. pi. 1, an- 

 terior choroid plexus forming roof of prosencephalon and diencephalon ; ch. pi. 2, aperture 

 in roof of mid-brain exposed by removal of middle choroid plexus ; ch. pi. 3, metacosle 

 exposed by removal of posterior choroid plexus ; crb. cerebellum ; crb. h. olfactory lobes ; 

 cr. crb. crura cerebri ; dien. diencephalon ; inf. infundibulum ; I. gn. hb. left ganglion" 

 habenulae ; med. obi. medulla oblongata ; Nv. 1, olfactory, Nv. 2, optic, Nv. 3, oculo-motor 

 Nv. 5, trigeminal, and Nv. S, auditory nerves ; olf. I. olfactory bulbs ; opt. I. optic lobes 

 pn. pineal eye ; r. gn. hb. right ganglion habenulse. (After Ahlborn.) 



portion, the olfactory bulb (olf. I.). The diacoele communicates 

 in front with a small prosocoele or common fore-ventricle, which is 

 roofed over by a choroid plexus (ch. pi. 1), and from which a trans- 

 verse passage goes off on each side and divides into two branches, 



