xm 



PHYLUM CHORDATA 



127 



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

 the right (r. gn. lib.) much larger than the left (I. gn. hb.) : 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 



B 



Nut 



rn.ed.obL 



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

 terior choroid plexus forming roof 9f prosencephalon and diencephalon ; ch pi 2 aperture 

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

 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 eanelion 

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

 Nr. 5, trigeminal, and Nv. S, auditory nerves ; olf. 1. olfactory bulbs : opt. I. optic lobes ' 

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



portion, the olfactory bulb (olf. 1). The diaccele communicates 

 in front with a small prosoccele 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, 



