XIII 



PHYLUM CHORDATA 



131 



rv^^^n^^^ System.— In the brain the small size of the cerebellum 

 (-tig. 797, crb.) is remarkable : it is a mere transverse band roofing 

 over the anterior end of the metaccsle. The optic lobes {opt. I.) 

 are very imperfectly differentiated, and the central region of the 

 roof of the mid-brain is formed merely of a layer of epithelium, 

 so that when the membranes of the brain are removed, an aperture 



ATutj: 



ATvl 



otfl 



fnedohl — t y ^ 



mci-ohl 



Fio. 797.— Petromyzon marinus. Dorsal (A) and ventral (B) views of brain, ch. pi. 1, an- 

 terior choroid plexus forming roof of prosencephalon and diencephalon ; ch. pL^, 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. cerebral hemispheres ; cr. crb. 

 crura cerebri ; dien. diencephalon ; ivf. infundibulum ; I. gn. hb. left ganglion habenula^ ; 

 vied. obi. medulla oblongata ; Nv. 1, olfactory, Nv. 2, optie, Uv. 3, oculo-motor, JVr. 5, tri-' 

 geminal, and Ne. S, auditory nerves ; olf. I. olfactorj' lobes ; opt. l. optic lobes ; pn. pineal eye ; 

 r. gn. hb. right ganglion habenulae. (After Ahlborn.) 



is left which is covered in the entire organ by a vascular thickening 

 of the pia, or choroid plexus {ch. pi. 2). On the dorsal border 

 of the lateral wall of the diencephalon are the two ganglia 

 habenulce, the right {r. gn. hb.) much larger than the left [1. gn. hb.) : 

 they are connected with the pineal apparatus. Below the dien- 

 cephalon is a small flattened pituitary' body (Fig. 798, pi'i/. b.). In 



I 2 



