XIII 



PHYLUM CHORDAT4 



131 



Nervous System. In the brain the small size of the cerebellum 

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

 over the anterior end of the metacoele. 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 



otf.l 



med-cbl - 



, obi- 



Fii;. T'.'T. Fetromyzon marinus. Dorsal (A) and ventral (B) views of brain, ch. pi. 1, an- 

 terior choroid plexus forming roof of prosencephalon and diencephalon; ch. pl.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. 

 crura cerebri ; dien. diencephalon ; inf. infundibulum ; I. yn. hb. left ganglion habenulse ; 

 med. ohl. medulla oblongata ; Ni\ 1, olfactory, Ni\ 2, optic, Nt\ 3, oculo-motor, Ni\ 5, tri- 

 geminal, and Nv. 8, auditory nerves ; olf. I. olfactory lobes ; opt. I. optic lobes ; pn. pineal eye ; 

 r. gn. lib. right ganglion habeiiulse. (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 

 habemdce, the right (r. gn. Jib.) 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, pty. b). In 



I 2 



