XIII 



PHYLUM C FIORD ATA 



131 



Nervous System. — In the brain the small size of the cerehelhiin 

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

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

 are very imperfectly tlifferentiated, 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 



J\/vt 



N'ui 



medchl — 



rnci-ohi 



Fii;. Ti'T.— Petromyzon marinus. Dorsal (A) and voutral (B) views of brain, ch. pi. 1, an- 



tei'ior choroid plexub forming roof of prosencephalon and diencephalon; ch. 2)1.2, aperture in roof 

 of mid-l.)rain exposed by removal of middle choroid plexus: ch. pi. 3, metaccele exposed by 

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

 crura cerebri ; dien. diencephalon ; hif. infundibulum ; I. tjn. hb. left ganglion halDenulis ; 

 med. obi. medulla oblongata ; Nr. 1, olfactory, Nr. ,', optic, Nc. S, oculo-motor, Nr. 5, tri- 

 geminal, and Nc. S, auditory nerves ; olf. I. olfactory lobes ; ojjt. I. optic lobes ;pn. pineal eye ; 

 /•. f/n. hb. right ganglion habcnula". (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 

 haheniUce, the right {r.gn.hh.) ranch, larger than the left (/. gn.hh.) : 

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

 cephalon is a small flattened pituitary body (Fig. 798, j^i'/- ^O- In 



I 2 



