PETROMYZON 



25 



there are three such canals, in planes at right angles to one 

 another, but Petromyzon has only two. 



Brain. — The anterior end of the nerve-tube is modified and 

 enlarged in connexion with the paired sense-organs to form the 

 brain. The brain can be divided into three main regions : 

 fore-, mid-, and hindbrain. The forebrain bears the olfactory 

 lobes in front, and on each side it connects with the eyes (which 

 are really part of it) by the optic nerves. The roof bears the 

 pineal and parapineal eyes. These lie in the middle line, the 

 pineal above the parapineal which is degenerate. The pineal 

 eye is a vesicle of which the dorsal wall forms a lens, and the 

 ventral wall a retina backed with pigment. The nerve-fibres 



ns. 00. Pf- Ff ■ 



hi oc - P b 



Fig. 15. — Petromyzon : view of median longitudinal section through the 



brain. 



c, cerebellum ; cp, choroid plexus ; hs, hypophysial sac ; n, notochord ; 

 nc, nerve-cord ; ns, nostril ; oc, optic chiasma ; 00, olfactory organ ; pb, 

 pituitary body ; pe, pineal eye ; ppe, parapineal eye. 



lead away from the underside of this retina, which is therefore 

 not inverted but erect. Above the pineal eye, the skull is thin 

 and the tissues are more or less transparent. Beneath the 

 forebrain is a simple pituitary body (see p. 399), the pars 

 intermedia of which is apposed to the feebly developed in- 

 fundibulum of the brain. The pituitary body in Petromyzon 

 has lost connexion with the hypophysial cavity ; the latter 

 extends backwards beneath the brain forming the hypophysial 

 sac, and connects with the exterior through the median dorsal 

 pore. 



The midbrain bears the optic lobes, and the roof .of the 

 hindbrain is modified into a rudimentary cerebellum. It is 



