128 ZOOLOGY SECT. 



a rhinoccele going directly forwards into the olfactory bulb, and 

 a paracoele backwards into the olfactory lobe. 



The pineal apparatus consists of two vesicles placed in a vertical 

 series : the dorsal-most of these is the vestigial pineal eye (Fig. 808, 

 pn. e.) : it has a pigmented retina, a flat and imperfectly formed 

 lens, and is connected with the right ganglion habenulae. The 

 lower vesicle (parapineal organ, pn.), of the same nature as the upper 

 but more imperfectly developed, is in connection with the small left 

 ganglion habenulae. The pineal eye is not an organ capable, like 

 the paired eyes, of forming definite images of objects, but probably 

 is capable of distinguishing differences in the intensity of the light. 

 The optic nerves differ from those of most of the higher classes in 

 the fact that the chiasma is not visible externally the intercrossing 

 of the fibres taking place beneath the surface. 



The spinal cord (Figs. 806 and 811, wy.)*is[flattened and band-like. 



na.afi. 



P n e , l.crn.hb 

 lificrb.h/ 



dien, <** 1 



cblm m.ed.obl 



Nv.4 tfv. 7, 



FIG. 808. Petromyzon. Side view of brain with olfactory and pituitary sacs, in section. 

 cblm. cerebellum ; crb. h. olfactory lobe ; dien. diencephalon ; /. fold in nasal tube ; g I. 

 nasal glands ; inf. infundibulum ; 1. gn. hb. left ganglion habenulse ; med. obi. medulla 

 oblongata ; na. ap. nostril ; nch. notochord ; Nv. 1, olfactory nerve ; Nv. 2, optic ; Nv. 3, 

 oculomotor ; Nv. 4, trochlear ; Nv. 5, trigeminal ; Nv. 6, abducent ; Nv. 7, facial ; Nv. 8, 

 auditory ; Nv. 10, vagus ; Nv. 12, hypoglossal ; olf. cp. olfactory capsule ; olf. I. olfactory 

 bulb ; olf. m. m. olfactory mucous membrane ; opt. 1. optic lobe ; pn. parapineal organ ; 

 pn. e. pineal eye ; pty. b. pituitary body ; pty. p. pituitary pouch ; sp. median septum of 

 olfactory sac ; sp. 1, dorsal root of first spinal nerve. (Combined from figures by Ahlborn 

 and Kaenische.) 



The dorsal roots of the spinal nerves alternate with the ventral 

 roots, and do not unite with them to form a trunk : the dorsal 

 roots are opposite the myocommas, the ventral opposite the 

 myomeres. A sympathetic is represented. The hypoglossal is 

 the first spinal nerve. 



Sensory Organs. The external nostril (Fig. 806, na", Fig. 808, 

 na. ap.) leads by a short passage into a rounded olfactory sac 

 (Fig. 806, na, Fig. 808) placed just in front of the brain and having 

 its posterior wall raised into ridges covered by the olfactory mucous 

 membrane (Fig. 808, olf. m. m.). From the bottom of the sac is 

 given off a large pituitary pouch (Fig. 806, na', Fig. 808, pty. p.) 



