132 



ZOOLOGY 



SECT. 



front of the dicncephalon arc paired bean-like mavsses, each con- 

 sisting (jf a small posterior portion, the cerebral Ii emispho'e (crb. k.), 

 and a larger anterior portion, the olfartort/ lohe {olf. I.). The 

 diacoele communicates in front with a small prosocoele or common 

 fore-ventricle, which is roofed over by a choroid plexus {{cl. pi. 1), 

 and from which a transverse passage goes off on each side and 

 divides into two branches, a rhinocoele going directly forwards into 

 the olfactory lobe, and a paracoele backwards into the hemisphere. 

 The innecd apparattis consists of two vesicles placed in a vertical 

 series : the dorsal-most of these is the vestigial p)incal eye (Fig. 798, 

 piv. c): it has a pigmented retina, a flat and imperfectly formed 

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

 loAvcr vesicle {parapincal organ, pn.) is in connection with the 



vnyUt, 



l.qn.hb 



A'u. to 



A'v.G 



pty.p 



Fjg. 79S.— Petromyzon. Side view of liraiii with olfactoi-j- and pituitary sacs, in section. 

 cblin. cuiclielluiii ; crb. h. cerebral hemisiihere ; ditn. diencephalon ; ./'. fold in nasal tube; 

 (jl. nasal glands; inf. infundibnlum ; I. ija. lib. left ganglion liabenulie; nu'd. oW. niediiUa 

 oblongata ; na. ap. nostril ; nch. notochord ; ifc. 1, olfactory nerve ; Nc. S, optic ; 7fv. -l, oculo- 

 motor ; A<'. i, trochlear ; A'r. .5, trigeminal ; A'c. 6, abducent ; Nr. ~, facial ; Si: S, auditory ; 

 Nr. 10, vagus ; Ni: 12, hypoglossal ; olj. cp. olfactory capsule : olf. I. olfactory lobe, with which 

 the olfactory bulb is amalgamated ; olf. m. m. olfactory mucous membrane ; opt. I. optic lobe ; 

 pn. parapineal organ ; /</». e. pineal eye ; pt)/. h. pituitary body : jitt/. />. pituitary pouch ; 

 up. median septum of olfactory sac ; ■''pi. i, dorsal root of first spinal nerve. (Combined from 

 figures by Ahlborn and Kaenische.) 



small left ganglion habenula-. The pineal eye is not an oi'gan 

 capable, like the paired eyes, of forming definite images of objects, 

 but probably is capable of distingniishing differences in the intensity 

 of the light. The optic nerves differ from those of most of the higher 

 classes in the fact that a rhiasma, or intercrossing of fibres between 

 the nerves of the right and left sides, is not conspicuously developed. 



The sjjinal cord (Figs. 796 and 801, mi/.) is fiattencd and 

 band-like. The dorsal roots of the spinal nerves alternate with 

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

 dorsal I'oots are opposite the myocommas, the ventral opposite 

 the myomeres. A sNaiipathetic is represented. The hy})oglossal is 

 the first s})inal nerve. 



Sensory Organs. — The external nostril (Fig. 796, na," Fig 798, 



