132 



ZOOLOGY 



SECT. 



front of the diencephalon are paired bean-like masses, each con- 

 sisting of a small posterior portion, the cerebral hemisphere {orb. h.), 

 and a larger anterior portion, the olfactory lobe {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 pineal apiparaUis consists of two vesicles placed in a vertical 

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

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

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

 lower vesicle {p)arap>ineal orgctn, pn.) is in connection with the 



ptv-fi 



Fjg. 70«. — Petromyzon. Side view of brain with olf.ictory and pituitary sacs, in section. 

 chba. cerebellum ; crb. h. cerebral hemisphere ; dien. diencephalon ; /. fold in nasal tube ; 

 ffl, nasal glands ; in/, infundibulum ; L ya. hb. left ganglion habenui» ; vied. ohl. medulla 

 oblongata ; na. ap. nosti'il ; vch. notochord ; Nr. 1, olfactory nerve ; Nr. S, optic ; Nv. 3, oculo- 

 motor ; iVr. i, trochlear ; Ni'. 5, trigeminal ; Nv. 6, abducent ; Nv. 7, facial ; Ni\ S, auditory ; 

 iV'/-. 10, vagus ; Nr. IS, hypoglossal ; olf. cp. olfactory capsule ; olf. L olfactory lobe, with which 

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

 pn. parapineal organ; pn. e. pineal eye; ptti. h. pituitary body ; pty. p. pituitary pouch; 

 up. median septum of olfactory sac ; sp. 1, dorsal root of first spinal nerve. (Combined from 

 figures by Ahlborn and Kaenische.) 



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 a rJiiasma, or intercrossing of fibres between 

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



The sjnnal cord (Figs. 796 and 801, my.) is flattened 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 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. 796, na"" Fig 798, 



