The most simple condition of the nervous central organ known in Vertebrates is found in _Branchiostoma_. In this fish the spinal chord tapers at both ends, an anterior cerebral swelling, or anything approaching a brain, being absent. It is band-like along its middle third, and groups of darker cells mark the origins of the fifty or sixty pairs of nerves which accompany the intermuscular septa, and divide into a dorsal and ventral branch, as in other fishes. The two anterior pairs pass to the membranous parts above the mouth, and supply with nerve filaments a ciliated depression near the extremity of the fish, which is considered to be an olfactory organ, and two pigment spots, the rudiments of eyes. An auditory organ is absent. The _spinal chord_ of the _Cyclostomes_ is flattened in its whole extent, band-like, and elastic; also in _Chimæra_ it is elastic, but flattened in its posterior portion only. In all other fishes it is cylindrical, non-ductile, and generally extending along the whole length of the spinal canal. The Plectognaths offer a singular exception in this respect that the spinal chord is much shortened, the posterior portion of the canal being occupied by a long cauda equina; this shortening of the spinal chord has become extreme in the Sun-fish (_Orthagoriscus_), in which it has shrunk into a short and conical appendage of the brain. Also in the Devil-fish (_Lophius_) a long cauda equina partly conceals the chord which terminates on the level of about the twelfth vertebra. The _brain_ of fishes is relatively small; in the Burbot (_Lota_) it has been estimated to be 1/720th part of the weight of the entire fish, in the Pike the 1/1305th part, and in the large Sharks it is relatively still smaller. It never fills the entire cavity of the cranium; between the dura mater which adheres to the inner surface of the cranial cavity, and the arachnoidea which envelops the brain, a more or less considerable space remains, which is filled with a soft gelatinous mass generally containing a large quantity of fat. It has been observed that this space is much less in young specimens than in adult, which proves that the brain of fishes does not grow in the same proportion as the rest of the body; and, indeed, its size is nearly the same in individuals of which one is double the bulk of the other. [Illustration: Fig. 41.--Brain of Perch. I. Upper aspect. II. Lower aspect. _a_, cerebellum; _b_, optic lobes; _c_, hemispheres; _e_, lobi inferiores; _f_, hypophysis; _g_, lobi posteriores; _i_, Olfactory lobes; _n_, _N._ opticus; _o_, _N._ olfactorius; _p_, _N._ oculo-motorius; _q_, _N._ trochlearis; _r_, _N._ trigeminus; _s_, _N._ acusticus; _t_, _N._ vagus; _u_, _N._ abducens; _v_, Fourth ventricle.]