. A manual of zoology. Zoology. ir. VERTEBRATA: PISCES. 56,- base of the fin, so that the dorsal part is reduced, the ventral greatly enlarged, the result being extremely asymmetrical, as seen from the exterior. The liomocercal fin is symmetrical externally, but in reality is extremely asymmetrical. The end of the vertebral column, the unossified notochord, is bent abruptly upwards, and hence the fin is almost entirely formed of the ventral portion, which is usually divided by a terminal notch into upper and lower halves. The homocercal fin begins with a diphycercal and passes through a hetero
. A manual of zoology. Zoology. ir. VERTEBRATA: PISCES. 56,- base of the fin, so that the dorsal part is reduced, the ventral greatly enlarged, the result being extremely asymmetrical, as seen from the exterior. The liomocercal fin is symmetrical externally, but in reality is extremely asymmetrical. The end of the vertebral column, the unossified notochord, is bent abruptly upwards, and hence the fin is almost entirely formed of the ventral portion, which is usually divided by a terminal notch into upper and lower halves. The homocercal fin begins with a diphycercal and passes through a heterocercal stage in development. In correspondence with the simple motions the musculature is simple and consists largely of longitudinal muscles divided into myotomes, which \v t^ ^-^i^^V'^. are conical witli the apex in front, and are so inserted in each otiier that a cross-sec- tion gives concentric circles. In a section there are at least two such systems, the muscles being divided by a lateral in- cision into dorsal and ventral halves. There are also smaller groups of muscles related to fins, gill arches, jaws, etc., but of much smaller size, derivatives from the larger mass. Electric and pseudelectric organs, which occur in different fislies, sometimes in the trunk, at others in the tail, are formed by the modification of mus- cles. Each organ consists of numerous closely packed vertical or horizontal col- umns, each column, like a Voltaic pile, eonsisting of layers of gelatinous plates (equivalents of muscle bundles) in which the nerves, with special end plates, termi- nate. The discharge is electronegative. The brain shows the low position of the class in the slight development of the cerebrum (tig. .591). This is especially true of the teleosts, in which, in place of a cortex, there is only a thin epithelial layer (Pall), what was formerly called cerebrum being only the corpora striata. The independent olfactory lobes lie either close to the cerebrum (most teleosts, Lol) o
Size: 1788px × 1398px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1902
